Your Guide to Planets, Stars, and Galaxies by Richard Talcott

FACTS500 INSIDE! Your guide to planets, stars, and galaxies by Richard Talcott

A supplement to Astronomy magazine

Saturn’s rings consist of icy particles ranging in size from tiny motes to house-sized icebergs. NASA/The Hubble Planets Heritage Team (STScI/AURA) of the solar system

arth may seem extraordinary to those who call it home, but it’s not a land of superlatives. Earth is neither too hot nor too cold, too big nor too small. It’s just right in so many ways — the perfect “Goldilocks” planet. Of course, as the only known abode of life in the universe, Earth doesE have one major claim to being special. The other planets in the solar system leave their marks in different ways. The planets divide into two broad categories: terrestrial and jovian. The small, rocky terrestrial planets include Mercury, Venus, Earth, and Mars. Mercury, the closest to the Sun, bakes Mars at temperatures up to 800° Fahrenheit at noon. But Mercury’s razor-thin atmosphere can’t hold heat; at night, the temperature plummets far below freezing. Venus most resembles Earth in mass and diameter, but a thick atmosphere of carbon dioxide has led to a runaway greenhouse effect. Venus’ surface remains a scorching 865° F year-round. Earth and Mars are the water worlds of the solar system. Our home planet is the only one with liquid water at the surface now, but spacecraft observations during the past 15 years leave no doubt that Mars once had loads of surface water. Even now, Mars has permafrost and permanent polar caps of water ice. Winds up to 70 mph blow around the ubiquitous martian dust, creating shifting seasonal patterns. FUN The jovian planets — Jupiter, Saturn, Uranus, and Neptune Mars boasts the largest vol- — are all gaseous behemoths. They consist mostly of hydrogen canoes in the solar system, and helium, the most abundant elements in the universe. Jupiter although they’re all extinct. The biggest — Olympus dwarfs the others: It contains more than twice as much matter Mons — spans nearly 400 as all the other planets combined. All the jovian planets pos- miles and rises 13 miles sess ring systems, but only Saturn’s appears bright. Its icy rings above the surrounding plains. span 170,000 miles and measure just 100 feet thick. Uranus FACT and Neptune are the true twin planets of the solar system, with nearly equal diameters, masses, compositions, and rotations. Most scientists no longer consider small, distant Pluto to be a major planet. A mixture of ice and rock, this world more closely resembles the thousands of so-called Kuiper Belt objects that lurk beyond Neptune. In 2006, astronomers demoted Pluto to a Mars’ ruddy appearance arises “dwarf planet,” a category that also includes the asteroid Ceres. because the sand on the planet’s surface consists largely of iron 2 Your guide to planets, stars, and galaxies oxides — rust. NASA/JPL/MSSS Jupiter

Jupiter is so big that it would take 11 Earths wedged side by side to cross the giant’s girth and more than 1,000 Earths to fill its volume. NASA/JPL/University of Arizona

Thick clouds blanket Venus, so astronomers use radar to see Venus its surface. The atmospheric pressure there is nearly 100 times that at Earth’s surface. NASA/JPL FUN Saturn has the lowest density of any planet. In fact, if you filled a solar-system-sized basin with water, the ringed world would float. FACT

Mercury Earth Pluto & Charon FUN Sunlight takes just eight min- utes to reach Earth but more than four hours to cross the void to Neptune and Pluto. FACT

Mercury's high density means Nearly three-quarters of more than half of it must be made Earth’s surface is covered with of the heavy elements iron and water. It’s what makes our home The orbit of dwarf planet Pluto (left; along with its moon, Charon) brings it closer nickel. NASA/JPL/USGS world conducive to life. NASA to the Sun than Neptune for 20 years out of its nearly 250-year-long circuit. ESA/NASA

