Look up at the night sky from anywhere on Earth and you'll see stars. Because these objects are so widespread, you'd think astronomers would know how they form. And, for the most part, researchers do.
A gas cloud collapses into something much smaller and hotter--a star. But questions arise when delving into the details of the formation process. Nearby massive stars, other sun-forming clouds, and even the Milky Way's spiral structure affect how stars form. And more problems arise when astronomers analyze the formation of groups of suns.
To help connect the dots, some scientists are simulating star formation using sophisticated computer models, while others are incorporating more observations in different wavelengths and piecing together three-dimensional images of what they view in the sky. Together, they are mounting an attack to figure out exactly how these familiar objects form.
It starts with a cloud
Clouds of gas and dust, called the interstellar medium (ISM), fill our galaxy. One of many processes triggers at least one slight overdensity in a giant molecular cloud, beginning the star formation process. "The overdensities in molecular clouds that we identify as 'cores' are almost certainly produced by the turbulent motions present in the clouds," says Mark Krumholz of the University of California, Santa Cruz.
If many hot, bright stars are close together, their intense radiation could create turbulence in the ISM. Close approaches of galaxies and interstellar clouds can also create instabilities. When a supernova explodes, it causes a shock front, a region of compressed gas directly ahead of the expanding shell. Spiral density waves sweep around the Milky Way, compressing material as they pass--a phenomenon that creates our galaxy's spiral structure. In fact, as astronomers observe other spiral galaxies, they see regions of young stars along these galaxies' arms. This compression can thus initiate star formation.
After the overdense seeds have been planted, the cloud of interstellar gas becomes unstable and fragments. Almost all cloud clumps that eventually become main sequence stars--those that fuse hydrogen to helium in their centers--do so within millions of years.
The rich get richer
Gravity pulls material into the denser region, so the cloud gets more massive and begins to contract. It spins faster due to conservation of angular momentum, the same way a figure skater's spins speed up as she brings her arms closer in.
The core's temperature increases as more material "settles" inward. Any region of hot gas will have some ions--particles that have lost or gained electrons and therefore have a positive or negative electric charge. Charged particles can move only in certain directions in a magnetized region. Hence, the galaxy's magnetic field combined with the gas cloud's ions cause the clump's rotation to slow but not stop--otherwise we wouldn't have stars.
Next, a cloud that will eventually become Sun-sized shrinks to a much smaller region--about the size of the solar system--after tens of thousands of years. The central portion's temperature is now about 10,000 kelvins. Astronomers call it a protostar.
Baby stars
A protostar is more luminous than the star it will become because it is much larger (some hundreds of times) and thus has a greater surface area from which to radiate energy. Although it's enrobed in dust, the baby star's bright luminosity still lets astronomers see it. The protostar continues to gravitationally attract more material, contract, and heat up internally. As the nascent star's outer surface shrinks, it becomes less luminous.
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The cloud and its central protostar spin faster, causing it to flatten into a "circumstellar disk." In some cases, the protostar produces a strong wind, which channels material out in the form of jets perpendicular to the disk. Magnetic fields also contribute to these jets, but researchers don't know exactly how. The radiation beams carry some mass away from the star and disk in a relatively short timescale (a few thousand years). The protostar also accretes much of the material that isn't blown off, leaving behind a residual disk likely to form planets.
The protostar's interior is now hot enough--about 1 million kelvins--to begin fusing deuterium to helium. Deuterium--also called heavy hydrogen because its nucleus contains one proton and one neutron, compared to hydrogen's one-proton nucleus--is the easiest nucleus to fuse. Hydrogen nuclei need higher temperatures to combine.
The protostar continues contracting, therefore increasing the density in its center. After its core reaches 10 million kelvins, it can fuse hydrogen nuclei into helium. It has then reached equilibrium, where the radiative energy from fusion perfectly balances the gravitational pull of its mass. It is now a full-fledged star.
Simulating a star
The star formation process includes many steps over millions of years. So how do outside factors affect the process? That's where computational modeling comes in.
Because scientists can't watch a star's entire formation, they simulate the process with supercomputers. The biggest question they hope these models will answer is why the mass distribution of newly formed stars varies little between sites of star formation--it's universal. But astronomers "do not understand where the average mass comes from, or what sets the distribution of masses around that mean," explains Krumholz.
