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With three instruments operating 24/7, the Solar Dynamics Observatory is gleaning new insights into how magnetic fields control solar activity. by W. Dean Pesnell

fter a year in orbit, NASA’s observatory into Earth orbit. Although Solar Dynamics Observatory the launch went smoothly, as SDO tra- (SDO) has started to fulfill its versed the surrounding atmosphere, it promise. The mission’s task: demonstrated how the observatory could to examine the Sun in such affect observations of solar phenomena. Adetail that astronomers will be able to A large winter storm over the eastern understand how our star’s magnetic field United States the previous day brought drives solar prominences, flares, coronal cold temperatures to central Florida. mass ejections (CMEs), and other solar Coupled with a thin layer of cirrus activity. And, just as important, the obser- clouds, the chill produced a sundog — a vatory will measure the changes in the rainbow-colored patch of light 22° from Sun that cause space weather, whose the Sun. A sundog arises when sunlight effects range from power outages and refracts through six-sided ice crystals all navigation problems on Earth to creating aligned with their broad sides down. drag on satellites in orbit. As SDO climbed through the cirrus

Pat Corkery/United Launch Alliance Corkery/United Pat SDO began its mission February 11, cloud deck, a sound wave emanating The Solar Dynamics Observatory launches 2010. That day, an Atlas V rocket roared from the rocket caused the sundog to from Cape Canaveral February 11, 2010, atop to life on Space Launch Complex-41 at disappear. Apparently, the wave either an Atlas V rocket with a Centaur upper stage. Cape Canaveral, Florida, and lofted the evaporated the ice crystals or destroyed These colorful Suns show our star’s appearance July 28, 2010, at five A massive solar prominence (upper left) erupts from the Sun March 30, wavelengths in the extreme ultraviolet portion of the electromagnetic 2010. Scientists created this false-color, multiwavelength image with data spectrum. By observing the Sun at different high-energy wavelengths, from the Solar Dynamics Observatory. The orange-red background images solar scientists will learn how the Sun converts energy in its magnetic on this page show the Sun’s chromosphere that same day in the light of fields into the heat that drives solar flares. NASA/SDO/AIA Science Team ionized helium (a wavelength of 30.4 nanometers). NASA/SDO/AIA Science Team

www.Astronomy.com 25 1:20 p.m. EDT 1:40 p.m. EDT 2:00 p.m. EDT

On March 30, 2010, a large prominence erupted on the Sun’s limb and spewed hot gas into space at speeds of approximately 435 miles per second (700 km/sec). This sequence of six images from the Solar Dynamics Observatory shows the eruption lasted only a couple of hours. The observatory captured these images in the light of singly ionized helium (a wavelength of 30.4 nanometers). NASA/SDO/AIA Science Team

their matching orientations. So, within maps the so-called vector magnetic field, offers more information, the line-of-sight minutes of launch, SDO already had the component directed across our line of measurements are easier to produce. impacted solar observations. But the real sight, every 15 minutes. SDO’s second instrument is the proof of the mission’s significance would The instrument makes these maps with Extreme ultraviolet Variability Experi- come only when the observatory began polarizing filters that measure how the ment (EVE). Developed at the University viewing the Sun in earnest from its perch velocity and magnetic field change. To of Colorado’s Laboratory for Atmospheric far above Earth’s atmosphere. gauge velocity, they measure the Doppler and Space Physics and the University of shift — the change in wavelength as the Southern California, EVE has three spec- SDO’s three-pronged attack distance between the Sun and SDO varies. trographs that measure the solar spectral The Solar Dynamics Observatory uses As the solar surface enlarges, and thus irradiance — the amount of energy the three instruments to study the Sun’s mag- moves toward SDO, the wavelength Sun emits at a given wavelength— for netic field. Scientists designed these tools decreases; as the solar surface shrinks, the wavelengths between 0.1nm and 105nm. to probe the Sun from below its surface wavelength increases. EVE also includes a out to the hot corona. Their goal: to find Astronomers track As soon as the small X-ray imager. how the magnetic field changes over time. these changes by observ- Extreme ultraviolet Scientists at Stanford University and ing one particular spec- detectors reached radiation from the Sun the Lockheed Martin Space Astrophysics tral line over time — in heats and ionizes the Laboratory (LMSAL) developed the Heli- this case, iron at 617.3 operating upper parts of Earth’s oseismic and Magnetic Imager (HMI). It nanometers — and temperature, the atmosphere to such a studies the behavior of the motions of the seeing how its incoming degree that scientists Sun’s surface and also the magnetic fields wavelength changes. The Sun put on a show. call it the “heartbeat of there. The Sun is a combination of gas velocity maps essentially space weather.” and its extremely hot, and thus ionized, build ultrasounds of the Sun that reveal The third instrument aboard SDO is counterpart known as plasma; its “sur- what is going on below its surface. Scien- the Atmospheric Imaging Assembly face” (or photosphere) is the region from tists use the same spectral line to chart the (AIA), which scientists at LMSAL also which light escapes into space. Every 45 behavior of magnetic fields at the surface. developed. It studies how the solar seconds, HMI makes maps of both veloc- Space-weather forecasters use the line- corona responds to the magnetic fields ities (the motions) and “line-of-sight” of-sight maps to anticipate solar flares that HMI observes near the Sun’s surface. magnetic fields at the solar surface. It also and CMEs. The vector-field maps, mean- AIA’s four telescopes focus light onto while, show the strength and direction of four CCD cameras and take images of the W. Dean Pesnell is the project scientist for the magnetic field as it emerges through Sun’s atmosphere at 10 wavelengths: one NASA’s Solar Dynamics Observatory. the surface. Although the vector field in visible light, two in the ultraviolet part

Wavelength: 450nm Wavelength: 33.5nm Wavelength: 30.4nm Wavelength: 21.1nm Wavelength: 19.3nm Wavelength: 17.1nm Wavelength: 13.1nm February 1, 2011 February February 14, 2011 February

The Sun’s character changes when viewed at different wavelengths. In visible light (far left), its photosphere typically reveals sunspots. But at shorter wavelengths in the extreme ultraviolet, the Sun’s outer atmosphere (its chromosphere and corona) come into view. NASA/SDO/AIA Science Team

26 Astronomy • May 2011 2:20 p.m. EDT 2:40 p.m. EDT 3:00 p.m. EDT

of the spectrum, and seven in the extreme HMI even saw a sunspot before the heit (–70° Celsius), an enormous promi- ultraviolet part that corresponds to the instrument’s door fully opened. Almost nence erupted off the Sun’s limb. (See the ionization states of iron and helium. Data immediately after the EVE doors opened sequence of images above and the large from the iron spectral lines allow SDO at 2:43 p.m. EDT March 26, active region photo on page 25.) scientists to map the corona’s temperature 11057 provided fireworks. The hot spot The images show that the ring-shaped from 0.6 to 20 million kelvins; the helium fired off four relatively small flares start- prominence sent a pulse of plasma rush- observations probe temperatures from ing at 5:08 p.m. EDT. ing away from the Sun at a speed of about 30,000 to 100,000 K. But the Sun saved its best for last. 435 miles per second (700 km/s). Before Because EVE and AIA fly together, Although AIA scientists opened their the eruption, this prominence was a long solar astronomers can associate most doors March 27, they kept their CCDs tube of magnetically contained material changes in the Sun’s irradiance with spe- hot to drive off contaminants. On March just above the visible surface. Then, by cific events, such as flares, simply by align- 30, just after the CCDs reached their some still poorly understood mecha- ing changes in EVE’s measurements with operating temperatures of –94° Fahren- nisms, it destabilized and created a small changes in the AIA images. This helps EVE scientists explain where their changes came from and helps calibrate AIA by determining the energy of each event. A grand first show After its launch, SDO took more than a month to reach its operating position. On March 16, 2010, the observatory reached its final geosynchronous orbit, in which it hovers above the longitude of its White Sands, New Mexico, receiving station. From this location, it can both observe the Sun and communicate with the ground 24 hours a day. Soon afterward, NASA engineers started to turn on the electronics for each SDO instrument before cooling down the CCDs and opening the instrument doors. HMI went first and then EVE, while AIA kept its heaters on until its doors opened. A double rainbow arcs above one of the 18-meter radio dishes at NASA’s White Sands Complex in As soon as the detectors reached oper- New Mexico. Because the Solar Dynamics Observatory lies in geosynchronous orbit, the antennas ating temperature, the Sun put on a show. here can receive all of the observatory’s data. NASA/Tim Gregor

Wavelength: 450nm Wavelength: 33.5nm Wavelength: 30.4nm Wavelength: 21.1nm Wavelength: 19.3nm Wavelength: 17.1nm Wavelength: 13.1nm February 14, 2011 February

A Valentine’s Day solar flare erupted from a sunspot group located just to the lower right of the Sun’s center. The group shows up nicely in visible light (far left). At shorter wavelengths, the flare’s brightness dominates. This February 14 event was the Sun’s largest flare in more than 4 years. NASA/SDO/AIA Science Team

www.Astronomy.com 27 4:45 a.m. EDT 5:15 a.m. EDT 5:45 a.m. EDT

Active region 11092 (the bright maelstrom halfway to the limb in the 10 o’clock direction) flared several times August 1, 2010, but that was just the start. Shock waves raced across the solar surface, disrupting the dark filaments visible above the Sun’s center. At about 4:40 a.m. EDT, one of these filaments launched a coronal mass ejection toward Earth. These images show gas glowing at 1.5 million kelvins (a wavelength of 19.3nm). NASA/SDO/AIA Science Team

CME. It is vitally important to under- magnetic energy into heat. Solar physi- EUV emissions last longer as well. Both of stand these mechanisms because they cists classify flares as B, C, M, or X (from these observations indicate that scientists produce all CMEs, which can launch up low- to high-energy X-rays); they also use need solar spectral measurements in to 10 billion tons of plasma into the solar flares’ appearances in Hydrogen-alpha many wavelengths to predict the effects of system and cause serious consequences images to categorize them. SDO provides space weather in our planet’s atmosphere. for any object, natural or man-made, that the additional ability to classify flares by happens to be in the way. their total energy (by combining AIA and The magnetic field’s behavior Since the first observations made EVE measurements) and by their devel- Solar physicists have not yet figured out through the AIA 30.4nm channel, SDO opment at many temperatures. cause and effect of flares, CMEs, and other scientists have observed new dynamics in The new observatory has started to magnetic phenomena. It’s a little like an how coronal filaments evolve. SDO teams give researchers a unique look at flare incident with my young nephew. After have studied why and how filaments form, evolution. Much of this is because SDO dinner one evening, he ran around the erupt off the surface, twist, and either eject takes fresh images of the Sun almost con- family room and hit the wall at full speed into the solar wind or return tinuously (AIA, for just as the furnace coincidentally turned to the lower solar atmosphere example, snaps eight on and warm air rushed into the room. He as coronal “rain.” Much of SDO takes fresh full-disk images every looked around and said, “It never did that what solar scientists see, they 12 seconds and oper- before. I just had to hit the wall harder.” don’t yet understand. For images of the ates nearly 24/7). But Scientists face a similar situation when example, why do some fila- Sun almost SDO also covers a they look at the Sun. We already know its ments escape and others have broad range of tem- magnetic field connects places on the Sun their plasma drain back onto continuously. peratures, and scien- that can be far apart. Flares, prominences, the Sun? Are the filaments tists can coordinate the and CMEs happen somewhere on our star twisted when they become measurements made almost every day. How can we know unstable, and do they twist more as they through its three instruments. The Sun whether any two events are related, or erupt? Does the filament heat or cool as it has produced flares of all classes for SDO, whether the “butterfly effect” is at work? If erupts? And how does the coronal rain including the strongest X-class flare in small changes in one place on the Sun can interact with the dense atmosphere once it more than 4 years on February 14, 2011. cause large changes in another place, then falls back to the surface? EVE data reveal that most of the understanding and predicting its magnetic energy radiated by a flaring region does field becomes far more complicated. The what and how of flares not consist of X-rays with wavelengths SDO scientists can look at this problem People have watched solar flares for more less than 7nm, but at longer extreme using high-resolution observations across than 150 years. (English astronomer Rich- ultraviolet (EUV) wavelengths around many wavelengths beamed to Earth at a ard Carrington spotted the first Septem- 27nm. This has consequences far beyond rate of nearly one new image every sec- ber 1, 1859.) During that time, researchers our understanding of solar flares. Earth’s ond. This allows them to see changes have learned that flares release enormous atmosphere absorbs EUV radiation at propagate across the Sun’s disk. They’ve amounts of energy by converting the Sun’s higher altitudes than it does X-rays, and already seen several examples, but the best