Uranus Neptune Solar system planets Planet Distance from Sun Orbital period Diameter Mass Density (Earth=1) (Earth=1) (Earth=1) (water=1) Mercury 0.39 87.97 days 0.383 0.055 5.43 Venus 0.72 224.70 days 0.949 0.815 5.24 Earth 1.00 365.26 days 1.000 1.000 5.52 Mars 1.52 686.98 days 0.532 0.107 3.93 Ceres 2.77 4.60 years 0.075 0.0002 1.93 Jupiter 5.20 11.86 years 11.209 317.832 1.33 Saturn 9.58 29.46 years 9.449 95.159 0.69 Uranus’ bland cloud tops mask Storms rage in Neptune’s atmos Uranus 19.20 84.01 years 4.007 14.536 1.27 the fact that its rotation axis lies in phere, as they do in the massive Neptune 30.05 164.79 years 3.883 17.147 1.64 its orbital plane, so night and day at atmospheres of most of the jovian Pluto 39.48 247.68 years 0.187 0.002 1.75 the poles last 40 years each. NASA/JPL planets. NASA/JPL Note: Ceres and Pluto are officially considered to be dwarf planets. Titan

Titan’s hazy atmosphere glows as it scatters incom- ing sunlight. The atmosphere of Saturn’s moon is thicker than Earth’s and, like ours, contains mainly nitrogen. NASA/JPL/SSI Small bodies Europa of the solar Ridges crack the surface of Jupiter’s moon Europa. Such ridges could be sites where slushy water erupted through the icy surface system and then froze. NASA/JPL fter the Sun and planets, there’s not much else in the Io solar system — certainly not in terms of mass. But More than 100 active volcanoes dot the surface in sheer number (and in a few notable instances, of Jupiter’s moon Io. The plumes can reach 100 prominence), the small objects hold their own. The miles high and spread debris over thou- biggest of the small bodies actually outrank the small- sands of miles. NASA/JPL est planet.A Both Ganymede, a Jupiter moon, and Titan, a Saturn moon, have diameters larger than Mercury. More than 170 moons have been discovered orbiting the eight planets, although the vast majority are little more than flying boulders. The smaller planets tend to have fewer moons. Earth has just one, which formed when an object the size of Mars struck the proto-Earth, ejecting debris that eventually coalesced. Mercury and Venus have no moons, and Mars possesses just two small ones. Oddly enough, Pluto’s large moon, Charon, is half the diameter of the dwarf planet — the largest ratio in the solar system. The hefty moons of the gas giants garner most of the atten- tion. Jupiter’s four big moons — Io, Europa, Ganymede, and Callisto — form a miniature solar system. Io ranks as the most volcanically active object in the solar system. Europa hides an ocean of liquid water — perhaps larger than all of Earth’s oceans — beneath its frigid ice crust. Giant Ganymede also may harbor an ocean and has a surface covered with intriguing grooved ter- rain. And Callisto sports more craters than any other object in the solar system. At the top of Saturn’s family of moons is Titan, which possesses a significant atmosphere and methane lakes. More than half a million asteroids also inhabit the solar system. The biggest, Ceres, has a diameter of 600 miles. Yet most are far smaller: If you add them all up, asteroids don’t equal the weight of Earth’s Moon. Most asteroids circle the Sun between the orbits of Mars and Jupiter, although a few wander into Earth’s vicinity. Perhaps the most spectacular small bodies are comets. Billions of these “dirty snowballs” lurk in the outer solar system. If their long, looping orbits bring them close to the Sun’s warmth, they shed gas and dust. The Sun then blows this material back to create a long tail. Although a comet’s nucleus may be only a mile or two across, its tail can stretch millions of miles.

4 Your guideguide to to planets, planets, stars, stars, and and galaxies galaxies Triton Callisto Geyser-like plumes deposited the dark streaks seen on Nep- tune’s moon Triton. At a FUN temperature of –390° F, Triton Saturn’s enigmatic outer has the coldest surface known in moon, Iapetus, has a split the solar system. NASA/JPL personality: Half of its surface appears as dark as freshly laid asphalt while the opposite Enceladus hemisphere reflects as much light as newly fallen snow. FACT

Saturn’s icy moon Enceladus reflects more than 90 percent of the sunlight Multi-ringed impact basins, some stretching more than that reaches it, the highest percentage 1,000 miles, formed on Jupiter’s Callisto when massive of any object in the solar system. NASA/JPL impacts left concentric fractures and faults. NASA/JPL

Io Hale-Bopp Eros One of the brightest comets of the past 40 years, Hale-Bopp wowed observers in 1997. It had a nucleus 25 miles wide and a tail that stretched more than 100 million miles. Bill and Sally Fletcher