Star formation models take into account the effects of thermodynamics, magnetic fields, radiative processes, and, of course, gravity. Recent simulations are also adding in "feedback" because stars affect their own environment. "This influence takes many forms: Young stars heat up the gas around them via their radiation, they push the gas around through their winds and jets, and massive stars can ionize the gas around them, causing it to blow away," says Krumholz.
In computer models, there is one key question. "Can you reproduce the properties of specific areas of star formation?" asks Alyssa Goodman of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. "Under certain physical conditions, if you have a big molecular cloud, what's the most massive star that's going to form there? ... How clustered are the stars going to be?" The solutions depend on chemical composition, magnetic fields, ionization, a cloud's age, and other factors.
Peering into the nursery
Astronomers use a range of wavelengths to observe star formation. The early stages (for example, as the interstellar cloud begins to contract) are observable in radio. The cores are too cold to emit visible and infrared radiation. (Higher energy, and shorter wavelength, corresponds to higher temperature; lower energy, and longer wavelength, to lower temperature.)
When looking for protostars, astronomers focus on infrared radiation. Stars emit in visible light, but we can't see that radiation because material surrounds the nascent star. Dust absorbs the light and radiates it in the infrared. So infrared satellites see the dust immediately encompassing the protostar.
The Spitzer Space Telescope, which looks for infrared radiation, has uncovered hundreds of protostars in large clouds. Now astronomers want to pierce even deeper and see the stages that come before protostars. But the gas is colder and therefore radiates at low energy, so the regions they're looking for are dim. They use slightly different tools, ones that detect "submillimeter" waves--far infrared into microwave.
Goodman's former graduate student Jaime Pineda and colleagues have succeeded in finding the best example yet of a core--the stage earlier than a protostar. The team detected a compact source embedded in cold gas that isn't visible in any wavelength more energetic than submillimeter.
Goodman's team is also at the forefront of reconstructing images of star-forming regions from observations. The team collects sky position data (which is 2-D from our view) and velocities for an entire molecular cloud. Then it pieces this information together into a 3-D model, which the researchers can analyze. Using this method, the team has discovered unexpected structures within clouds and even previously unseen star-forming regions.
Other tripping points
From observations, astronomers know that stars tend to form most often in multiples. They think star clusters--which contain hundreds to thousands of stars--start out as a large cloud and fragment into many smaller clouds. But this creates a problem when divvying out the gas. Do individual cores steal material from different areas, or does each accrete gas only from its nearby vicinity? These two theories are called competitive accretion and gravitational collapse, respectively. As often is the case, scientists are split regarding which theory is right. And others say each of these processes may happen occasionally. Figuring out if one method beats the other is a hotly debated topic.
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High-mass stars are also poorly understood, especially their formation. "Do these very massive stars form in a process that is simply a scaled-up version of the process by which low-mass stars form, or do they require some fundamentally different process to form?" adds Krumholz. Astronomers expect that these high-mass stars would blow away surrounding material, halting star formation.
Astronomers also aren't sure what the upper limit to the biggest stars' masses are, and even if there is one. For years, scientists believed stellar masses would max out at about 150 times the mass of the Sun. But this summer, a team of astronomers announced that it had found evidence of a mammoth exceeding the 150-solar-mass limit.
Scientists are also concerned that the galaxy's star formation rate seems awfully low. The rate they're finding is between 10 and 100 times lower than expected considering the amount of material available in giant molecular clouds. Are some star-forming regions more or less efficient? Astronomers aren't sure. But with new observational techniques, and constant upgrades to their simulations, they're on their way to answering this and other questions of star formation.
How to make a star
1 A shock wave moves through a giant molecular cloud and causes slight overdensities in the gas--these are "cores.
2 Each slightly dense core attracts more material (a result of gravity) and begins to contract.
3 The core gains more mass, becomes brighter, and spins faster. The surrounding material flattens into a circumstellar disk.
4 Jets perpendicular to the disk channel mass and radiation away from the baby star, thus helping to clear out the area.
5 Nuclear fusion kicks in as the star's primary energy source. A thin residual dust disk surrounds the full fledged star; this material will eventually form planets.
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Liz Kruesi is an associate editor of Astronomy. Her June 2010 article "What makes stars tick?" explained what goes on inside stars.
Source Citation
Kruesi, Liz. "How stars form: stars begin as clouds of cold gas that transform into blazing hot fireballs. Here's what scientists know about the process--and what they're missing." Astronomy Dec. 2010: 26. General OneFile. Web. 13 Nov. 2010.
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