If there is anything you think astronomers would have figured out by now, it is how stars form. The basic idea for how stars form goes back to Immanuel Kant and Pierre-Simon Laplace in the 18th century, and the details of how they shine and evolve were worked out by physicists in the first half of the 20th century. Today the principles that govern stars are taught in middle school, and exotica such as dark matter dominate the headlines. It might seem that star formation is a problem that has been solved. But nothing could be further from the truth. The birth of stars remains one of the most vibrant topics in astrophysics today. In the simplest terms, the process represents the victory of gravity over pressure. It starts with a vast cloud of gas and dust floating in interstellar space. If the cloud—or, more often, a dense part of such a cloud called a core—is cool and dense enough, the inward pull of its gravity overpowers the outward push of gaseous pressure, and it begins to collapse under its own weight. The cloud or core becomes ever denser and hotter, eventually sparking nuclear fusion. The heat generated by fusion increases the internal pressure and halts the collapse. The newborn star settles into a dynamic equilibrium that can last millions to trillions of years. The theory is self-consistent and matches a growing body of observations. Yet it is far from complete. Every sentence of the above paragraph cries out for explanation. Four questions, in particular, trouble astronomers. First, if the dense cores are the eggs of stars, where are the cosmic chickens? The clouds must themselves come from somewhere, and their formation is not well understood. Second, what causes the core to begin collapsing? Whatever the initiation mechanism is, it determines the rate of star formation and the final masses of stars. Third, how do embryonic stars affect one another? The standard theory describes individual stars in isolation; it does not say what happens when they form in close proximity, as most stars do. Recent findings suggest that our own sun was born in a cluster, which has since dispersed [see “The Long-Lost Siblings of the Sun,” by Simon F. Portegies Zwart; Scientific American, November 2009]. How does growing up in a crowded nursery differ from being an only child? Fourth, how do very massive stars manage to form at all? The standard theory works well for building up stars of as much as 20 times the mass of the sun but breaks down for bigger ones, whose tremendous luminosity should blow away the cloud before the nascent star can accumulate the requisite mass. What is more, massive stars blast their surroundings with ultraviolet radiation, high-velocity outflows and supersonic shock waves. This energy feedback disrupts the cloud, yet the standard theory does not take it into account. The need to address these shortcomings has become increasingly pressing. Star formation underlies almost everything else in astronomy, from the rise of galaxies to the genesis of planets. Without understanding it, astronomers cannot hope to dissect distant galaxies or make sense of the planets being discovered beyond our solar system. Although final answers remain elusive, a common theme is emerging: a more sophisticated theory of star formation must consider the environment of a fledgling star. The final state of the new star depends not only on initial conditions in the core but also on the subsequent influences of its surroundings and its stellar neighbors. It is nature versus nurture on a cosmic scale. Swaddled in Dust If you look at the sky from a dark site, far from city lights, you can see the Milky Way arching over you, its diffuse stream of light interrupted by dark patches. These are interstellar clouds. The dust particles in them block starlight and make them opaque to visible light. Consequently, those of us who seek to observe star formation face a fundamental problem: stars cloak their own birth. The material that goes into creating a star is thick and dark; it needs to become dense enough to initiate nuclear fusion but has not done so yet. Astronomers can see how this process begins and how it ends, but what comes in the middle is inherently hard to observe, because much of the radiation comes out at far-infrared and submillimeter wavelengths where the astronomer’s toolbox is relatively primitive compared with other parts of the spectrum. Astronomers think that stars’ natal clouds arise as a part of the grand cycle of the interstellar medium, in which gas and dust circulate from clouds to stars and back again. The medium consists primarily of hydrogen; helium makes up about one quarter by mass, and all the other elements amount to a few percent. Some of this material is primordial matter barely disturbed since the first three minutes of the big bang; some is cast off by stars during their lifetimes; and some is the debris of exploded stars. Stellar radiation breaks any molecules of hydrogen into their constituent atoms [see “The Gas between the Stars,” by Ronald J. Reynolds; Scientific American, January 2002]. Initially the gas is diffuse, with about one hydrogen atom per cubic centimeter, but as it cools it coagulates into discrete clouds, much as water