Potato-shaped asteroid Eros, some 20 miles long, looks like a lot of other modest-sized asteroids, but this object might one day wander dangerously close to Earth. NASA/JHUAPL

FUN Major moons Io’s active volcanoes and Europa’s underground ocean Moon Planet Distance from Orbital period Diameter Density of liquid water both stem planet (miles) (days) (miles) (water=1) from enormous tidal forces — which flex and heat the Moon Earth 238,900 27.32 2,160 3.3 moons’ interiors — exerted Io Jupiter 262,100 1.77 2,263 3.5 by Jupiter’s massive gravity. Europa Jupiter 417,000 3.55 1,940 3.0 FACT Ganymede Jupiter 665,100 7.15 3,271 1.9 Callisto Jupiter 1,169,900 16.69 2,994 1.8 Enceladus Saturn 147,900 1.37 313 1.6 Tethys Saturn 183,100 1.89 660 1.0 Dione Saturn 234,500 2.74 698 1.5 Rhea Saturn 327,500 4.52 949 1.2 Titan Saturn 759,200 15.95 3,200 1.9 Iapetus Saturn 2,212,600 79.33 913 1.1 Ariel Uranus 118,600 2.52 719 1.7 Phobos Umbriel Uranus 165,300 4.14 727 1.4 Like most small moons in the solar Titania Uranus 271,100 8.71 980 1.7 system, Mars’ Phobos measures just a Oberon Uranus 362,600 13.46 946 1.6 few miles across and has an irregular Triton Neptune 220,400 5.88 1,681 2.1 shape. NASA/JPL/MSSS Charon Pluto 12,200 6.39 753 1.9 The Sun

Stars in our galaxy

ll stars begin their lives in the vast clouds of gas and dust that litter galaxies like the Milky Way. A single cloud can produce hundreds, or even thousands, of stars. Something triggers the cloud to start collaps- ing — perhaps strong winds from a massive star or Most people think of the Sun as the anchor of our solar system — and a nearbyA supernova explosion — and gravity works its magic. that’s certainly true. It contains 99.8 percent of all the matter in the solar The cloud fragments, and each pocket of material continues to system. But to astronomers, the Sun has even more importance. It is the contract and heat up. only star in the universe that appears as more than a point of light The contracting star becomes stable when it starts to gener- through a telescope. Detailed observations of the Sun led scientists to understand how stars shine, how they radiate energy, and even how ate energy by nuclear fusion. Four hydrogen atoms combine to huge storms wrack their surfaces. NASA/SOHO form one helium atom. Because one helium weighs slightly less than the four hydrogens combined, the reaction creates energy according to Einstein’s equation E=mc2. N49 The biggest stars contain up to about 120 times as much material as the Sun. They burn hot and use their fuel rapidly. These luminaries may have a surface temperature of 70,000° F, radiate nearly a million times the Sun’s light, and survive only a few million years. The Sun shines at about 10,000° F and will last some 10 billion years (it’s about halfway through now). The smallest stars have 8 percent of the Sun’s mass and glow at only 3000° Heavy elements forged to 4000° F — so dim that they can shine for a trillion years. in a massive star spread Once a star exhausts its hydrogen fuel, the end is nigh. First, out at thousands of miles it swells into a red giant, expanding to a diameter of hundreds of per second in supernova remnant N49. One day, millions of miles and cooling to a few thousand degrees. It may these elements may be tap into more nuclear reactions, converting helium to carbon, included in a new stellar for example, but eventually those generation. NASA/The Hubble

Heritage Team (STScI/AURA) fuels run out as well. Stars with up FUN to about eight times the Sun’s mass Astronomers divide stars into eventually puff off their outer layers seven main spectral classes. and form glowing gas clouds known Generations of students have The life of a Sun-like star as planetary nebulae. The star itself learned the sequence by settles down as a white dwarf. using the first letters in the sentence: “Oh, Be A Fine Girl More massive stars typically die (or Guy), Kiss Me.” in supernova explosions. Such explo- FACT sions scatter the heavy elements built up during the star’s life, forming the raw material for new stars and, perhaps, planets. The collapsed remnant of the exploded star becomes either a rapidly spinning neutron star or a black hole, whose gravity is so strong that not even light can escape.