28 Astronomy • May 2011 6:15 a.m. EDT

was the August 1, 2010, flare and double CME. (See the images above, which show the Sun in AIA’s 19.3nm channel.) Active region 11092 flared several times early that morning. Although this was only a C-class flare, it affected two filaments whose “foot points” (where the ends enter the Sun’s surface) were 185,000 miles (300,000 kilometers) and 280,000 miles (450,000 km) away, respectively. Soon after the flare, a dimming in the corona spread across the Sun. When this The August 1, 2010, flare (white area at upper left) launched a shock wave whose edge appears at dimming reached the longer filament, it upper right. This view combines data from three extreme ultraviolet wavelengths. NASA/SDO/AIA Science Team began to lift off the Sun. At about 5:40 a.m. EDT, the filament erupted and launched a CME toward Earth. Afterward, a beautiful close proximity, SDO scientists still haven’t increase in data from missions such as arcade of post-eruption loops formed in determined cause and effect. Some think SDO with ever-faster computer speeds to AIA images and a prairie fire of emission the spreading of coronal dimming repre- run solar models, solar scientists should spread out below where the filament had sents a wave or pulse that links remote take big steps forward before Solar Cycle been. Later that day, the second filament areas of the Sun, but others aren’t so sure. 25 starts around 2020. also erupted. Although all three events The vector magnetograms produced occurred on the same day and in relatively Looking in the crystal ball by HMI should help with their predic- Given our current knowledge, predicting tions. For the first time, solar scientists future solar activity is a huge challenge. will have measurements of the strength 150 What’s ahead for Depending on the prediction you read, and direction of the Sun’s magnetic field the Sun? the current solar cycle (number 24) could over its visible disk. These data are fast be extremely large or nearly absent. enough for scientists to see changes in 100 (Solar cycles run on a roughly 11-year the field that coincide with or precede period from minimum to maximum and flares and CMEs. back; the current cycle began in 2009 and From launch to the present, SDO has Sunspot number 50 should end around 2020.) yielded information about the Sun and the Solar Cycle Solar Cycle 23 24 Looking at the Sun’s present state, it world around us. Will pulses spreading 0 appears that Solar Cycle 24 will prove from central sites serve as the link between 2000 2005 2010 2015 2020 Year below average in terms of the number of the events witnessed at SDO’s launch and sunspots, and also longer than average. the solar science it conducts? Time will No one knows how big Solar Cycle 24 will be. This would follow a pattern in sunspot tell. After all, SDO’s 5-year mission to Black dots show cycle 23 observations and red numbers where the Sun’s activity level study Solar Cycle 24 has just begun. dots cycle 24. The author predicts the current cycle will peak with an average sunspot number pauses every 100 years or so. But we of 80. He predicts the peak will occur in late 2013 don’t have a clue what Solar Cycle 25 Watch SDO videos of the Sun at www.Astronomy.com/toc. or early 2014. Astronomy: Roen Kelly, after W. Dean Pesnell will look like. By combining the rapid

www.Astronomy.com 29 Stellar astrophysics

A billion tons of gas erupts from the Sun during a coronal mass ejection. This blast occurred January 8, 2002, near the peak of solar cycle 23. SOHO (NASA and ESA)

© 2013 Kalmbach Publishing Co. This material may not be reproduced in any form without permission from the publisher. www.Astronomy.com Is the an oddball star? Accepted wisdom holds that the Sun is an ordinary, run-of-the-mill star. But astronomers are having a hard time finding true solar twins. by Bruce Dorminey

’m OK. You’re OK. But the Sun? It any age. By contrast, a solar twin must normal solar maximum, you would have could be having a midlife crisis. be nearly the same age. It also must have dozens of sunspots on any given day.” Like aging parents anxiously tak- a mass, chemical composition, and tem- Records of sunspots date to the 4th ing stock of their adult offspring, perature nearly identical to the Sun’s. century b.c. But in 1610, Galileo observed solar physicists must continually them in detail as they moved across the tweak their theoretical models Spots before our eyes solar sphere. That happened some 30 I to explain our middle-aged star’s some- In time, our true stellar soul mates will years before the sunspots all but disap- times confounding behavior. Most solar provide a basis of comparison to help peared during the Maunder minimum, researchers confidently declare our Sun scientists fine-tune their current solar a 70-year period of diminished solar to be perfectly normal; average even. But models. Until then, a large part of our activity that lasted from 1645 to 1715. then in the next breath, they protectively solar-learning curve lies in the varying Then, 2 centuries later, German ama- argue that the Sun is unique or peculiar magnetic forces that surround the Sun’s teur astronomer Heinrich Schwabe made in its own right. 11-year sunspot cycles. an extraordinary discovery during a Among the questions these research- During the past 100 years alone, lengthy search for a planet inside Mer ers continually ask are: How normal is observers have reported between 40,000 cury’s orbit. Schwabe noticed that his the Sun for a star of spectral type G2? Is and 50,000 sunspots. These numbers wax observations of sunspots seemed to show its behavior fine-tuned for life, or is our and wane with a fairly steady 11-year a peak about once a decade. For obscure evolution here something of a fluke? And period. Yet the Sun’s latest sunspot cycle, historical reasons, astronomers slapped if the Sun is average, where are all its ana- solar cycle 24, began almost a year late. the label of “solar cycle 1” on an unevent- logs among the hundreds of billions of In June and July of 2009, some anemic ful cycle that peaked in 1760. stars in our galaxy? sunspot activity finally kicked in after a Since then, observers have watched For decades, astronomers have hunted 2008 in which more than 260 days had 23 cycles come and go. “In March 2007, for a solar twin that would best match the no visible sunspots. I predicted cycle 24 would be at least a chemical and astrophysical characteris- “Most of 2008 and 2009 were as dead year late,” says UCLA solar physicist tics of our own aging star. By definition, as a doornail,” says astronomer Gary Roger Ulrich, who also began observing a solar analog is a spectral type G star of Chapman, director of California’s San the Sun in 1986. “We’ve beat the 1905 Fernando Observatory, who has been extended solar minimum. The end of the Frequent contributor Bruce Dorminey is running solar observations since 1986. “A Maunder minimum was the last time we a science journalist and author of Distant typical sunspot is 2 or 3 times the size of had a cycle this long. But I think we’re Wanderers: The Search for Planets Beyond Earth. We’re seeing one or two tiny ones [now] coming into a period of rising the Solar System (Springer, 2001). a month, smaller than Earth. During a solar activity.”

www.Astronomy.com 25 Christoph Scheiner observed the Sun starting in 1611, about the same time as Galileo. The two scientists described dark spots moving across our star’s surface. Linda Hall Library of Science, Engineering, and Technology

flow migration of the most recent cycle had slowed. “This jet stream is moving down toward the equator at a slower rate than the previous solar cycle,” says Hill, “and the extra length of time it’s taking is equivalent to the extended solar minimum. We conclude that they’re related.”

Multiple sunspot groups dot the Sun’s sur- A new Maunder minimum? face October 28, 2003, when activity from solar Whether this jet stream is causing the cycle 23 remained quite high. SOHO (NASA and ESA) extended solar minimum remains open A single small sunspot mars an otherwise to debate. Even so, stellar astronomer bland surface on the Sun February 2, 2010. Mark Giampapa, deputy director of NSO, The current solar cycle (number 24) got off to is one of the first solar researchers who a later-than-normal start and has yet to ramp thinks we may be entering a long-period, up significantly. SOHO (NASA and ESA) Maunder-type minimum, or a so-called grand solar minimum. This anomalously long solar mini- 2700° Fahrenheit (1500° Celsius) cooler “My gut feeling is that we’re heading mum raises the question of whether than their surroundings. Such a region into the next Maunder minimum,” says solar scientists actually understand the radiates less energy and looks darker — Giampapa. “Our observational sunspot Sun’s inner workings. “If you don’t worry and we see a sunspot. data archive since the 1980s points to a about the magnetic fields, we do,” says “According to the dynamo theory,” long-term trend of decreasing mean Ulrich. “The magnetics is where the big says Ulrich, “the polar field generates the [average] magnetic field strengths in sun- puzzle is these days.” toroidal [circular] field, and the toroidal spots.” Could these changes affect Earth Solar scientists understand that solar field generates the sunspots. The toroidal in some way? After all, Giampapa notes magnetic fields start in the Sun’s interior. field is like a big rubber band wrapped up that mean global temperatures have been Dynamic flows of plasma generate elec- inside the Sun. It has kinks that pop up as declining from their maximum in 1998. trical currents that give rise to the Sun’s sunspot pairs.” Hill doesn’t think we’re going into a active dynamo. This internal process But that’s not all that’s happening just Maunder minimum, although he suggests gives birth to magnetic fields. beneath the Sun’s surface. East-west jet such long-term minima could originate The Sun concentrates these fields, streams, known as torsional oscillations, from random dynamo action at the base twisting and turning them in the process. slowly migrate from the Sun’s middle lati- of the Sun’s convection zone. Such distorted fields inhibit the ability of tudes to the equator and its poles. Even if we face an extended mini- rising and falling gas cells to transport Normally, their migrations take place mum, Hill doesn’t think it would have a energy, a process scientists call convec- over a 17-year period. But in 2009, solar major effect on Earth. “I would guess a tion. Where the magnetic fields break physicists Rachel Howe and Frank Hill at grand minimum would change global through the solar surface, or photo- the National Solar Observatory (NSO) in temperatures by a degree at the most,” he sphere, temperatures can be as much as Tucson, Arizona, found that the zonal says. But some solar researchers believe

