The Turbulent Birth of Stars and Planets
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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
T
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-
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.
Photo: NASA/JPL-Caltech
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
beginning to end. In their search for
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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
Photos: NASA/ESA (top); NASA / JPL-Caltech (bottom)
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.
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
Photo: Hardy Müller (right)
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
made use of this principle when observ- ious masses. “We can study in great de- external trigger, consider the star forma-
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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.
Direct evidence for this model was,
Are such external influences – candi- however, lacking – at least until last
dates include not only young stars, but year, when an international research
also expanding clouds of matter result- group headed by Christian Thalmann
ing from so-called supernova explo- and Johan Olofsson from the Max
sions, in which more massive stars end Planck Institute for Astronomy finally
their lives – necessary in order for new succeeded in producing a direct image
stars to form? Thomas Henning is con- of such a disk. Thalmann and his col-
vinced that external triggers play a sup- leagues utilized a special camera on the
porting role at best: “Turbulence in the Japanese Subaru telescope in Hawaii to
clouds is perfectly sufficient to cause observe the young star LkCa 15. The
10,000 years affected regions to contract and achieve image showed a disk with a large cen-
critical density.” But how does this tur- tral gap. In fact, the gap is so large that
bulence come about? it could quite comfortably accommo-
Stars and nebula in our cosmic date the orbits of all the planets in our
neighborhood orbit the center of our own solar system.
home galaxy, the Milky Way, with ob- A second set of observations with
jects closer to the center moving faster Subaru, again involving the Heidelberg
than their more distant cousins. Since astronomers, led to the discovery of a
interstellar gas clouds can span up to smaller type of disk. The disk around
several light-years, different regions the star AB Aurigae, at a distance of 470
within those clouds will move at differ- light-years from Earth, is about the size
ent speeds, leading to turbulent flows of our solar system. It consists of nest-
of gas and dust. ed rings that are arranged somewhat
100,000 years In fewer than one million years, the asymmetrically around the star.
protostellar cores of G011 will have con- Another discovery, again by Max
densed sufficiently for protostellar disks Planck researcher Olofsson and col-
to form around nascent stars. These disks leagues, involves the young star T
are the birthplaces of planets. Many oth- Chamaeleontis (T Cha), at a distance
er young stars in the galaxy have already of 330 light-years from Earth. Using
1 million years reached this stage, and have been re- the Very Large Telescope of the Europe-
warding targets for observation. an Southern Observatory (ESO) in
In many cases, the existence of Chile, the astronomers found that a
these disks can be deduced only indi- portion of the disk material has formed
rectly from characteristic infrared radi- a thin dust ring at a distance of just 20
10 million years
ation emitted by their constituent dust million kilometers from the star –
Graphic: Subaru
Stages of stellar evolution: First, a nearly spherical cloud collapses. Since it rotates,
centrifugal forces will pull it apart, resulting in a disk. In the center, matter collapses
100 million years further to form a star, while the disk becomes the birthplace of planets.
50 MaxPlanckResearch 4 | 11
Insights: Using the Japanese Subaru telescope in Hawaii, Heidelberg-based Max Planck astronomers and their colleagues have
managed to capture astounding images of protoplanetary disks, such as the ring-shaped disk around LkCa 15 (above) and the smaller disk
around the star AB Aurigae with its much more complicated structure.
about one-third the orbital radius of “That, of course, is the most interest- carbon dioxide in their atmospheres.
Mercury, the planet closest to our Sun. ing possibility,” says Thomas Henning. But they have not yet succeeded in pin-
Behind this ring extends a wide, dust- Early this year, an international ning down the evolutionary link – a
free region, up to about 1.1 billion ki- team actually found evidence for this young planet inside the disk that
lometers from the star. That is where latter scenario. Near the outer edge of served as its birthplace.
the outer part of the disk begins. the large gap in the disk around T Until recently, the astronomers had
Chamaeleontis, they found what could observed only the dust component of
CLOSE TO THE be an orbiting body. It is located about protoplanetary disks. But this accounts
LIMITS OF OBSERVATION a billion kilometers away from its cen- for only about a hundredth of the to-
tral star, a distance comparable to that tal mass. Most of the disk mass is in
Even before these direct sightings of between Jupiter and the Sun. But these the form of gas, which is very difficult
donut-shaped disks, astronomers had observations are at the limits of what is to observe, since the radiation emitted
discussed the possible causes for the technically feasible for the astronomers. by the atoms and molecules is so ex-
formation of such gaps. One possibili- They were thus unable to determine be- traordinarily weak.