6 Your guide to planets, stars, and galaxies Protostar Main sequence Red giant Asymptotic-giant-branch star Protoplanetary nebula Planetary nebula White dwarf solar-type star Cat’s Eye Nebula Surface of the Sun

FUN To shine as brightly as it does and nourish life on Earth, the Sun must convert 600 million tons of hydrogen into helium every second. FACT

When the Sun dies in 5 billion years, it may resem- ble the symmetric Cat’s Eye Nebula. Here, glowing strands of ionized gas mark where a dying star repeatedly shed its outer layers. NASA/ESA/HEIC/

The Hubble Heritage Team (STScI/AURA)

Cone Nebula

An intricate honeycomb on the Sun’s surface marks regions where heat (bright areas) rises and cooler material (dark areas) sinks in a process called convection. Royal Swedish Academy of Sciences

Star characteristics

Spectral Mass Temperature Main sequence Examples class (Sun=1) (Fahrenheit) radius (Sun=1)

O 20–120 greater than 55,000° 12–25 Zeta (ζ) Puppis B 4–20 17,100°–55,000° 4–12 Rigel, Spica A 2–4 12,300°–17,100° 1.5–4 Sirius, Vega F 1.05–2 10,300°–12,300° 1.1–1.5 Canopus, Procyon G 0.8–1.05 9,000°–10,300° 0.85–1.1 Sun, Capella New stars form from clouds of gas and dust such as the Cone Nebula. Hot stars ionize the surrounding K 0.5–0.8 6700°–9000° 0.6–0.85 Aldebaran, Arcturus hydrogen gas, which glows with a characteristic red M 0.08–0.5 3100°–6700° 0.1–0.6 Antares, Betelgeuse color. NASA/ESA/The ACS Science Team

Stars like the Sun condense out of a gaseous cloud. The growing protostar develops a disk (which may form planets) and shoots out material before settling down as a main sequence star, converting hydrogen to helium. Once the hydrogen runs out, the star swells to a red giant and becomes unstable as an asymptotic-giant-branch star before puffing off its outer layers as a planetary nebula. The star’s core remains as a dense white dwarf. ASTRONOMY: ROEN KELLY

Protostar Main sequence Red giant Asymptotic-giant-branch star Protoplanetary nebula Planetary nebula White dwarf solar-type star Star formation in Cygnus

The Milky

A stellar nursery in Cygnus harbors many massive young stars. Invisible in optical light, the DR21 complex shows up when viewed in dust-penetrating Way Galaxy infrared radiation. NASA/JPL-Caltech/A. Marston (ESTEC/ESA) ead outside on a clear, dark summer’s night, and your The Pleiades M3 eyes will be greeted by thousands of stars. All of them belong to our galaxy, as does virtually everything else you can see with the naked eye. If you let your eyes adjust to the darkness, you’ll see a gauzy, whitish bandH running across the sky. This is the Milky Way — the combined light of countless stars — and the feature that lends its Open star clusters like the Pleiades Globular star clusters have existed name to our galaxy. contain dozens to hundreds of stars. as long as the Milky Way. M3 packs The Milky Way is a giant barred spiral galaxy that stretches These groups lie in our galaxy’s spi- 500,000 stars in a sphere 160 light- about 120,000 light-years from end to end but whose disk mea- ral arms and disperse over billions years across. S. Kafka and K. Honeycutt, of years. Jason Ware Indiana University/WIYN/NOAO/NSF sures only some 1,000 light-years thick. The central bar extends 28,000 light-years. The Sun lies about halfway between the gal axy’s center and edge and revolves at approximately 150 miles per second, taking roughly 225 mil- Structure of the Milky Way

lion years to complete one circuit of FUN the galactic hub. Most naked-eye stars are The most obvious sights of the massive and highly luminous galaxy are stars. Astronomers esti- ones that shine across great mate between 200 and 400 billion distances. But this gives a dis- populate the Milky Way Galaxy torted view of the galaxy as a Arm whole. In actuality, cool, dim, rus (most are hidden from view or tau M-type stars make up about en -C extremely faint, so a precise count m two-thirds of all stars in the tu cu rma isn’t possible). Milky Way. S No Arm Because the hottest, brightest stars FACT are also short-lived — and the spiral arms are the only place in the galaxy Central bar