26 Astronomy • June 2010 Earth’s own climate mechanisms could amplify any temperature changes in ways that solar physicists and climatologists still don’t completely understand. Steve Tobias, an applied mathematician at the University of Leeds in the United Kingdom, says “differential rotation” — the fact that the solar atmosphere rotates at different speeds at different latitudes and depths — generates the toroidal field that leads to the formation of sunspots. Changes in differential rotation could, in turn, weaken the solar dynamo. This could instigate an interval where the formation of active regions simply switches off, says Tobias. This also might modify the solar dynamo and cause the Sun to go through a Maunder-type minimum. Looking to the stars No matter what causes such events, Giampapa says he sees tentative evidence for Maunder-type cycles in some of the solar analogs he’s observed. Giampapa and his Italian colleagues studied the open star cluster M67, located some 2,700 light-years from Earth in the con- A sunspot group may comprise dozens of individual spots. Each spot typically contains a dark stellation Cancer. If scientists confirm central region called the umbra, which is up to 2700° F (1500° C) cooler than the surrounding this trend, he says, it would indicate that photosphere, and a lighter outer region called the penumbra. Royal Swedish Academy of Sciences

Europe suffers a deep freeze Students of art history will readily identify a string of late-Renaissance Netherlandish paintings that depict what look like unusually harsh winters. From 1500 to 1850, a Little Ice Age engulfed much of the world, but its effects proved particularly strong in Northern Europe. Cool summers and colder-than-average winters were the norm. The unusually cold winters allowed “frost fairs” to be held on London’s frozen River Thames as late as 1814. In addition to the 70-year Maunder minimum from 1645 to 1715, two other solar minima affected Northern Europe during this epoch: the Spörer minimum from approximately 1415 to 1510 and the Dal- ton minimum from about 1795 to 1820. Some researchers think volcanic ash might have triggered the Little Ice Age, and then the Maunder minimum amplified the effect. That’s a view that Gary Chapman, a solar physicist and director of The Little Ice Age brought unusually cold weather to much of north- California’s San Fernando Observatory, sees as plausible. “The Little Ice ern Europe from 1500 to 1850. In this 1565 painting by Pieter Bruegel, a crowd skates on a frozen river. The lack of sunspots during the Age actually began before the Maunder minimum and continued into Maunder minimum may have exacerbated this 350-year cold spell. the Maunder minimum,” says Chapman. “Maybe the Little Ice Age was bigger than it should have been if the Sun [hadn’t been] involved.” Contrary to some reports, this period of solar inactivity was well- Maunder minimum, says Chapman, we may start seeing some cool- documented at the time. Researchers at the Paris Observatory alone ing effect. However, scientists still debate the specific effects of such made some 8,000 observations between 1660 and 1719. And, in grand minima. Part of this debate hinges on lingering questions 1887, after constructing a table of sunspots recorded between about how Earth’s atmosphere amplifies such solar variabilities. 1672 and 1699, German astronomer Gustav Spörer reported that As a result, the National Solar Observatory’s Frank Hill says, he found fewer than 50 spots. there’s a disconnect between solar physicists and climatologists. It’s hard not to speculate how a grand minimum might affect our “Both solar physicists and climatologists need to come together present climate. But if solar activity cuts out like it did during the about the impact of solar activity on climate,” says Hill. — B. D.

www.Astronomy.com 27 quiescent than most stars,” says Giam- papa. “But cycles in Sun-like stars in M67 could be quite similar to our Sun.” And a little quiescence might not be so bad. “We’re near the end of our ropes here now,” says astronomer Edward Guinan of Villanova University in Pennsylvania. That’s because even though the Sun is only about halfway through its 10-billion- year hydrogen-burning lifetime, its increasing luminosity will make Earth uninhabitable much sooner — perhaps within the next 500 million years. A better place for life? Because of its relatively short life, the Sun may not be the best candidate star for sup- porting planets with life. Guinan says that spectral type K stars may be better hosts. Hot gas traces the Sun’s magnetic field lines as they arc above the solar surface. Essentially all solar Such stars possess about 80 percent of the activity arises from magnetic forces acting beneath, at, and above the surface. TRACE/NASA/GSFC Sun’s mass and burn cooler, so they fuse hydrogen stably for much longer. K stars such grand minima occur in Sun-like stars are fairly common. But researchers have fixed habitable zones, regions where stars only 10 to 15 percent of the time. also want to find stars with similar ages, liquid water could exist on a planet’s sur- “There is no doubt that the Sun expe- masses, and magnetic cycles as the Sun. face. Theoretically, intelligent life there rienced many more Maunder-type min- One of Earth’s nearest neighbors, could survive for 40 to 50 billion years. ima during the past 10,000 years,” says Alpha Centauri A, is a true solar analog. Even cooler M stars make up some Jürg Beer, a physicist at Switzerland’s It belongs to a triple star system that lies 75 percent of the galaxy’s inhabitants. Eawag Research Institute. His team used only 4.36 light-years away. With an esti- Although M stars live a long time, they the radioactive elements Beryllium-10 mated age of 6 billion years, however, it’s glow dimly. Their habitable zones lie so and Carbon-14 found in polar ice cores about 1.5 billion years older than the Sun. close in that any planet orbiting within and tree rings to reconstruct solar history But Alpha Centauri A is just one star. one would be tidally locked, with one over the past 10,000 years. “The minima Giampapa’s team is studying 15 solar ana- side continuously facing the star and the themselves do not show a clear periodic- logs in M67 with the European Southern other in constant darkness. ity. However, they seem to be clustered Observatory’s Very Large Telescope in K and M stars have a further liability. with a [spacing] of about 200 years.” Chile. M67 offers a unique laboratory to Both types have more efficient dynamos But the Sun’s magnetics, which drive search for solar analogs because its stars’ than the Sun, so they produce more mag- both short-term solar magnetic cycles chemical compositions and ages (roughly netic energy. Strong magnetic fields pose and perhaps even grand minima, present 3.5 to 4.8 billion years old) are nearly the more danger than no magnetic fields, says more of a puzzle. That’s why observers same as the Sun’s. Giampapa says that Ulrich. He notes that the largest recorded and theorists alike need to compare the after 6 years of observations, none of the eruption of charged particles from the Sun with close solar analogs. So far, sci- stars his team has observed appears to Sun, a so-called coronal mass ejection, entists have turned up only a couple have cycles that last less than 6 years. came during the 1859 solar superstorm. dozen. The trick is not in finding stars “I’m seeing some brightness variability “The aurorae were so bright that people that make a nice chemical match. Such data that suggests the Sun might be more camping in Colorado awoke and decided

200 A history of sunspots

100 Dalton minimum Maunder minimum Sunspot number

1600 1650 1700 1750 1800 1850 1900 1950 2000 Year Typical solar cycles last about 11 years, but the strength of each cycle varies considerably. This illustration plots the yearly average sunspot numbers since 1610, when detailed observations began. During the Maunder minimum from 1645 to 1715, virtually no spots appeared. Astronomy: Roen Kelly

28 Astronomy • June 2010 Helioseismology reveals the Sun’s structure and dynamics by measuring This obscure star in Draco, HIP 56948, is the closest thing to a solar twin sound waves generated in the interior. This computer representation astronomers have discovered. The 9th-magnitude star lies just north of the shows rising gas in blue and sinking gas in red. NSO/AURA/NSF Big Dipper’s bowl. Bill and Sally Fetcher it was dawn,” says Ulrich. “Sparks flew will link eight new telescopes spread over Thus far, the closest to a solar twin out of all the telegraph instruments.” five continents, including the U.S. main- Meléndez’s team has found is the obscure Such extraordinary events demon- land and Hawaii. star HIP 56948. This 9th-magnitude strate how volatile our Sun can be. It also The network will offer full-sky 24- object lies some 217 light-years from gives pause to wonder: Does the Sun’s hour observational coverage. SONG Earth in the constellation Draco. evolution and its often tumultuous solar researchers hope the first prototype, But NASA’s Kepler mission should cycles qualify it as normal? located in the Canary Islands, will see create an asteroseismological shift in the To answer this question, astronomers first light by early 2011. Giampapa says search for solar twins. During a 4-year need to probe the interiors of the Sun SONG-like global networks are what period, Kepler will observe 100,000 stars and other stars. The science of helioseis- scientists need to confirm whether in a single 100-square-degree field. The mology studies the Sun’s interior structure these solar analogs are indeed operating spacecraft’s instruments are sensitive and dynamics by measuring sound waves on solar-type cycles and experience enough to measure the passage of indi- emanating from our star. The best obser- Maunder-type grand minima. vidual starspots across the disks of its vations so far have come from the Global stellar targets. Oscillation Network Group (GONG). Find a solar twin Travis Metcalfe, an astronomer at the An asteroseismological version of Meanwhile, astronomer Jorge Meléndez High Altitude Observatory in Boulder, GONG — the Stellar Oscillations Net- at Portugal’s University of Porto leads an Colorado, predicts that over Kepler’s life- work Group (SONG) — plans to study international collaboration looking for time, the spacecraft will find at least 100 solar-like oscillations in nearby bright solar twins. “My group has already stud- solar analogs. However, he emphasizes stars. This Danish-led international effort ied about 75 percent of stars similar to the that the key to understanding the Sun in Sun in the whole Hipparcos million-star detail is the broader study of other stars. catalog,” says Meléndez. “Surprisingly, we “There’s always the danger that if we see that the Sun is actually chemically study this one object in great detail, we’re different from most solar twins.” just going to fine-tune our model to fit That’s in part because our star is lack- that one Sun,” says Metcalfe. “But that ing in refractory elements — those that same model has to work for other stars.” vaporize at high temperatures — says At the end of the Kepler mission, if Meléndez. These missing elements prob- not before, researchers should be able to ably formed dust, which then accreted partially settle our age-old quandary over into planetesimals and ultimately into what constitutes solar normality. And in the terrestrial planets: Mercury, Venus, the process, astronomers might just find Earth, and Mars. Meléndez notes that a solar twin orbited by its own family of Open star cluster M67 holds at least 15 close an estimated 15 percent of stars seem to habitable terrestrial siblings. solar analogs — stars with similar masses, ages, and compositions as the Sun. Observations so far have a chemical composition similar to show that none of these analogs has a stellar the Sun’s, raising the possibility that such To learn more about the Sun’s biggest blasts, visit www.Astronomy.com/toc. cycle shorter than 6 years. Anthony Ayiomamitis stars also may possess terrestrial planets.

www.Astronomy.com 29 A star’s final voyage How the Sun When Sun-like stars exhaust their fuel, they cast off shells of gas, creating colorful will die fireworks. by Bruce Balick ur Sun has lived half its life. Five billion years from now, its inner workings will trigger a transformation. For a brief span, distant observers will not see a star, but a colorful, expanding cloud of gas called a planetary nebula. OAstronomers pay close attention to planetary nebulae, and these objects have gotten more popular since the mid-1990s, when the Hubble Space Telescope began delivering spectacular photographs of them. In fact, a new planetary nebula has probably flared to life somewhere in

the Milky Way since Hubble went into service. The object This infrared view of the Helix Nebula from may be too far, too small, or too faint to detect, but it’s out NASA’s Spitzer Space Telescope shows features called “cometary knots” with blue-green heads. The there waiting to return our gaze. knots glow brightly — because of shock fronts or ultraviolet radiation — at wavelengths between Despite the efforts of astronomers using Hubble and 3.2 and 4.5 microns, to which astronomers many other instruments that have probed planetary nebu- assigned the colors blue and green, respectively. NASA/JPL-Caltech/J. Hora (Harvard-Smithsonian CfA) lae in every wavelength, there remain important aspects of these enigmatic objects we don’t understand.

