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The Turbulent Birth of Stars and Planets

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The Turbulent Birth of Stars and Planets PHYSICS & ASTRONOMY_Protoplanetary Disks The Turbulent Birth of Stars and Planets Exoplanets – planets that orbit stars other than the Sun – used to be a matter of science fiction. Some 15 years ago, with the first detection of an exoplanet, they became a matter of observational astronomy. Since then, exoplanet observations have provided astronomers with intriguing clues as to the formation of stars and planets. This is invaluable information for researchers interested in planetary and star formation, such as the team led by Thomas Henning, Director at the Max Planck Institute for Astronomy in Heidelberg. TEXT THOMAS BÜHRKE he birth of planets and stars begins with clouds of gas and dust measuring many light- years in size. Such clouds can be found throughout our ga- T lactic home, the Milky Way, and for billions of years, they have acted as cosmic nurseries. In broad terms, what happens next has been known for de- cades: when a suitably large part of such a cloud exceeds a certain density, it begins to contract under its own grav- ity. Typically, such a region will not be perfectly motionless; instead, it is like- ly to rotate, if only ever so slightly. That is why, once contraction starts, there are two significant physical effects: contraction will reduce the cloud’s overall size. But at the same time, the rotation becomes faster and faster, due to what physicists call the conservation of angular momentum – think of a fig- ure skater who pulls her arms close to her body in order to execute a pirou- ette. As the speed of rotation increases, the interplay between gravity and the centrifugal force pulls the collapsing cloud into the shape of a disk. THE BIRTH FOLLOWS A PREDEFINED CHOREOGRAPHY In the center of this protoplanetary disk, the gas reaches sufficient density and temperature for nuclear fusion to commence: a star is born. In the outer regions, gas and dust continues to swirl, with dust particles repeatedly colliding and sticking together, form- ing objects of rock-like consistency that grow continually in size. When these objects have reached a few kilo- meters in size, gravity takes over and ensures ever further growth as objects attract each other and coalesce. This is how planets, such as our own Earth, come into being. This general picture of planet for- mation has been around for a number of years. But the devil is in the details, and there is as yet no physical model that can explain planet formation from Photo: NASA/JPL-Caltech beginning to end. In their search for 4 | 11 MaxPlanckResearch 47 PHYSICS & ASTRONOMY_Protoplanetary Disks answers, astronomers have turned to The Spitzer Space Telescope, one of NASA’s to Herschel’s predecessor, the ISO satel- ever more refined observations. “The Great Observatories, was launched in lite. Incidentally, this expertise also Spitzer and Herschel space telescopes 2003 and has been trailing the Earth in gained the Heidelberg group generous have supplied us with a wealth of ex- its orbit around the Sun ever since. access to Spitzer observational data, cellent new data. It will take years to Spitzer is an infrared telescope, sensitive which is quite unusual for a European evaluate and analyze what we have ob- for radiation that is typically associated research group and a strictly American served – but when we are done, we with warm objects. Just as Spitzer ran project. “We were involved in a key should have a much clearer picture of out of the coolant it needs for proper program for the observation of proto- what happens,” says Thomas Henning, operation, in May 2009, its European planetary disks, for which we carried a Director at the Max Planck Institute successor Herschel was launched. With out practically the entire analysis of the for Astronomy in Heidelberg and head a mirror measuring 3.5 meters in diam- spectroscopic data,” says Henning. of the institute’s Planet and Star Forma- eter, Herschel is the largest space tele- tion department. scope yet, and just like Spitzer, it special- WITH INFRARED RADIATION izes in collecting and analyzing the INTO THE HEART OF DARKNESS infrared radiation emitted by objects such as planets and nebulae. When Herschel began its operations, The Max Planck Institute for As- Henning and his colleagues found an tronomy played a significant role in ideal opportunity to build on their ear- the construction of one of the three lier Spitzer observations in the frame- Herschel instruments, the far-infrared work of the Herschel program “Early camera PACS – building on a long his- phases of star formation.” The astron- tory of expertise in infrared astronomy, omers set their sights on a cloud of gas which also features key contributions and dust with the name