r with active star formation — the arms stand out. The clouds Sun u p S of gas and dust from which stars form also call the spiral arms n io r home, as do the open star clusters that emerge from them. O S a g The nuclear bulge of the galaxy consists mostly of old stars. i tt ar It measures about 12,000 light-years across. At the galaxy’s heart iu rm s A A rm us lies a supermassive black hole that weighs approximately 4 mil- rse Pe lion Suns. Surrounding the bulge and disk is a vast spherical halo that stretches some 300,000 light-years. The most prominent members of the halo are globular clusters. These ancient collections of up to a million stars each were born at the same time as the galaxy, some 12 billion to 13 billion years ago. They contain few heavy elements because they formed before supernova explosions had enriched the inter stellar medium with them. The Sun lies in the Orion Spur, one of several arms and smaller appendages where our galaxy creates stars. Astronomers name the spiral arms after the 8 Your guide to planets, stars, and galaxies constellation where they appear prominent. NASA/JPL-Caltech/R. Hurt (SSC-Caltech) Star formation in Cygnus FUN In measuring distances in the galaxy and the universe, astronomers use a unit known as the light-year. It represents the distance a beam of light travels in one year. At 186,000 miles per second, light traverses 5.9 trillion miles in a year. FACT

FUN Much of the Milky Way Galaxy and its structure remain hidden to earthbound observers because dust chokes the spiral arms. It’s like being in the woods and trying to discern M3 the forest’s form. FACT

FUN A century ago, astronomers thought the Sun occupied the center of the galaxy. But careful studies of globular clusters, which orbit the Milky Way’s center and tend to gather in the constellation Sagittarius, show we live Structure of the Milky Way halfway to the edge. FACT

FUN How do astronomers know a black hole resides at the Milky Way’s center? They have found stars near the central hub orbiting so fast that they must be circling an invisible object containing 4 million solar masses. FACT r u p S n io r O

rm A us rse Pe

A band of dust cuts through the Milky Way, blocking light from distant stars. If not for all the dust, the galaxy’s center would shine brighter than the brightest star. Steve Thornton NGC 4414 Galaxies

Multiple spiral arms wind out from the nucleus of in the NGC 4414. Young blue stars throng the arms while older, redder stars populate the nuclear universe bulge. NASA/The Hubble Heritage Team (STScI/AURA) M84 & M86 he collections of stars, gas, and dust known as galaxies form the building blocks of the observable universe. Roughly 125 billion galaxies populate the cosmos, and they come in all shapes and sizes. Astronomers divide galaxies into three major categories: spirals, ellipticals, andT irregulars. A spiral has a broad disk containing clouds of gas and dust and from two to several spiral arms, a nuclear bulge of Giant elliptical galaxies M84 (left) and M86 (right) old stars, and a spherical halo that envelops both. each contain a trillion stars. These two dominate the central region Approximately one-third of spirals exhibit central bars — a of the nearby Virgo cluster, a collection of some 2,000 galaxies. NOAO/AURA/NSF symmetric concentration of stars, and sometimes gas and dust, that crosses the nucleus and connects with the outer spiral arms. (Recent studies show the Milky Way possesses a significant bar.) The Mice The diameters of spirals range from roughly 20,000 to more than 100,000 light-years, and they contain anywhere from several billion to several hundred billion stars. Elliptical galaxies appear spherical or flattened in shape. They possess little of the gas and dust seen in the disks of spiral galaxies, so they don’t generate any new stars. Ellipticals show the widest range in size of any galaxy type. Giant ellipticals can span 1 million light-years and contain several trillion stars; dwarf ellipticals may be only a few thousand light-years across and have millions of stars. An important intermediate type of galaxy has a disk like a spiral galaxy but contains no gas or dust. Astronomers call this type of galaxy a lenticular. Irregular galaxies don’t show any symmetry or organized The “Mice” are two spiral galaxies in the process of merging. Gravity has spiral structure. The category exists basically as a catchall for pulled material out of each to form long tails while compressed gas clouds galaxies that don’t fit either the spiral or elliptical classification. fuel new star formation. NASA/ESA/the ACS Science Team Irregulars can be big, containing up to 100 billion stars, or as small as dwarf ellipticals. Astronomers think most irregulars result from the collisions or mergers of two or more galaxies. The gravitational interactions disrupt normal spiral or elliptical structure, leaving behind a chaotic appearance. Most galaxies belong to groups with dozens of members, or to clusters with up to thousands of members. The Milky Way joins with the slightly larger Andromeda Galaxy to form the cornerstones of the Local Group, a collection of roughly 50 galaxies that spans several million light-years. The vast majority of Local Group galaxies are dwarf ellipticals and irregulars. Small groups generally have a few dozen member galaxies, but clusters can contain several thousand galaxies. The Virgo cluster, located M87 50 million light-years away, is the nearest large cluster to Earth. A high-speed jet shoots from the heart of the giant elliptical galaxy M87 (upper left) in the Virgo cluster. A black hole of some 10 Your guide to planets, stars, and galaxies 3 billion solar masses drives this activity. NASA/The Hubble Heritage Team (STScI/AURA) FUN If you examine the brightest galaxies, some 75 percent are spirals, 20 percent ellipticals, and 5 percent irregulars. Including faint dwarfs skews the numbers to 30 percent spirals, 60 percent ellipticals, and 10 percent irregulars. FACT