© 2013 Kalmbach Publishing Co. This material may not be reproduced in any form 38 Astronomy • Decemberwithout 08permission from the publisher. www.Astronomy.com The Helix Nebula (NGC 7293) in Aquarius is one of the best known planetary nebulae. It’s also one of the closest, lying only 650 light-years from Earth. NASA/NOAO/ESA/The Hubble Nebula Team/M. Meixner (STScI)/T. A. Rector (NRAO)

www.Astronomy.com 39 The Ant Nebula (Menzel 3) resembles a garden-variety ant. This Hubble image reveals the ant’s body as a pair of fiery lobes protruding from a dying Sun-like star. The Ant Nebula lies in the southern constellation Norma approximately 3,000 light-years away. NASA/ESA/The Hubble Heritage Team (STScI/AURA)

The majority rules images along their major axes form in the The story of a planetary nebula starts at final helium flash. This material cools as the end of a star’s red giant phase. That’s it expands. Based on observations, when the star’s core finally dies. It astronomers surmise dust particles becomes a dense, Earth-sized lump of quickly condense at the base of the out- carbon containing about 50 percent of flow just before the gas disperses. the Sun’s original mass. The core has no New dust makes up about 1 percent of way to replenish its heat, so it cools like the mass ejected into the interstellar an ember in a fireplace. medium. This enriches the surrounding Some life remains in the layers outside area with carbon- and silicate-rich par- the core, however. Gravity has com- ticles along with a variety of carbon- pressed shells of fresh hydrogen and based molecules. The dust particles are helium to their thermonuclear fusion small (0.001 millimeter or so) and reflect points. They burn furiously, but only This cosmic jellyfish actually is planetary light from the nearby dying star. nebula OH231.8+4.2, sometimes called the briefly. For example, the Sun will be Rotten Egg Nebula. Shown in blue is light The newly born preplanetary nebulae about 30 times brighter than now when it from hydrogen and ionized nitrogen arising (sometimes called protoplanetary nebu- enters the red giant stage. At the most from supersonic shocks where the gas lae) are small and far away on average. luminous stage of its evolution, however, stream collides with surrounding material. Because they appear tiny, Hubble is the it will be 150 times larger and 2,100 times This image showed, for the first time, these observing tool of choice. complex gas structures predicted by theory. more luminous than it is now. ESA/Valentin Bujarrabal (Observatorio Astronomico Nacional, Spain) Only the most elite of all stars — those Within stars that will become plan- whose mass puts them in the upper 1 etary nebulae, carbon is the ultimate percent — become supernovae. The rest fusion byproduct. Such a star’s atmo- ward at about 36,000 mph (58,000 km/h). settle for “15 minutes of fame” before spheric carbon settles down and adds its Astronomers can see these bubbles as they fade away, and we see the results as mass to the inert carbon core. The re- concentric rings of luminous gas in many planetary nebulae. That time span — maining fuel is too far from the core, so Hubble images of planetary nebulae. when the star ejects and ionizes its outer not enough mass remains above it to The star’s final helium flash is a doozy. layers in a final fiery, smoky breath — is compress it to its fusion point. Instead, Instead of another bubble, we get a dense what attracts astronomers’ attention. the last spasmodic fits of helium “burn- and highly organized spray of gas and Planetary nebulae ultimately expand ing” — a colloquial term substituted for fresh dust particles. This event creates the and disperse into the ocean of galactic “fusion” by astronomers — fling these planetary nebula’s shape — that is, its gas known as the interstellar medium. All outer layers into space, resulting in a complex inner structure and organiza- that remains are ever-cooling white planetary nebula a thousand years later. tion. The “superwind” is too organized dwarfs — nature’s burnt cherry pits — Each of the spasmodic “sneezes” pro- and symmetric to be the chaotic remnant too faint to see after a billion years and duces a new bubble of gas flowing out- of an explosion. Rather, there’s method too numerous to name. — or perhaps a few methods — in the Bruce Balick is Chairman of the Department mad out-rush of material. Revealing a planetary of Astronomy at the University of Washington, In many planetary nebulae, the dark It takes about a thousand years for a new Seattle. He serves on the design team for WFPC 3. dust lane and the bright lobes seen in planetary nebula to become visible. Its

40 Astronomy • December 08 gas has to expand to a size we can detect, and the star at its center must shed its cool outer layers and reach a temperature of about 30,000 kelvins (K). Before this happens, the nascent planetary nebula shines only because its newly formed dust particles happen to reflect starlight in our direction. Indeed, the dust may hide the visible light entirely until the nebula expands for a while and light finds pathways out of the opaque cocoon. At 30,000 K, ultraviolet photons start to strip electrons from neutral atoms. This ionizes the gas, rendering the planetary nebula visible. The object’s spectrum shows lines of hydrogen, helium, and other elements. Using filters that iso- late these emissions, Hubble and other telescopes can take the spectacular color images we’ve grown accustomed to. No chain is stronger than its weakest link, and no theory of stellar evolution can be complete without understanding the origin of the shapes of planetary nebulae. It’s a thrill for an observer like me to get imaging time on Hubble. However, there’s a sobering — and, for the really The Retina Nebula (IC 4406) exhibits a high degree of symmetry; the left and right halves are nearly spectacular nebulae, gut-wrenching — mirror images of each other. Gas and dust form a vast doughnut of material streaming outward from realization that we have not yet explained the dying star. One perplexing feature of IC 4406 is the irregular lattice of dark lanes that crisscross the structure of these amazing objects. the nebula’s center. We see them in silhouette because their density is 1,000 times greater than the rest of the nebula. NASA/The Hubble Heritage Team (STScI/AURA)/C. R. O’Dell (Vanderbilt Univ.) From a scientific viewpoint, images of preplanetary nebulae pose a problem. All known stellar wind acceleration mecha- The stellar superwind may be the last degrees or more by the fast wind. At its nisms convert light energy from the star big event in a star’s life, but there’s more perimeter, we find gas that has been into outward motions of dust particles. to come. Ultraviolet observations of the scooped up into a thin rim. Beyond the Just like the Sun shining on a comet’s tail, emerging star clearly show the presence rim lies a slow wind that has yet to sense light pushes on the dust particles and of a “fast wind” whose speed is truly the bubble’s presence. Ultimately, the forces them to flow outward. If the star is impressive — up to 350,000 mph bubble pops when it reaches the outer round, then the outflows also should look (560,000 km/h) — but whose mass den- edge of the slow wind, emptying its con- round. Almost none of the preplanetary sity is smaller than that of the superwind. tents into the interstellar medium. As it nebulae behave this way, however. The fast wind quickly smashes into does, it may scoop up much older gas the star had deposited when it was still a red giant. This can create large halos or pairs A planetary nebula “gets its certificate” of bubble-like lobes. when the central star becomes hot enough A planetary nebula “gets its certificate” when the central star becomes hot to ionize the preplanetary nebula. enough to ionize the preplanetary nebula. The fast wind peels away the star’s Spanish radio astronomers uncovered the slower wind ejected earlier, pushes cool outer layers and exposes hotter a second problem in 2001. They mea- the gas out of its way, and sears whatever inner regions. So the star’s surface sured the Doppler shifts from carbon gas it contacts. This effect creates the becomes increasingly blue. This takes a monoxide (CO) emissions (CO is always appearance of an empty cavity between few hundred to a few thousand years, a bountiful molecule in cool, dusty gas) the central star and the rest of the nebula. after which the star’s surface temperature and found the momentum of the out- The cavity is no more empty than an reaches up to 100,000 K. For comparison, flowing gas is more than 10,000 times inflated tire; it just looks that way. the Sun’s surface is currently 5,800 K. larger than light-pressure based accelera- Inside this illusionary cavity, we detect The small, hot surface of the dying tion mechanisms had predicted. X rays from gas heated to a million star emits ultraviolet light before it starts

www.Astronomy.com 41 How astronomers classify planetary nebulae CLASS Round Elliptical Butterfly

Early

IC 3568 NGC 6891 M1–92 TYPE Middle

Abell 39 NGC 3918 NGC 650–1

Late

Edge of outer halo Outer halo Inner halo Central cavity NGC 2438 NGC 6886 NGC 6302 Bright rim