vapor condenses into clouds in Earth’s atmosphere. The gas cools by radiating heat, but the process is not straightforward, because there are only a limited number of ways for the heat to escape. The most efficient turns out to be far-infrared emission from certain chemical elements, such as the radiation emitted by ionized carbon at a wavelength of 158 microns. Earth’s lower atmosphere is opaque at these wavelengths, so they must be observed using space-based observatories such as Herschel Space Observatory, launched last year by the European Space Agency, or telescopes mounted in airplanes, such as the Stratospheric Observatory for Infrared Astronomy (SOFIA). As the clouds cool, they become denser. When they reach about 1,000 atoms per cubic centimeter, they are thick enough to block ultraviolet radiation from the surrounding galaxy. Hydrogen atoms can then combine into molecules through a complicated process involving dust grains. Radio observations have shown that molecular clouds contain compounds ranging from hydrogen (H2) up to complex organics, which may have provided the wherewithal for life on Earth [see “Life’s Far-Flung Raw Materials,” by Max P. Bernstein, Scott A. Sandford and Louis J. Allamandola; Scientific American, July 1999]. Beyond this stage, however, the trail goes cold. Infrared observations have revealed nascent stars deeply embedded in dust but have trouble seeing the earliest steps leading from molecular cloud to these protostars. The situation for the very earliest stages of star formation began to change in the mid-1990s, when the Midcourse Space Experiment and the Infrared Space Observatory discovered clouds so dense (more than 10,000 atoms per cubic centimeter) that they are opaque even to the thermal infrared wavelengths that usually penetrate dusty regions. These so-called infrared dark clouds are much more massive (100 to 100,000 times the mass of the sun) than clouds that had been previously discovered at optical wavelengths. Over the past several years two teams have used the Spitzer Space Telescope to make a comprehensive survey of them: the Galactic Legacy Infrared Midplane Survey Extraordinaire (GLIMPSE) led by Edward B. Churchwell of the University of Wisconsin–Madison and the MIPSGAL survey led by Sean Carey of the Spitzer Science Center. These clouds appear to be the missing link between molecular clouds and protostars. In fact, dark clouds and dense cores could represent the crucial formative stage of stars when their masses are determined. The clouds come in a range of masses; small ones are more common than large ones. This distribution of masses closely mimics that of stars—except that the clouds are systematically three times more massive than stars, suggesting that only one third of the mass of a cloud ends up in the newborn star. The rest is somehow lost to space. Whether this similarity in distributions is causal or just coincidental remains to be proved. Whatever sets the mass of a star determines its entire life history: whether it is a massive star that dies young and explodes catastrophically or a more modest star that lives longer and goes more gently into that good night. What Pulled the Trigger? Astronomers are also making some progress on the second major unresolved problem, which is what causes a cloud or core to collapse. In the standard model of star formation, a core begins in beautiful equilibrium, with gravity and external pressure balanced by internal thermal, magnetic or turbulent pressure. Collapse begins when this balance is upset in favor of gravity. But what triggers the imbalance? Astronomers have proposed many different ways. An outside force such as a supernova explosion might compress the cloud, or the internal pressure might ebb as heat or magnetic fields dissipate. Charles Lada of the Harvard-Smithsonian Center for Astrophysics (CfA), João Alves of the European Southern Observatory (ESO) and their co-workers have argued for the slow dissipation of thermal support. By mapping molecular clouds at millimeter and submillimeter wavelengths, which straddle the radio and infrared bands, they have been able to identify a large number of relatively quiescent, isolated cores in nearby clouds. Some show evidence of slow inward motions and may be on their way to making stars. An excellent example is Barnard 335, located in the constellation Aquila. Its density structure is just what would be expected if the cloud’s thermal pressure were nearly in equilibrium with external pressure. An infrared source in the center may be an early-stage protostar, suggesting that the balance recently tilted in favor of collapse. Other studies find evidence for external triggering. Thomas Preibisch of the Max Planck Institute for Radio Astronomy in Bonn and his collaborators have showed that widely distributed stars in the Upper Scorpius region all formed nearly in unison. It would be quite a coincidence for the internal pressure of different cores to dissipate at the same time. A likelier explanation is that a shock wave set off by a supernova swept through the region and induced the cores to collapse. The evidence is ambiguous, however, because massive stars disrupt their birthplaces, making it difficult to