M84 & M86

FUN The Andromeda Galaxy may seem a good neighbor to The Mice the Milky Way, but it won’t always be so. Astronomers think that in approximately 5 billion years, our two galaxies will collide and merge. FACT

Andromeda Galaxy FUN Slightly bigger than the Milky Way, the Andromeda Galaxy (M31) contains The Large and Small some 500 billion stars. Located 2.5 million light-years away, the galaxy can be Magellanic Clouds (LMC and SMC) are satellite galax- glimpsed with the naked eye. Michael Stecker ies to the Milky Way. They lie deep in the southern sky and were not seen by Europeans Leo I Leo II until Magellan’s around-the- The LMC 818 kly 750 kly world voyage. Ursa Minor Dwarf 225 kly FACT Sextans Draco Dwarf Dwarf 248 kly 293 kly Milky Way LMC 160 kly SMC 189 kly NGC 185 2,020 kly NGC 147 The Large Magellanic NGC 6822 2,460 kly Cloud (LMC) is an irregular The Local Group 1,760 kly galaxy about 160,000 M87 light-years from Earth. The The Milky Way and Andromeda galaxies reddish cloud at top right rule the Local Group, accounting for more is the Tarantula Nebula, than half its mass. Distances from our galaxy Andromeda Galaxy (M31) Pinwheel Galaxy (M33) 2,510 kly the largest known region are given in thousands of light-years (kly). 2,770 kly of star formation. Luke Dodd ASTRONOMY: ROEN KELLY Probing

FUN For decades, astronomers the distant have known that the universe contains lots we can’t see. This mysterious dark matter surrounds galaxies and binds clusters. Scientists suspect universe exotic subatomic particles are the culprit. ow did the universe get to be the way it is today? It FACT might seem a hopeless question at first, at least until scientists invent a time machine to take us back. But astronomers have invented such an instrument — they call it a telescope. Here’s how it works: Because light travelsH at a finite speed (186,000 miles per second), the light we receive on Earth left its place of origin some time ago. The farther we look into space, the further we peer back in time. When astronomers first studied galaxies in the early 20th century, they found the farther a galaxy was from Earth, the faster it appeared to be moving away. If you think of this expansion as a movie and run it backward, then all of the galaxies must have been much closer together in the past. This led to the idea of the Big Bang — the theory that all matter in the universe started out together and then something triggered a rapid expansion that continues today. But is there any proof of such an extraordinary beginning? Yes — lots of it. Astronomers looking ever deeper into space find that the universe was surprisingly different. Instead of the stately spiral and elliptical galaxies we see now, there were lots of galactic fragments that were colliding, merging, and creating general havoc. The activity dumped fuel onto supermassive black holes at the centers of nascent galaxies, creating highly luminous quasars. All this action took place — in fact, could only take place — in a universe much smaller than today’s. Radio astronomers have even discovered the echo of the initial fireball, FUN which has cooled to a few degrees Galaxies do not spread above absolute zero. evenly, but gather in clusters An even more shocking discovery that themselves form vast came at the end of the 20th century. filaments, leaving huge voids in between. By looking at supernova explosions in the distant universe, astronomers FACT discovered that the blasts did not appear as bright as expected. The conclusion: The universal expansion is accelerating. In essence, a long-range repulsive force must be driving the universe to expand at ever greater speeds. As the Scottish geneticist J. B. S. Haldane once famously said: “The universe is not only queerer than we imagine, it is Abell 1689 queerer than we can imagine.” Thousands of galaxies cluster together in Abell 1689. The concentration of luminous matter and dark matter (stuff we can’t see but which adds to grav- 12 Your guide to planets, stars, and galaxies ity) creates a fun-house mirror of arcs and wisps. NASA/ESA/the ACS Science Team Cosmic microwave background