Edge of outer halo Astronomers have identified three types of planetary nebulae, along with Outer halo three classes. Combinations allow for nine distinct shapes among these Inner halo objects. The key image to the right shows the nebulae’s main parts. All planetary nebula images courtesy of NASA/ESA/STScI; artwork: Astronomy: Roen Kelly after Bruce Balick Central cavity Bright rim to cool and fade away. The radiation strips electrons from atoms in the nebula. Suddenly, the nebula fluoresces and becomes easy to observe. The object’s luminosity rises abruptly. Instead of reflecting a small portion of the star’s total light, the nebula converts about half of the radiation into visible light. This brightening, coupled with their longer lifetimes and ever-growing diameters, make most planetary nebulae much easier to find than preplanetary nebulae. Back to the puzzle board The success of the first models of plan- Magnetic fields (left, yellow lines) become twisted as a star about to form a planetary nebula etary nebulae elated astronomers in the rotates. Charged particles spiral along the yellow lines as they flow outward. The fields guide these 1990s, but it didn’t last long. As Hubble particles along the star’s spin axis. The white and red regions of the right panel show where the particles will flow. The white rectangular area at the center is probably the only region sufficiently dense produced more images, the complexity of enough for Hubble to detect. Both images: Sean Matt (University of Virginia)/Adam Frank and Eric G. Blackman (Rochester University) the structures soon led to humility. We’ve made a good start, but the quest to understand the shapes of planetary nebu- dense. Highly aligned outflows can arise fluids, reveal strange behaviors. MHD lae continues. Of particular interest are from any accretion disk, even those in models require huge computers, clever the small knots, which don’t readily form young stars, as the swirling material gen- programming methods, and shrewd in the lovely, regular patterns. Another erates magnetic fields. guesses about the ways magnetic fields area of interest is planetary nebulae with If the red giant swallows its compan- permeate the gas and exit the star’s sur- more than one symmetry axis. And then ion, the small star acts like an eggbeater face. The enterprise is in its infancy, and there’s the question of shapes. as it follows a spiral descent into the star’s results don’t always agree, so it is prema- The same shapes appear for planetary heart. This, too, will form a disk that ture to draw vast conclusions. However, nebulae as for preplanetary nebulae — shapes the nebula. But the merger of two the range of outcomes explains some of round, elliptical, and butterfly. However, orbiting stars can also account for other the more complex nebular shapes, except half of the preplanetary nebulae are ellip- puzzling shapes of the final ejection. for one thing: The energies and momen- ticals with dust lanes cutting through First, the merger provides a way of tums of the predicted outflows are their centers. For planetary nebulae, only extracting and harnessing the momen- smaller than the observations require. 10 percent are bipolar, and none show tum of the companion’s orbit and trans- dust lanes. Something happens to morph ferring it into the dense winds from Looking forward or distort their shapes, providing astron- which the planetary nebula forms. Sec- The beauty and symmetry of planetary omers another challenge. Astronomers ond, the merger will disrupt the red nebulae please the eye and challenge the also wonder why planetary nebulae have giant’s core. This might explain why the brain. We largely understand stellar evo- such a narrow range of shapes. largest ejection is also the final one. In lution until the end. That’s when we hit The shapes are also strikingly sym- particular, an entire star’s worth of the limit of our understanding of the metrical. One popular idea is that a com- hydrogen fuel landing on the extremely shapes and energies of mass-ejection. panion star exerts a gravitational tug on hot core might trigger a conflagration. Stellar mergers and magnetized winds the loosely bound outer layers of a red Another mechanism for shaping plan- can solve some problems. New ideas will giant star, or perhaps the giant swallows etary nebulae may be magnetic fields enter the discussion. For now, even with its companion when it starts to bloat. released by convection. Red giants have Hubble and other tools, it’s too soon to In the former case, the tidal forces on deep convection, but generally not deep tell if we’re barking up the right trees. the outer layers drag them toward the enough to dredge material out of the Future instruments, especially the rapidly orbiting companion star for the core. This changes during a helium flash. Atacama Large Millimeter Array duration of the final helium flash. How- The extreme heat acts like a gas flame (ALMA) under construction in Chile, ever, by the time the giant’s material under a pot of water. The flash causes will allow us to map the dust and possibly reaches the companion, it has moved on. deep material to rise quickly. If that the magnetic fields in preplanetary nebu- So most of the material just keeps going material consists of magnetized gas, as lae. Such studies will provide essential outward in a spiral pattern, like water we expect, then, as the gas surfaces, the data and propel the research for years to from a sprinkler. This forms a tightly star ejects and distorts it while it’s still come. But for now, public, amateur, and wound one-armed spiral in a thin disk. connected to the star’s spinning surface. professional astronomers will continue to Moreover, some of the material falls onto But field lines stretch like rubber enjoy the poetry of planetary nebulae the companion and forms an accretion bands. Magnetohydrodynamic (MHD) even if we haven’t figured out how stars disk around it if the companion star is models, which study moving magnetized manage to write it.

www.Astronomy.com 43 Space weather

by Sten Odenwald THE COMING In 1859, the Sun unleashed its biggest storm in 450 years. We’re more vulnerable than ever to its next blast.

ny disaster capable of causing large-scale blackouts, above a sunspot- disrupting communications networks, and damaging Hot plasma arcs over the Sun’s limb. The ultravio satellites is something to prepare for. These are just a let scene, captured in late 2006, reveals fine-scale structure few of the effects that follow powerful eruptions extending from the mottling, or granulation, on the Sun’s surface from the Sun. and into its lower atmosphere. Hinode, the most advanced solar Scientists now recognize that we haven’t seen the worst telescope ever flown in space, A launched September 22, 2006, the Sun can throw at us. Solar flares in 1859 kicked off a and returned this view. JAXA/NASA/Hinode storm so massive that, if it happened today, could cripple much of the technology we’ve come to rely on.

© 2013 Kalmbach Publishing Co. This material may not be reproduced in any form 34 Astronomy • Septemberwithout permission 08 from the publisher. www.Astronomy.com by Sten Odenwald THE COMING In 1859, the Sun unleashed its biggest storm in 450 years. We’re more vulnerable than ever to its next blast. ens of instruments around the world recorded its passage as a minor mag- As solar cycles go, Cycle 23 was well netic glitch. Yet, even by modern standards, this was a major storm. behaved. Its worst blasts, which Normally, a storm’s north-directed included a record-breaking flare magnetic field squelches any auroral Solar cycles November 4, 2003, spawned intense displays. But despite its incorrect mag- The Sun’s storminess rises and falls every activity now known in space-weather netic orientation, the 1859 storm was 11 years, on average. Cycle 23 is now circles as the Halloween Storms. They so intense it spawned aurorae seen as drawing to a close. Since 1996, this cycle crippled instruments on some space- far south as Athens, Greece. Why? has produced more than 21,000 flares — craft, including NASA’s Mars Odyssey, Some hours before the bubble of hot giant magnetic explosions on the Sun’s and triggered a blackout in Sweden. surface. It has launched 13,000 coronal But that, researchers say, was nothing. gas reached Earth, it emitted a gale of fast-moving protons. These particles mass ejections (CMEs) — billion-ton Once every few centuries, the Sun erupts raced ahead of the cloud and were clouds of ionized gas (plasma) that race with an event unlike anything modern absorbed by Earth’s atmosphere. These through interplanetary space. scientists have ever experienced. Both events can trigger spectacular particles, not the CME itself, produced auroral displays (the so-called northern the light show. The 1859 superstorm A few days later, a second CME raced and southern lights). But the same radi- Between August 28 and September 6, past our planet. British astronomers ation that produces these beautiful 1859, the Sun produced one of its most Richard Carrington and Richard Hodg- glows can damage satellite electronics. spectacular solar storms in the last 450 son, who happened to be observing the Radiation from solar flares also can ion- years. Scientists who reconstructed this Sun, witnessed the launch of this mas- ize the atmosphere so strongly that radio event from historical data put it in a sive cloud by a rare and powerful solar communication at some frequencies is class by itself. They call it a superstorm. flare (see “Stars on the Sun,” p. 36). impossible for hours. Electrical currents Here’s what it was like. flowing at the top of the atmosphere can The first CME arrived at 7h30m Uni- induce currents on the ground in long versal Time (UT) August 28, 1859. Doz- conductors, like the national power grid, and damage and destroy transformers and other equipment.

www.Astronomy.com 35 Stars on the Sun

On September 1, 1859, two noticed “two patches of the spots themselves, and British astronomers inde- intensely bright and white concluded that “the phe- pendently observed a rare light” near the largest group. nomenon took place at an and powerful flare on the Hodgson described the elevation considerably Sun’s disk. It was this erup- flare as a “brilliant star of above the general surface tion that launched round light, much brighter than of the Sun.” two of the solar superstorm. the Sun’s surface, and most What these observers Richard Carrington and dazzling to the protected couldn’t see, of course, was Richard Hodgson were eye.” The show ended 5 the massive CME this flare observing sunspots by pro- minutes later. launched toward Earth. But jecting the Sun’s image onto In that time, Carrington Carrington suggested the Magnetic loops arch above solar active regions a screen. Carrington had noted, the patches of light flare might be related to the June 9, 2007. These areas of intense magnetism completed drawing sunspot moved some 35,000 miles exceptional magnetic storm often are sources of solar storms. NASA/STEREO groups when, at 11:18 a.m. (56,000 km) across the Sun’s that followed 17 hours later. Greenwich Mean Time, he disk. He saw no changes to — Francis Reddy over some celestial battle. They stood and gawked by the millions and wrote detailed eyewitness accounts to newspapers. Miners awoke after midnight and broke camp, thinking dawn had arrived. Thousands of people comfortably read newspapers under the night sky’s waver- ing crimson glow. In this pre-Civil War era, telegraph outages and instrument fires proved to be the only technological impact. The currents generated underground by the storm’s shifting magnetic and electric fields were so powerful they set fires in telegraph offices on both sides of the Atlantic. In Washington, D.C., they nearly electrocuted telegraph operator Frederick Royce. Today, the calamity such a storm would cause would have no historical Richard Carrington sketched the massive white-light flare he witnessed September 1, precedent. At no other time has the web 1859. The eruption began at the locations marked A and B. The eruption subsided 5 min- of technology so completely engulfed our utes later at points C and D. Linda Hall Library day-to-day lives. Billions of people are in close personal contact with the technolo- gies most prone to space weather’s This CME had everything going for affected the ozone layer, reducing this effects. Luckily, superstorms seem to be it. It was fast: In just 17 hours, the cloud ultraviolet-absorbing gas so much — by rare events for a star like our Sun. swept across the entire inner solar sys- about 5 percent — that it took years to tem at a speed of 5 million mph (8 mil- recover to pre-storm levels. Deep freeze lion km/h). The dense wall of plasma The night-side portion of Earth’s Deep within ice crystals in Greenland also possessed a southward-pointing magnetic field became a complex, tan- and Antarctica, nitrate molecules have magnetic field, which enhanced its gled web of field lines trying to sort collected in trapped gases since 1561. potential impact. At 4h40m UT Septem- themselves out within an enormous vol- Scientists Michael Smart, Donald Shea, ber 2, part of this monster plasma cloud ume of space. This magnetic upheaval, and Kenneth McCracken at the U.S. Air brushed past Earth. invisible to the human eye, may have Force Research Lab at Wright-Patterson Our planet resides in a protective bub- continued for more than a day. Air Force Base, Ohio, and the University ble created by its magnetic field and ions From the ground, spectacular crimson of Maryland discovered that nitrate con- trapped inside it. Within minutes of the aurorae could be seen as far south as centrations rise and fall with solar activ- cloud’s impact, the entire Sun-facing equatorial Central America and Bombay, hemisphere of Earth’s magnetic bubble India. Predictably, people mistook the Sten Odenwald is a professor of astronomy at was compressed until it reached the outer lights for distant cities on fire, or imag- Catholic University in Washington. He is an avid atmosphere’s fringes. The blow instantly ined them as specters dancing in glee science popularizer and author.