G011.11-0.12, A portrait of two generations: Older stars in the central region of the object RCW 34, at a distance of 8,000 light-years from Earth, and younger stars (white-edged box near the top). False-color image based on infrared data taken with the Spitzer Space Telescope. Photos: NASA/ESA (top); NASA / JPL-Caltech (bottom) 48 MaxPlanckResearch 4 | 11 Heavenly glory: The Carina Nebula, at a distance of some 7,500 light-years from Earth, is one of the most beautiful star formation regions in the Milky Way. It contains at least a dozen young stars, each of which is 50 to 100 times as massive as the Sun. Thomas Henning, Director at the Max Planck Institute for Astronomy in Heidelberg, is investigating the early phases of star birth – although not with this telescope, which he uses to share the fascination of astronomy with the public and, in particular, with students. or G011 for short. Previous observa- ing G011. Spitzer had already provided tail the physical conditions at the very tions had shown that G011 contains a some images at relatively short wave- beginning of gravitational collapse,” number of newly born stars, making it lengths: at 8 micrometers, the cloud says Thomas Henning. an ideal target for studying the early still appears dark, but in observing at phases of star formation. successively longer wavelengths, the as- 20 YOUNG STARS An ideal target, that is, for observa- tronomers could peer ever more deep- IN A HOT GAS BUBBLE tions in the infrared: In visible light, ly into the cloud. The unique images clouds like this are completely opaque, obtained with Herschel at wavelengths Another interesting finding is that star showing up as solid black against a star- between 70 and 350 micrometers re- formation proceeds in a very economi- ry background. Infrared radiation, on vealed, hidden deep in the interior, 24 cal manner. Only around 10 percent of the other hand, can penetrate gas and regions of increased density, so-called the available gas mass is turned into dust. What you see of such a cloud de- pre- or protostellar cores. stars, leaving plenty of raw material for pends heavily on the wavelength. The temperatures of these cores, as the formation of subsequent genera- Following a fundamental law of derived from the infrared data, varied tions of stars. physics, all bodies emit thermal radia- between 16 and 26 Kelvin, only slight- While, in G011, a few regions have tion, and at temperatures between abso- ly warmer than their environment, at already taken the first steps toward lute zero (0 Kelvin, which corresponds 12 Kelvin. The masses of these cores collapsing under their own gravity, the to minus 273 degrees Celsius) and ranged from 1 to 240 solar masses. Ev- question remains whether such a col- about room temperature, most of this idently, within the next million or so lapse will happen spontaneously or re- radiation is emitted at infrared wave- years, the cloud will transform into a quires some external trigger. lengths. Henning and his colleagues star cluster, teeming with stars of var- As to the possible nature of such an Photo: Hardy Müller (right) made use of this principle when observ- ious masses. “We can study in great de- external trigger, consider the star forma- 4 | 11 MaxPlanckResearch 49 PHYSICS & ASTRONOMY_Protoplanetary Disks » Since interstellar gas clouds can span up to several light-years, different regions within those clouds will move at different speeds, leading to turbulent flows of gas and dust. tion region RCW 34, at a distance of Using spectroscopy, the astronomers particles. Since the disks are heated by some 8,000 light-years from Earth: a were able to determine the age of the the central object, the temperature of large, hot gas bubble containing some stars and stars-to-be in RCW 34. Sur- the disk’s inner regions is significantly 20 young stars. In fact, the stars are like- prisingly, there is a systematic trend, higher than at the outer rim. By the ly responsible for the existence of the with objects inside the bubble being a fundamental laws of thermal radiation, bubble, blowing away the surrounding few million years older than those at this means that infrared radiation from gas with their intense radiation and a the edge. The researchers hypothesize the interior region will be emitted steady stream of particles known as that stars of the first generation in this mostly at shorter wavelengths than is “stellar wind”. Near the top of the bub- nebula pushed matter outward, trigger- the case for the outer zones. In cases ble is a cloud of gas and dust in which ing the formation of a second genera- where emission at shorter infrared additional stars are being born. tion of stars near the edge of the cloud. wavelengths was missing, astronomers concluded that the disks in question PROTOSTELLAR DONUTS must feature a large central hole.
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