The microwave background glows at a nearly constant temperature of 4.9° F above absolute zero. Tiny variations in the glow are subtle density fluctuations that gave rise to the galaxies and voids we see in the universe. NASA/WMAP Science Team

What is the universe made of? Dark energy 72%

Atoms 5% Dark matter 23% The atoms that make up stars, planets, and us add up to just 5 percent of the universe. Invisible dark matter makes up 23 percent more, while the dark energy that drives the accelerating cosmos accounts for 72 percent. ASTRONOMY: ROEN KELLY FUN Black holes are so dense that their gravity prevents even light from escaping. They The Great Attractor range in size from monsters in galactic cores to star-sized remnants of supernovae. Some scientists think there even may be mini-black holes left over from the Big Bang. FACT

FUN Quasars — short for quasi- stellar radio sources — are the brightest objects in the universe. They radiate as much energy as an entire galaxy from a volume no bigger than our solar system. Abell 1689 FACT

The biggest concentration of matter in the nearby universe is a string of huge galaxy clusters known as the Great Attractor, which is pulling the Local Group and the Virgo cluster in its general direction. European Southern Observatory Conjunctions

When a crescent Moon passes a bright star or planet during the twilight You and the hours, it’s a sight that thrills any skywatcher, experienced or not. Randall Wehler universe Solar eclipses

ou can make a connection to the universe at large on any clear night. Simply head outside and look up. You don’t need binoculars or a telescope (although they won’t hurt) — all you need are your eyes. The simplest thing to do is trace the patterns of stars. In winter, Y look for the commanding figure of Orion the Hunter. Spring brings the Big Dipper, probably the most recognizable stellar group in the sky. In the summertime, look for the Northern Cross and trace the path of the Milky Way, which appears most prominent this time of year. The Great Square of Pegasus beckons on autumn eve- nings. You can find these patterns and, from them, hunt down the less conspicuous constellations with the help of the circular star map that appears at the center of every issue of Astronomy. Next, use “The Sky this Month” in the magazine to home in on the most visually arresting events in a given month. It could When worlds align, observers can expect a treat. Here, the Moon passes in be a nice meteor shower, where you might see a “shooting star” front of the Sun, blocking the brilliant solar disk and revealing the corona. Such every minute or so. Or maybe there’ll be a pretty conjunction displays are possible because, by cosmic coincidence, the Sun is 400 times farther from Earth than the Moon and 400 times bigger than the Moon. Don Folz of the Moon with a bright planet or two (events like this usually happen several times a month). Or perhaps you’ll be lucky and get to witness a solar or lunar eclipse. After all, in the world of backyard astronomy, the sky is the limit. Lunar eclipses

Meteor showers Annual showers Name Peak date Quadrantids Jan. 3 Lyrids april 22 Eta Aquarids may 6 Perseids aug. 12 Orionids oct. 21 Leonids nov. 17 Geminids Dec. 14

Meteors storm from the sky at speeds of up to 44 miles per second. Fric- A ruddy Moon signals a total lunar eclipse, when the Moon dips completely tion with Earth’s atmosphere heats the tiny dust particles until they flare into Earth’s shadow. Earth’s atmosphere acts like a filter, scattering out blue light into incandescence. John Chumack and bending red light into the shadow, producing the striking color. Jason Ware