36 Astronomy • September 08 This Hinode image provides the sharpest-ever look into a modest sunspot about 4 times Earth’s size. Rising and falling cells of hot gas create the mottled appearance, called granulation, of the Sun’s normal surface. Sunspots appear dark because their strong magnetic fields suppress rising columns of hot gas and allow the spots to cool. JAXA/NASA/Hinode ity. Nitrates are a particularly good So, if you wanted to build satellites to barometer of powerful radiation storms endure the rigors of the space environ- called solar proton events (SPEs). In the ment, the solar storms of the past 40 450 years covered by the frozen record, years’ are the wrong examples to use. In the biggest SPE was the 1859 superstorm. truth, things can get much nastier than The satellite industry uses the storm these storms indicate. event that occurred August 4, 1972, as Scientists can’t predict when one of its worst case. According to the nitrate these superstorms will occur. Here’s Enormous sunspots more than 10 times Earth’s record, 19 events more intense than this what we can expect if Cycle 24 launches diameter peppered the Sun in October 2003. storm have occurred since 1561. In one our way. These spots launched the record-breaking Hal- addition, they occurred, on average, in loween solar storms. NASA/ESA/SOHO 23-year intervals. The next superstorm The August 1972 event, which was The superstorm likely would come some- during the equinoxes, when solar storms one-fourth as strong as the superstorm, time between 2010 and 2012, near the can more easily impact Earth. is the only one that even comes close to peak of the Sun’s activity cycle. The most The first warning sign might be the its power since 1965. In fact, during the favorable months would be March presence of a large, distorted sunspot last 4 decades, the Sun has produced the or September, group. Scientists expect dozens of small fewest large SPEs of any 40-year span X-ray flares (classed as X-1 and up) from back to 1670. such systems. They also can produce a few moderate-intensity (X-10 and up) flares. These occurrences signal highly disturbed magnetic conditions within the The August 1972 solar storm serves as the spot. Each flare would cause notable short-wave radio blackouts, but only satellite industry’s worst case, but the 1859 amateur radio operators and some emer- superstorm packed 4 times its punch. gency services might take notice. www.Astronomy.com 37 This composite of Solar and Heliospheric Observatory images shows the Sun’s disk in ultraviolet light and the tail end of a coronal mass ejection (CME) blasting into space. These enormous eruptions launch billions of tons of ionized gas across the solar system. Those directed Earth’s way can affect satellites, radio communications, and power systems. NASA/ESA/SOHO

Unless the sunspot group launched short-wave broadcasts across the hemi- the following years. And the night sky a white-light flare visible to lucky ama- sphere facing the Sun. would blaze bright enough to confuse teur and professional solar observers, The entire Arctic region, perhaps animals — and let humans read beneath the first to see the eruption would be an extending to the Great Lakes, would eerie auroral glows. armada of aging solar research satellites, experience a Polar Cap Absorption If the superstorm arrived as a so- such as Hinode, STEREO, and the Solar event, where a flood of solar protons called double-barreled CME, the first Dynamics Observatory. knocks out most radio communications. would dazzle Northern Hemisphere sky- An intense blast of X rays and ener- This would be an instant hazard to air watchers with spectacular aurorae rival- getic particles would black out every travel and lead to days of delays and ing those produced by 2003’s Halloween sensor system in space on Earth’s day- costly flight rerouting. Storms. The second punch, launched light side. The X-ray pulse also would As in 1859, the atmosphere would perhaps a day later, would race through destroy the atmosphere’s ionized D- lose 5 or 10 percent of its ozone layer, the inner solar system and arrive at layer. This would instantly black out which would raise skin cancer rates in Earth within 20 hours.

38 Astronomy • September 08 At impact, all geosynchronous satel- tricity for days or even weeks. Insidious lites outside Earth’s protective magnetic magnetic storm currents would damage bubble would find themselves immersed transformers. Replacements would be in a turbulent magnetic plasma they hard to come by because no domestic weren’t designed to endure for long. suppliers exist. Accelerated by shock waves in the CME, A 2003 blackout in the northeastern a tremendous pulse of fast-moving pro- United States involved 50 million people tons would race ahead of the cloud and and 12 states and Canadian provinces, arrive 12 hours earlier. The radiation and cost $6 billion over a 24-hour instantly would invade satellite circuitry. period. During a superstorm event, such Satellite controllers would record thou- effects might linger for a week or more. sands of glitches, some serious enough to The cost could exceed $20 billion a day end the hardware’s operational life. in lost salaries, spoiled food, and other On December 5 and 6, 2006, as solar cycle 23 In a few hours, the effects on satellites collateral effects. approached its activity minimum, two power- alone could result in a loss of $20 billion ful X-ray flares exploded on the Sun’s limb. The in revenue and resources. U.S. Defense Making headlines X-ray imager aboard the GOES 13 weather sat- Department satellites also would be Newspaper accounts often described the ellite captured this view of the first flare, rated blinded by varying degrees. The Global last century’s solar storms. On March 25, X-9, and experienced slight damage during the event. NGDC/NOAA Positioning System would report inaccu- 1940, the Boston Globe ran the headline rate positions for a day or more, impact- “U.S. Hit by Magnetic Storm” in 2-inch ing precision navigation, oil drilling, and type above the fold. of satellite outages and losses totaling, by some military operations. some estimates, nearly $3 billion. Satel- As the proton storm’s particles lite industry trade journals spoke about enter Earth’s insurance brokers struggling under billion-dollar annual payouts. Collec- tively, commercial satellites lost about 3 years of their operational lifespan, which The next superstorm’s effects on satellites translates to tens of billions of dollars in alone could reach $20 billion a day lost profit. There also were several incidents with U.S. electrical power grids, in lost revenue and resources. some of which found themselves pushed Editors banked heavily to near-blackout conditions. on the average reader knowing what a Yet, over the past 5 years, the U.S. atmosphere, they magnetic storm was. Back then, many Congress has steadily reduced National would initiate nuclear reactions people did. Oceanic and Atmospheric Administra- with oxygen and nitrogen atoms. The Since the 1950s, solar storms have all tion funding for maintaining and result would be showers of high-speed but vanished from the front page. Those improving our space-weather forecasting neutrons, many of which would reach the that make it to print lack details of spe- ability. The $6 million annual budget ground. Computer systems would crash cific incidents, which mutes their human pays for operation of the Space Weather as so-called “sudden event upsets” violate impact. For instance, the historic March Prediction Center in Boulder, Colorado. the integrity of binary information stored 13, 1989, storm blacked out Quebec for Its forecasts, which are used by thousands in memory. 12 hours and cost several billion dollars. of companies and government agencies Yet this remarkable cosmic event was each year, save an estimated $200 million Lucky break? never mentioned in major U.S. papers. annually. If this doesn’t seem like a bar- Despite this grim scenario, we might It’s ironic that we’re now far more vul- gain, factor in safeguarding the $500 bil- actually luck out. Half the time, a CME’s nerable to major solar storms. They now lion in annual revenues generated by the magnetic field is oriented north, which tend to be regarded by the public as power industry and satellite commerce. minimizes its interaction with Earth’s entertainment and nighttime spectacle, Our next national blackout could magnetic field. Such a storm would pass rather than their historically demon- come as a surprise, even though the tech- by with only moderate impact. strated capacity to do serious harm. For nology needed to alert us was used dur- Then again, our luck could go the instance, the solar storm of September ing the previous solar cycle. If the cosmic other way. According to John Kappen- 18, 1946, caused navigation errors that dice fall the wrong way when the Sun’s man, an electric power engineer at led to a famous plane crash in Gander, activity peaks again, we may well look MetaTech Corporation in Goleta, Califor- Newfoundland, which killed 26 passen- back at Cycle 23 as the good old days. nia, a solar storm as severe as one that gers and crew. occurred in 1921 would affect all of North The previous sunspot cycle — Cycle See movies of the Sun from the America in an unprecedented blackout. 22 — was quite eventful. It led to Hinode solar observatory at www.astronomy.com/toc. More than 150 million would lose elec- a costly period The evolving solar system Earth’s deadly future

© 2013 Kalmbach Publishing Co. This material may not be reproduced in any form without permission from the publisher. www.Astronomy.com “BLACK SMOKERS” are bastions of life at hydrothermal vents in today’s oceans. They get their names from the soot-like look of the mineral-rich material they eject. NOAA

he first things to go will be A brightening Sun will boil the Earth’s glaciers and polar ice caps. Warming surface seas and bake the continents a temperatures will turn ice to water, leading to a billion years from now. But that’s slow but steady rise in sea levels. But it Tdoesn’t stop there. Eventually, tempera- nothing compared with what tures will rise high enough for seawater to boil away, leaving Earth bereft of this we can expect further down vital substance. With that, life on our world will need to relocate underground the road. ⁄⁄⁄ BY Richard Talcott or emigrate from our home planet. This apocalyptic scenario is more than an inconvenient truth — it’s our inevitable destiny. And it has nothing to do with changes humans may work on our fragile environment. The agent for this transformation is far beyond our control. The culprit: our current life-sustaining source of heat and energy, the Sun. Ask most people familiar with astronomy when to expect this coming apocalypse, and you’ll hear answers of around 5 billion years — once the Sun swells into a red giant. But the end is nearer than that. The Sun is currently growing brighter, and has been since the day it was born. Life on the main sequence A BILLION YEARS FROM NOW, the Sun’s When the Sun was a baby, it was rather increasing luminosity will have boiled off miserly by today’s standards. It emitted most of Earth’s water. In this view, water roughly 30-percent less energy then exists only in deep ocean trenches, where than it does now. The Sun officially thermophilic bacteria cling to life. Lynette Cook became a star when it started fusing

www.astronomy.com 29 Planets on the move Today

Sun Mercury Venus Earth Mars 0.38 AU 0.72 AU 1.00 AU 1.52 AU

Sun and planetary orbits shown to scale; planet sizes not to scale

6.5 billion years from now

Sun as red giant 0.88 solar-mass Venus Earth Mars 0.93 AU 1.17 AU 1.85 AU

6.7 billion years from now

Sun as asymptotic giant 0.66 solar-mass Earth Mars 1.61 AU 2.46 AU

As the SUN ages, it will lose some of its mass. This trend will accelerate when it becomes a red giant, and grow even greater when it swells into an asymptotic-giant-branch star. This mass loss will cause the orbits of the planets to migrate outward. Astronomy: Roen Kelly hydrogen into helium in its core. These And that’s the rub. The nuclear reactions weren’t much, if any, colder. That’s good nuclear reactions release energy according in the Sun’s core essentially convert four news as far as life is concerned. The first to Einstein’s famous equation: E=mc2. This hydrogen atoms into one helium atom. Gas single-cell organisms arose some 3.5 billion energy source defines any star’s main pressure, however, depends in part on the years ago, and they presumably required sequence life — where it spends the vast number of particles in the gas. The ongoing liquid water. But the Sun wasn’t hot enough majority of its days. fusion reduces the number of particles, so by itself to melt terrestrial ice until roughly We tend to think of a main sequence the pressure drops. To maintain hydrostatic 2 billion years ago. star like the Sun as constant, but it’s not. equilibrium, the Sun must compensate. The We can thank our lucky stars for the It maintains what astronomers call hydro- core shrinks, raising both the temperature greenhouse effect. The presence of water static equilibrium — the outward pressure and density. That, in turn, increases the rate vapor and carbon dioxide in the atmos exerted by the core’s hot gas balances the of nuclear reactions, and the Sun generates phere warms our planet well above what it inward crush of gravity. If the Sun’s central even more energy. would otherwise be. Even today, Earth is temperature were to drop slightly, for These changes operate slowly. Although some 60° Fahrenheit (33° Celsius) warmer example, the gas pressure would also fall. a hundred million years may sound like a than it would be without greenhouse Gravity then would force our star to con- long time, for the Sun, it’s a blip on the warming. In the distant past, when Earth’s tract and heat up, restoring its equilibrium. radar screen, representing 1 percent of its interior was hotter and volcanic eruptions The Sun started life as a uniform mix life span. And in a hundred million years, likely belched significantly more green- of approximately 73-percent hydrogen, the Sun’s luminosity rises less than 1 per- house gases into the atmosphere, the effect 25-percent helium, and 2-percent heavier cent. The energy increase prompts the Sun would have been greater. elements, by mass. The outer parts of the to expand at a comparably lethargic pace. The push to higher solar luminosities Sun still maintain that balance. But in the Its diameter grows at about the same rate as continues. Roughly 1 to 2 billion years core, where nuclear fusion rules, helium human fingernails: 1 to 2 inches per year. from now, Earth’s surface temperature levels continuously rise. Since the Sun’s will approach the point of no return, birth, about 5 percent of its total mass has Crystal-ball gazing when water will start evaporating and been converted into helium. If the Sun is warmer now than it was in the herald an end to above-ground life. past, what were conditions like on Earth a Several unknowns affect the timing. Richard Talcott is a senior editor of Astronomy. few billion years ago? Surprisingly, they Most important: The fraction of greenhouse