ty involves the central star’s radiation yond all doubt exactly what this object Yet the gas plays a major role in the
causing the dust particles to evaporate: is. It may be a newly formed planet. development of the overall disk and in
Alternatively, a nearby (and as yet un- That planets grow in protoplane- the formation of planets. For instance, it
discovered) star could have swept up tary disks is, by now, an undisputed exerts friction on the tiny dust particles,
the dust. The most likely option, how- fact. In the case of some older stars, sci- slowing them down at different rates de-
Photos: Subaru (3)
ever, is that a planet has formed entists have managed to observe a few pending on particle size. The resulting
around the star, sweeping up gas and exoplanets directly. In a few cases, it different speeds of the particles increase
dust and, in the end, incorporating was even possible to detect substances the likelihood of collisions, thus accel-
whatever material it has attracted. such as water, sodium, methane and erating the growth of the particles. >
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PHYSICS & ASTRONOMY_Protoplanetary Disks
in, at most, 10 million years. “That puts Detecting the more complex building
very strict limits on the timescale of blocks of life lies beyond the capabilities
planet formation,” explains Henning. of today’s telescopes. However, using
And, as all these statements should ap- some sensitive radio telescopes, as well
ply equally to the formation of our own as Spitzer and Herschel, the researchers
solar system, it imposes strict limits on at the Max Planck Institute for Astrono-
the timescale of our own planetary or- my have at least been able to take some
igins, as well. baby steps toward that goal: they man-
aged to observe simple molecules such
150 TYPES OF MOLECULES IN as carbon monoxide and water in pro-
INTERSTELLAR CLOUDS toplanetary disks. Now they are putting
their hope in the next generation of tele-
Last but not least, there is an aspect of scopes, notably the Atacama Large Mil-
the analysis of the gaseous compo- limeter Array (ALMA). ALMA is a com-
nents of disks that has a direct bearing pound telescope consisting of nearly 70
on questions about the origins of life separate radio telescopes that can be
on Earth and elsewhere. In large inter- moved into different positions. ALMA is
stellar clouds, astronomers have thus currently under construction in Chile
far found around 150 types of mole- and will gradually be put into operation
cules, including complex organic sub- in the coming years.
stances. Not so in the protoplanetary Hubble’s successor, the James Webb
disks, where, so far, no such chemical Space Telescope, promises another
compounds have been observed. This shift in cosmic perspective. The Hei-
leaves highly interesting questions un- delberg-based Max Planck researchers
answered: Can the complex precursor are involved in this project, too, de-
molecules of life survive the early phas- veloping key components of the MIRI
es of planet formation? Are they pres- instrument, which will work in the
ent in the circumstellar disks, and can medium infrared range – and is ex-
they possibly make it to the surfaces of pected to be good for more than one
At the Max Planck Institute in Heidelberg, the young planets unharmed? observational surprise.
astronomers and engineers are also involved
in developing and constructing new instru-
ments, including adaptive optics systems
that can mitigate the effects of atmospheric
turbulence, resulting in ground-based images GLOSSARY
of unparalleled sharpness. Adaptive optics
plays a key role in the search for extrasolar Galaxy Protostellar cores
planets and disks. The collection of billions and billions of stars Stars form inside interstellar clouds of
to which our own planetary system belongs. gas and dust that collapse under their
The Milky Way Galaxy (from the Greek word own gravity. Ultimately, a concentration
gala for milk) consists of 150 to 200 billion of mass known as a protostellar core de-
Furthermore, gas is the key ingredient velops, which attracts additional matter
suns, as well as interstellar gas and dust
of giant planets, such as Jupiter and Sat- clouds. It features several spiral arms and, if and contracts ever further, transforming
urn, which gather significant amounts viewed from the top, would look like a disk gravitational energy into heat. The result
of gas when forming their enormous with a diameter of about 100,000 light- is a protostar that emits infrared radia-
years. Viewed from the side, the disk is only tion. Around such a protostar, matter
atmospheres. For these reasons, as-
about 1,000 light-years thick. At the heart of will typically contract to form what is
tronomers are very interested in learn- the Milky Way sits a gigantic black hole. The known as a protoplanetary disk: the
ing the typical lifetimes of such gas- central bulge is surrounded by a spherical re- birthplace of future planets.
eous disks. The result would impose a gion (the so-called halo) that contains 150
James Webb Space Telescope
strong constraint on the timescale of globular clusters.
The James Webb Space Telescope (JWST) is
planet formation. Very Large Telescope (VLT) the intended successor to the Hubble
The astronomers in Heidelberg dug An array of four telescopes, each with a main Space Telescope. With its 6.5-meter-mir-
mirror measuring 8 meters in diameter, com- ror, it is set to peer much deeper into
deep into their bag of observational
plemented by several auxiliary telescopes. space than its predecessor, receiving in-
tricks to address this question. In sever- The VLT is designed for observations in visi- frared light from planetary systems-in-
al star clusters with members aged be- ble light up to the mid-infrared. Light collect- the-making, as well as the most distant
tween about 1 million and 20 million ed by these telescopes can be combined, in galaxies. However, due to financial rea-
years, they determined the dust and gas effect creating a single virtual telescope with sons, the project was recently put on
ultra-high resolution – the Very Large Tele- hold by the American space agency,
Photo: Hardy Müller
quantities in the disks. What they scope Interferometer (VLTI). The observatory is NASA. It is questionable whether the
found was surprising: the amounts of situated atop the 2,635-meter-high Cerro Pa- JWST can start as planned in 2018.
gas and dust decreased almost exactly ranal in the Chilean Andes, and is operated
in parallel, and rather quickly at that: by the European Southern Observatory (ESO).
disk matter largely disappeared with-
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