reconstruct the conditions under which they formed. Another limitation has been the difficulty of seeing lower-mass stars (which are dimmer) to confirm that they, too, formed in synchrony. Spitzer has made progress on these questions. Lori Allen of the National Optical Astronomy Observatory, Xavier P. Koenig of the CfA and their collaborators have discovered a striking example of external triggering in a region of the galaxy known as W5. Their image shows young protostars embedded in dense pockets of gas that have been compressed by radiation from an earlier generation of stars. Because compression is a rapid process, these widely scattered objects must have formed almost simultaneously. In short, the triggering of star formation is not an either-or situation, as once thought. It is case of “all of the above.” Life in a Stellar Nursery Leaving aside the above deficiencies, the standard model explains observations of isolated star-forming cores fairly well. But many, perhaps most, stars form in clusters, and the model does not account for how this congested environment affects their birth. In recent years researchers have developed two competing theories to fill in this gap. The great advance in the computing power available for simulations has been crucial in honing these theories. Observations, notably by Spitzer, are helping astronomers to decide between them. In one, interactions between adjacent cores become important. In the extreme version, many very small protostars form, move rapidly through the cloud and compete to accrete the remaining gas. Some grow much bigger than others, and the losers may be ejected from the cluster altogether, creating a class of stellar runts that roam the galaxy. This picture, called competitive accretion, has been championed by Ian Bonnell of the University of St. Andrews, Matthew Bate of the University of Exeter, and others. In the alternative model, the main external influence is not interactions among cores but turbulence within the gas. The turbulence helps to trigger collapse, and the size distribution of stars reflects the spectrum of turbulent motions rather than a later competition for material. This turbulent-core model has been developed by Christopher McKee of the University of California, Berkeley, Mark Krumholz of the University of California, Santa Cruz, and others. Observations seem to favor the turbulent-core model [see “The Mystery of Brown Dwarf Origins,” by Subhanjoy Mohanty and Ray Jayawardhana; Scientific American, January 2006], but the competitive-accretion model may be important in regions of particularly high stellar density. One very interesting case is the famous Christmas Tree Cluster (NGC 2264) in the constellation Monoceros. In visible light, this region shows a number of bright stars and an abundance of dust and gas—hallmarks of star formation. Spitzer observations have revealed a dense embedded cluster with stars in various stages of development. This cluster provides a snapshot of precisely those stages when either turbulence or competitive accretion would leave its mark. The youngest stars, identified as those with the largest proportion of emission at long wavelengths, are clumped in a tight group. Paula S. Teixeira, now at ESO, and her collaborators have shown that they are spaced roughly every 0.3 light-year. This regular pattern is just what would be expected if dense cores were gravitationally collapsing out of the general molecular cloud, suggesting that the initial conditions in the cloud are what determine the road to collapse. And yet, even though the observations support the turbulent model, the images have good enough resolution to tell that some of the supposed protostars are not single objects but compact groups of objects. One consists of 10 sources within a 0.1-light-year radius. These objects have such a high density that competitive accretion must be taking place, at least on a small scale. Therefore, as with triggering mechanisms, the effect of the stellar environment is not an either-or choice. Both turbulence and competitive accretion can operate, depending on the situation. Nature seems to take advantage of every possible way to make a star. Supersize This Star Massive stars are rare and short-lived, but they play a very important role in the evolution of galaxies. They inject energy into the interstellar medium via both radiation and mass outflows and, at the end of their lives, can explode as supernovae, returning matter enriched in heavy elements. The Milky Way is riddled with bubbles and supernova remnants created by such stars. Yet the standard theory has trouble explaining their formation. Once a protostar reaches a threshold of about 20 solar masses, the pressure exerted by its radiation should overpower gravity and prevent it from growing any bigger. In addition to the radiation pressure, the winds that so massive a star generates disperse its natal cloud, further limiting its growth as well as interfering with the formation of nearby stars. Recent theoretical work by Krumholz and his collaborators offers one way out of this problem. Their three-dimensional simulations show stellar growth in all its unexpected intricacy. The inflow of material can become