30 astronomy ⁄⁄⁄ july 07 gases the atmosphere will contain. Most scientists expect the level of atmospheric carbon dioxide to drop in the distant future. This will come about as photosynthetic organisms extract carbon dioxide from the atmosphere and weathering incorporates some of it into silicate rocks, which then are subducted into the mantle. As the oceans start to evaporate, the Sun’s high-energy ultraviolet radiation will break the water molecules into their constituents, hydrogen and oxygen. The lightweight hydrogen gas will escape Earth’s gravitational hold and bleed into space. It might take another billion years for ocean water to disappear completely, but by then, any remaining life will have had to make other plans. One viable option might be Mars. As Earth becomes too warm for most life to survive, the Red Planet should be getting balmy. If humans can make it till then, Mars would offer some attractive real estate. Into the deep future To this distant point, the Sun and Earth have taken nearly opposite paths. Even a billion or two years from now, the Sun will look basically the same on the outside as it ICY EUROPA could prove to be a watery haven in the distant future, when increasing does now — a little bigger and brighter, but solar radiation will render the inner planets uninhabitable. NASA/JPL still recognizable. The Sun’s internal structure, however, will have changed markedly. Location, location, location Its center will be largely helium, although When the South Pole feels more like the Amazon jungle a few billion years from now, any lots of hydrogen will exist in the core. The life on Earth will be looking for a way out. The Sun’s increasing luminosity will render Earth hydrogen continues to fuse into helium and uninhabitable, and worried eyes will look skyward. add to that element’s growing abundance. In a reversal of science-fiction proportions, the first stop may well be Mars. Unlike H. G. For Earth, on the other hand, the sur- Wells’ classic novel, in which dying Martians looked longingly toward a more hospitable face would hardly be recognizable. Our Earth, earthlings may decide to head for cooler martian climes. Mars has a distinct advan- “pale blue dot” will be more of a muted tage: Not only will it likely serve as humans’ first permanent outpost in the solar system, brown, and blistering temperatures will but it also holds the promise of being clement for an extended period. make it uninhabitable. But the deep interior But even Mars will grow too hot once the Sun becomes a red giant. Then, the only won’t see much effect. Although it will have reasonable outposts will be on the moons of the gas-giant planets. Several of them — including Jupiter’s Io, Europa, and Ganymede, and Saturn’s Enceladus, Rhea, and Dione cooled modestly as the total mass of radio- — already come with huge complements of ice. Raise the Sun’s temperature significantly, active elements decreases, a 21st-century and all may afford ocean-front property at some future point. geologist would still recognize it. But the reality of the Sun’s demise is that by the time Jupiter or Saturn become viable But as time continues to march on, abodes, any surviving civilization should seek other solar systems. After several billion changes in the Sun and the rest of the solar years of calling Sol home, a few million extra years won’t seem like much. It will be time system will become more pronounced. The to become citizens of the galaxy. — R. T. real changes start roughly 5 billion years from now, when the Sun exhausts the hydrogen fuel in its core and prepares to of water) there, but not extreme enough to jumps, the overlying layers will expand and leave the main sequence. As the Sun takes ignite helium. Meanwhile, hydrogen in the cool. The star will be on its way to becom- its first tentative steps into old age, it will outer core will continue to burn. ing a red giant. shine some 70-percent brighter than it does With no source of energy at the center, now. That won’t last long, however. the core will contract and heat up. Like Monster star The Sun’s inner core then will contain adding gasoline to a fire, the increased heat It will take the Sun between 1 and 1.5 billion only helium. It’ll be hot (some 50 million will cause the hydrogen-burning shell to years to evolve from the close of its main Kelvin) and dense (10,000 times the density kick into overdrive. As the Sun’s luminosity sequence life to a full-fledged red giant. By

www.astronomy.com 31 A RED-GIANT SUN looms over a dead and waterless planet Earth some 6 billion years in the future. Lynette Cook for Astronomy

32 astronomy ⁄⁄⁄ july 07 then, its surface temperature will have dropped to around 3,500 K, just over half of what it was on the main sequence. The cool surface will mean the star radiates most of its energy at longer wavelengths, in the red part of the spectrum. Still, the Sun will put out 1,000 times more energy than today. To release this much energy from a cooler surface requires the Sun to swell dra- matically. As a red giant, it will appear 100 times bigger than today, taking it beyond Mercury’s orbit and swallowing the inner- most planet. If any people were to visit Earth on a spaceship from the more tem- perate outer solar system, they would see the Sun as a bloated red sphere spanning some 50° of the sky. If our planet still rotated once every 24 hours, it would take the Sun more than 3 hours to rise and set. In reality, Earth’s rotation will have slowed significantly by then, lengthening sunrise and sunset further. In the red giant’s distended outer layers, gravity will be so weak that the solar wind will blow a million times stronger than it WHEN THE SUN DIES, it will puff off its outer layers in a final blaze of glory. The resulting does today. During the course of its red- planetary nebula, like NGC 2440 seen here, will last about 50,000 years. NASA/ESA/K. Noll (STScI) giant phase, the Sun will lose approximately 10 percent of its total mass. This gradual mass loss will reduce the it’s déjà vu all over again. Carbon ash will The Sun’s internal instability during this Sun’s overall gravitational pull, so it no build up in the center, surrounded by a asymptotic-giant-branch stage will cause longer will hold the planets as tightly. The helium-burning shell which, in turn, will our star to pulsate with a period measured planets will spiral outward a bit — except be surrounded by a hydrogen-burning in hundreds of days. It will be a Mira vari- for Mercury, of course, which already will shell. Once more, the core will contract, able star, named after the prototype star in have succumbed to the Sun’s appetite. heating the interior and spiking the nuclear- the constellation Cetus. As hydrogen continues burning in a reaction rates. The star swells again; but In just a few tens of thousands of years, shell, it’ll dump more helium “ash” onto the this time, it’ll grow even bigger and more the Sun will puff off its outer layers. The inner core. Eventually, the temperature at luminous than on the first go-round. It is Sun’s core, made of carbon and oxygen, the center will rise to 100 million K — hot now an “asymptotic-giant-branch star.” will be left behind as a white-dwarf star. enough to ignite helium. The Sun will tap At the height of this phase, the Sun will The star then will contain more than half into this second energy source with a ven- be 500 times its current diameter and swell the Sun’s current mass compressed into a geance, fusing helium into carbon and beyond the current orbit of Mars. Its outer sphere the size of Earth — a density equiva- some oxygen in its core while still fusing layers will claim their second victim as they lent to crushing a car to the size of a grape. hydrogen to helium in a surrounding shell. swallow Venus. But the Sun also loses mass The white dwarf will have an initial tem- Ironically, the initiation of helium fusion at a greater rate this time around, turning perature of 100,000 K, so it’ll emit lots of will lower the Sun’s luminosity as it causes the solar wind into a full-blown hurricane. ultraviolet light. This high-energy radiation the core to expand and cool. The star as The Sun’s mass will drop to two-thirds of will energize the expanding shell that was a whole will shrink, and its surface will what it is now, and Earth’s orbit will grow previously the Sun’s outer layers, causing it warm. It will stay in this stable configura- by approximately 60 percent. to glow. This planetary nebula will light up tion for approximately 100 million years. Current computer models can’t tell for about 50,000 years before the shell dis- Two bright stars visible from Earth — whether Earth will survive the onslaught sipates into the interstellar medium. Mean- Aldebaran and Arcturus — are at this stage or not — it looks to be a close call. Mars while, the remnant white dwarf will slowly of evolution now. should make it easily, although its days of but steadily cool off, eventually extinguish- relative tranquility will be long over. The ing the light that nurtured billions of years Supersize me best place to be could be on one of the of life in the solar system. As with all nuclear reactions, a small tem- moons of the outer planets. They may enjoy To watch a simulation of the Sun’s perature increase causes a big jump in the a brief period of springlike weather. And ONLINE reaction rate. That’s why the Sun will burn with large stores of ice currently on some EXTRA evolution in the distant future, visit www.astronomy.com/toc. through its helium fuel so rapidly. Then, of them, precious water could be plentiful.

www.astronomy.com 33 Turning up the heat

Can we send a spacecraft to the Sun? NASA’s proposed Solar Probe stretches technology and material science to their torrid limits. ⁄⁄⁄ BY david j. mccomas

© 2013 Kalmbach Publishing Co. This material may not be reproduced in any form without permission from the publisher. www.Astronomy.com ne day — perhaps as early as 2014 — a This mission would answer some of the most vex- spacecraft will depart Earth on a voyage ing questions about our star: What heats the corona of solar exploration. Measuring 8.9 feet (the Sun’s outer atmosphere) to millions of degrees? O(2.72 meters) across and 30.8 feet (9.4m) long, Solar How does the corona transition into the supersonic Probe would encounter solar-corona temperatures outflow of ionized gas known as the solar wind? averaging 3,600,000° Fahrenheit (2,000,000° Celsius) as it passes within 1.3 million miles (2.1 million kilo- SOLAR PROBE’S close approach to the Sun will test its thermal- protection system. Here, the glowing cone points toward the Sun meters) of the Sun’s surface. and shields the spacecraft’s instruments. LYNETTE COOK FOR ASTRONOMY

www.astronomy.com 43 SOLAR ECLIPSES once were the only way to study the Sun’s THIS CLOSE-UP of the Sun’s lower corona shows loops of hot corona. During totality, the Moon covers the solar photosphere plasma traveling along magnetic field lines. Without special equip- (the Sun’s visible disk), leaving only its delicate outer atmosphere ment (or a total solar eclipse), however, we can’t see the corona in view. During the February 26, 1998, eclipse, observers could see because it’s only one-millionth as bright as the Sun’s photosphere. inclined streamers and polar plumes. NCAR/HIGH ALTITUDE OBSERVATORY STANFORD-LOCKHEED INSTITUTE FOR SPACE RESEARCH/NASA

To the Sun — finally cast, the changing radiation environment in If NASA and Congress fund Solar The Sun remains our solar system’s only which future space explorers will work. Probe, it stands to become the first space- unvisited realm, half a century after the Thanks to unprecedented advances in craft to venture into the inner heliosphere, Space Age began. We’ve sent probes to all imaging, theory, and modeling, we now where the solar wind is born. Through the planets, and Voyagers 1 and 2 are now know more about the corona and the solar direct measurement and imaging of the exploring the solar system’s boundary, the wind than ever before. And yet, solar phys- region’s plasma, energetic particles, and heliosphere. But so far, no mission has got- ics’ two fundamental questions remain fields, Solar Probe would provide the data ten close to our star. unanswered. The only way to find out why needed to solve the twin mysteries of coro- Technology finally exists to change that. the corona is so much hotter than the Sun’s nal heating and solar-wind acceleration. Consider the immense design challenges surface, and how the solar wind accelerates, for such a spacecraft. It must endure the is to collect data near the Sun. Keeping Solar Probe cool extreme temperatures and radiation near The Solar Probe mission is rooted in the At its closest approach to the Sun, Solar the Sun. At Solar Probe’s planned orbit to National Research Council’s 2003 decadal Probe will endure solar energy much more within 3 solar radii of the Sun’s surface, the survey of solar and space physics. This intense than at Earth. The design and eval- spacecraft must endure more than 3,000 study stressed the scientific importance of a uation of a thermal-protection system times the Sun’s heat at Earth’s distance. solar mission and recommended that it be (TPS) that will shield the spacecraft in this The space scientists and engineers plan- implemented as soon as possible. environment are the central foci of the ning the Solar Probe mission borrowed In late 2003, NASA formed a Solar Solar Probe team’s efforts. heat-resistant-material technology from Probe Science and Technology Definition The planned TPS consists of three com- missiles and the space shuttle to fashion a Team. The team’s official report, issued in ponents. The primary heat shield is a hol- cone-shape thermal-protection system. The September 2005, outlined a $1 billion mis- low cone about 9 feet (2.7m) in diameter cone will shade the spacecraft as it pen- sion that could launch in about a decade and 17 feet (5.1m) tall made of carbon- etrates the solar corona. and provide an acceptably low-risk survey carbon composite with a ceramic coating. This proximity to the Sun will allow rev- of the near-Sun environment. Funding of a This shield protects the sensitive instru- olutionary measurements and images that mission of this scale will require special ments and spacecraft subsystems from will greatly expand our knowledge of the congressional appropriations. direct exposure to solar radiation. corona and solar wind. Solar Probe would make direct, on-the-scene measurements right where the Sun energizes its most haz- ardous solar particles. The spacecraft also would help us characterize, and even fore-

David J. McComas is the senior executive director of the Space Science and Engineering IMAGES FROM the Solar and Heliospheric Observatory (SOHO) show the corona’s evolu- Division at the Southwest Research Institute in tion during the rising phase of a solar cycle. As activity increases, streamers originate from San Antonio. higher solar latitudes, not just the Sun’s equator. SOHO

44 astronomy ⁄⁄⁄ December 06 Protecting Solar Probe’s instruments 9 Rs

Primary shield + 8h

7.4 Rs Support struts + 6h

5.9 Rs + 4h

4.6 R Secondary shield s Approaching + 2h the Sun THIS SCHEMATIC of the Solar Probe spacecraft shows the three main components of the thermal-protection system: the primary and secondary shields and the struts. Engineers constructed each component using a carbon-carbon composite material, which is ultra-heat-resistant. astronomy: rOEN KELLY, AFTER JOHNS HOPKINS UNIVERSITY APPLIED PHYSICS LABORATORY/NASA Closest 4 Rs approach

The secondary shield is a disk of the protective shadow for carbon-foam layers encased in a carbon- the spacecraft bus and carbon jacket and attached to the primary instruments. Solar Probe Solar Probe Sun shield’s base. It insulates the instruments will retain this orientation orbit from the primary shield, which will reach throughout its encounter. solar probe’s heat temperatures approaching 3,000° Fahren- The instrument package –2h shield points at the Sun heit (1,650° Celsius) at perihelion (Solar includes instruments both 4.6 Rs during closest approach, Probe’s closest approach to the Sun). for direct measurement of keeping its instruments The third component consists of the the solar environment and for cool. (RS = solar radii; h = carbon-carbon attachment struts. They remote-sensing observations of hours before (–) or after (+) support the TPS and separate it from the coronal structures. Most of the – 4h closest approach.) astronomy: ROEN KELLY, AFTER spacecraft bus. The separation prevents the instrument package will be mounted 5.9 Rs secondary shield heat from reaching the safely inside the heat-protected space- JOHNS HOPKINS UNIVERSITY APPLIED PHYSICS LABORATORY/NASA bus while providing maximum stability. craft body. Three instruments, however Mission scientists conducted thermal — a fast-ion analyzer, a fast-electron ana- – 6h analysis using computer models of the TPS, lyzer, and an ion-composition analyzer — combined with experimental testing of can- will be on a movable arm. This platform will 7.4 Rs didate materials. The tests demonstrated extend instruments out to nearly the cone’s – 8h the TPS can maintain spacecraft systems at edge then retract them incrementally as the their normal operating temperatures — less spacecraft approaches the Sun and the “safe 9 Rs than 120° F (49° C) — even during closest zone” becomes smaller. Instruments on the elliptical approach to the Sun. arm will gather data just inside the edge of orbit. Even During the proposed — and still to be the thermal-protection system’s shade. at that speed, funded — second phase, further testing of Meanwhile, a retractable, heat-resistant the Sun is so TPS materials and components will be per- imaging periscope will intermittently poke large that the space- formed, and a full-scale prototype will be out beyond the shadow for quick views of craft’s pole-to-pole pas- fabricated, assembled, and tested. the solar wind’s source regions. sage takes about 14 hours. With the trajectory the Solar Probe team Following its initial pass, One tough spacecraft has designed, the spacecraft will make two Solar Probe will sail out to Jupiter’s Solar Probe is a gyroscope-stabilized vehi- close approaches to the Sun separated by orbit before the Sun’s gravity pulls it cle designed to survive and operate in about 41⁄2 years. The first will come as Solar back for a return visit along a similar path. intense heat. Its most prominent feature Probe shoots past the Sun’s south pole, These two approaches, and the interval is the cone. Twenty days before closest dives to its closest approach near the solar between them, allow the spacecraft to com- approach, when Solar Probe is near Venus’ equator at 670,000 mph (1.09 million kilo- pare solar-wind and coronal measurements orbital distance, the craft’s thrusters will meters per hour), and then travels out- during opposite phases of the 11-year solar turn the cone toward the Sun. This provides bound over the north pole in a highly cycle. (Every 11 years, on average, the Sun

www.astronomy.com 45 TRACE — the Transition Region and Coronal Explorer spacecraft — the corona at a time when the Sun was moderately active, with some currently orbits Earth. TRACE’s field of view covers only a fraction of hot (red) active regions in both hemispheres, surrounded by cooler the solar disk, but by repeatedly repointing, it can create an image of (green and blue) coronal plasma. In contrast to TRACE, which images the Sun’s entire inner corona. This image shows the solar corona on the Sun from Earth orbit, Solar Probe will provide the first-ever close- August 2, 1999, in a false-color, 3-layer composite. Imaging revealed up views of the Sun’s corona. STANFORD-LOCKHEED INSTITUTE FOR SPACE RESEARCH/NASA

46 astronomy ⁄⁄⁄ December 06 solar probe: a Long time coming Scientists first recommended the idea of a near-Sun mission in 1958, less than a year after Sputnik awoke the world to the possi- bilities of sending human-made hardware into orbit and beyond. In that year, the National Academy of Science’s Simpson Commit- tee issued recommendations for future missions that the newly organized NASA should undertake. They included sending a spacecraft inside Mercury’s orbit to measure the Sun’s particles and magnetic fields. Four years later, the Mariner 2 probe to Venus made the first definitive solar-wind measurements. It confirmed what had been a controversial theory describing the solar corona’s supersonic expansion. The next decade saw various missions measure solar- wind properties from different regions of space — but none ventured inside Venus’ orbit. In the mid-1970s, the joint U.S.-German Helios project placed two spacecraft into highly elliptical orbits that ventured a little closer to the Sun than Mercury. These data remain the closest measurements of the Sun. But their closest approach was 60 solar radii — far outside the corona and the region where solar- wind acceleration occurs. Since then, as new heat-shield materials and spacecraft pro- pulsion systems were developed, NASA conducted several solar- THE SUN’S PHOTOSPHERe, or visible surface, is the region we’re mission studies. Some manner of solar probe has remained most familiar with. Only the best solar telescopes, however, deliver consistently at or near the top of various National Science Acad- an image like this one. Besides sunspots, dark areas roughly 3,000° emy and NASA priority lists. One short-lived plan even proposed F (1,650° C) cooler than the surface, the telescope also resolved a joint U.S.-Russian mission in the 1990s. — D. J. M. granules, the tops of convective plasma-current cells. G. SHARMER AND

K. LANGHANS, INSTITUTE FOR SOLAR PHYSICS OF THE ROYAL ACADEMY OF SCIENCES, SWEDEN moves from one period of high activity to on vast scales — play roles in coronal heat- Solar Probe will make the first direct the next.) For example, if Solar Probe ing and solar-wind acceleration. measurements of the near-Sun region, sam- launches in 2014, its first flyby will take Scientists now want to know the geom- ple the environment where solar plasma place in 2018, around the solar minimum. etry and dynamics of the expanding mag- particles are energized, and identify the The second flyby, in 2023, will occur dur- netic fields and particle distributions that seed populations for these dangerous par- ing a time of increased solar activity. lie at the sources of the fast and slow solar ticles. The spacecraft also will provide criti- winds. Solar Probe will trace the energy cal data for predictive models that — along Why we need to get close flow that heats the corona and accelerates with solar and heliospheric monitoring — Thanks to years of observations coupled the solar wind. It also will determine what will enable scientists to forecast the space- with complex modeling and sophisticated mechanisms accelerate and transport these radiation environment in support of theoretical research, scientists already have charged particles around the Sun. human exploration. a general picture of how the Sun’s corona As a bonus, Solar Probe’s unique path Solar Probe’s visit to a never-before- and the solar wind operate. They know that close to the Sun will answer some questions explored region of our solar system will at times of lower solar activity, the solar about the size and mass distribution of answer questions that simply can’t be wind has two components: a dominant, mysterious dust grains orbiting near the answered any other way. The answers will steady, high-speed wind and a more vari- Sun. This dust, believed to originate from help space scientists understand what’s able, low-speed wind. Each emanates from comets and asteroids, interacts with the going on in a region that has awed and specific coronal zones. corona and may influence the solar wind frightened humans since we first witnessed As solar activity increases to solar maxi- and the formation of energetic particles. the Sun’s beautiful and mysterious corona mum, this orderly configuration breaks Finally, even though Solar Probe will go during a total eclipse. down into a complex mixture of fast and where humans cannot, what scientists learn Moreover, as with any voyage into slow winds from all areas of the corona. from the mission will play an important uncharted territory, Solar Probe’s journey The energy that heats the corona and drives role in piloted missions to the Moon and also holds the promise of unanticipated the wind manifests itself as motions of Mars. On these expeditions (well beyond discoveries — new mysteries that will chal- plasma within and around the Sun. Mag- the protective shield of Earth’s magnetic lenge humanity’s ever-expanding knowl- netic fields channel, store, and then dissi- field), astronauts may be exposed to intense edge of our home in the universe. pate these plasma motions. Heliophysicists blasts of the Sun’s energetic particles. Such believe waves, instabilities, magnetic recon- events present serious threats to humans ONLINE See a Solar Probe animation at nections, and turbulence — all operating living and working beyond Earth. EXTRA www.astronomy.com/toc.

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