quite nonuniform; dense regions alternate with bubbles where the starlight streams out. Therefore, the radiation pressure may not pose an obstacle to continued growth after all. The dense infalling material also readily forms companion stars, explaining why massive stars are seldom alone. Observers are now looking for confirmation using Spitzer surveys of massive star-forming regions. But verifying the model will be tricky. The rarity and short lives of these stars make them hard to catch in the act of forming. Fortunately, new facilities will soon help with this and the other questions posed by star formation. Herschel and SOFIA, a Boeing 747 that flies above 99 percent of the obscuring water vapor of Earth’s atmosphere, will observe the far-infrared and submillimeter wavelengths where star formation is easiest to see. They have the spatial and spectral resolution needed to map the velocity pattern in interstellar clouds. At longer wavelengths, the Atacama Large Millimeter Array (ALMA), now under construction in the Chilean Andes, will allow mapping of individual protostars in exquisite detail. With new observations, astronomers hope to trace the complete life cycle of the interstellar medium from atomic clouds to molecular clouds to prestellar cores to stars and ultimately back into diffuse gas. They also hope to observe star-forming disks with enough angular resolution to be able to trace the infall of material from the cloud, as well to compare the effects of different environments on stellar birth. The answers will ripple out into other domains of astrophysics. Everything we see—galaxies, interstellar clouds, stars, planets, people—has been made possible by star formation. Our current theory of star formation is not a bad one, but its gaps leave us unable to explain many of the most important aspects of today’s universe. And in those gaps we see that star formation is a richer process than anyone ever predicted. Note: This story was originally printed with the title “Cloudy with a Chance of Stars”
In the simplest terms, the process represents the victory of gravity over pressure. It starts with a vast cloud of gas and dust floating in interstellar space. If the cloud—or, more often, a dense part of such a cloud called a core—is cool and dense enough, the inward pull of its gravity overpowers the outward push of gaseous pressure, and it begins to collapse under its own weight. The cloud or core becomes ever denser and hotter, eventually sparking nuclear fusion. The heat generated by fusion increases the internal pressure and halts the collapse. The newborn star settles into a dynamic equilibrium that can last millions to trillions of years.
The theory is self-consistent and matches a growing body of observations. Yet it is far from complete. Every sentence of the above paragraph cries out for explanation. Four questions, in particular, trouble astronomers. First, if the dense cores are the eggs of stars, where are the cosmic chickens? The clouds must themselves come from somewhere, and their formation is not well understood. Second, what causes the core to begin collapsing? Whatever the initiation mechanism is, it determines the rate of star formation and the final masses of stars.
Third, how do embryonic stars affect one another? The standard theory describes individual stars in isolation; it does not say what happens when they form in close proximity, as most stars do. Recent findings suggest that our own sun was born in a cluster, which has since dispersed [see “The Long-Lost Siblings of the Sun,” by Simon F. Portegies Zwart; Scientific American, November 2009]. How does growing up in a crowded nursery differ from being an only child?
Fourth, how do very massive stars manage to form at all? The standard theory works well for building up stars of as much as 20 times the mass of the sun but breaks down for bigger ones, whose tremendous luminosity should blow away the cloud before the nascent star can accumulate the requisite mass. What is more, massive stars blast their surroundings with ultraviolet radiation, high-velocity outflows and supersonic shock waves. This energy feedback disrupts the cloud, yet the standard theory does not take it into account.
The need to address these shortcomings has become increasingly pressing. Star formation underlies almost everything else in astronomy, from the rise of galaxies to the genesis of planets. Without understanding it, astronomers cannot hope to dissect distant galaxies or make sense of the planets being discovered beyond our solar system. Although final answers remain elusive, a common theme is emerging: a more sophisticated theory of star formation must consider the environment of a fledgling star. The final state of the new star depends not only on initial conditions in the core but also on the subsequent influences of its surroundings and its stellar neighbors. It is nature versus nurture on a cosmic scale.
Swaddled in Dust If you look at the sky from a dark site, far from city lights, you can see the Milky Way arching over you, its diffuse stream of light interrupted by dark patches. These are interstellar clouds. The dust particles in them block starlight and make them opaque to visible light.
Consequently, those of us who seek to observe star formation face a fundamental problem: stars cloak their own birth. The material that goes into creating a star is thick and dark; it needs to become dense enough to initiate nuclear fusion but has not done so yet. Astronomers can see how this process begins and how it ends, but what comes in the middle is inherently hard to observe, because much of the radiation comes out at far-infrared and submillimeter wavelengths where the astronomer’s toolbox is relatively primitive compared with other parts of the spectrum.
Astronomers think that stars’ natal clouds arise as a part of the grand cycle of the interstellar medium, in which gas and dust circulate from clouds to stars and back again. The medium consists primarily of hydrogen; helium makes up about one quarter by mass, and all the other elements amount to a few percent. Some of this material is primordial matter barely disturbed since the first three minutes of the big bang; some is cast off by stars during their lifetimes; and some is the debris of exploded stars. Stellar radiation breaks any molecules of hydrogen into their constituent atoms [see “The Gas between the Stars,” by Ronald J. Reynolds; Scientific American, January 2002].
Initially the gas is diffuse, with about one hydrogen atom per cubic centimeter, but as it cools it coagulates into discrete clouds, much as water vapor condenses into clouds in Earth’s atmosphere. The gas cools by radiating heat, but the process is not straightforward, because there are only a limited number of ways for the heat to escape. The most efficient turns out to be far-infrared emission from certain chemical elements, such as the radiation emitted by ionized carbon at a wavelength of 158 microns. Earth’s lower atmosphere is opaque at these wavelengths, so they must be observed using space-based observatories such as Herschel Space Observatory, launched last year by the European Space Agency, or telescopes mounted in airplanes, such as the Stratospheric Observatory for Infrared Astronomy (SOFIA).
As the clouds cool, they become denser. When they reach about 1,000 atoms per cubic centimeter, they are thick enough to block ultraviolet radiation from the surrounding galaxy. Hydrogen atoms can then combine into molecules through a complicated process involving dust grains. Radio observations have shown that molecular clouds contain compounds ranging from hydrogen (H2) up to complex organics, which may have provided the wherewithal for life on Earth [see “Life’s Far-Flung Raw Materials,” by Max P. Bernstein, Scott A. Sandford and Louis J. Allamandola; Scientific American, July 1999]. Beyond this stage, however, the trail goes cold. Infrared observations have revealed nascent stars deeply embedded in dust but have trouble seeing the earliest steps leading from molecular cloud to these protostars.
The situation for the very earliest stages of star formation began to change in the mid-1990s, when the Midcourse Space Experiment and the Infrared Space Observatory discovered clouds so dense (more than 10,000 atoms per cubic centimeter) that they are opaque even to the thermal infrared wavelengths that usually penetrate dusty regions. These so-called infrared dark clouds are much more massive (100 to 100,000 times the mass of the sun) than clouds that had been previously discovered at optical wavelengths. Over the past several years two teams have used the Spitzer Space Telescope to make a comprehensive survey of them: the Galactic Legacy Infrared Midplane Survey Extraordinaire (GLIMPSE) led by Edward B. Churchwell of the University of Wisconsin–Madison and the MIPSGAL survey led by Sean Carey of the Spitzer Science Center. These clouds appear to be the missing link between molecular clouds and protostars.
In fact, dark clouds and dense cores could represent the crucial formative stage of stars when their masses are determined. The clouds come in a range of masses; small ones are more common than large ones. This distribution of masses closely mimics that of stars—except that the clouds are systematically three times more massive than stars, suggesting that only one third of the mass of a cloud ends up in the newborn star. The rest is somehow lost to space.
Whether this similarity in distributions is causal or just coincidental remains to be proved. Whatever sets the mass of a star determines its entire life history: whether it is a massive star that dies young and explodes catastrophically or a more modest star that lives longer and goes more gently into that good night.
What Pulled the Trigger? Astronomers are also making some progress on the second major unresolved problem, which is what causes a cloud or core to collapse. In the standard model of star formation, a core begins in beautiful equilibrium, with gravity and external pressure balanced by internal thermal, magnetic or turbulent pressure. Collapse begins when this balance is upset in favor of gravity. But what triggers the imbalance? Astronomers have proposed many different ways. An outside force such as a supernova explosion might compress the cloud, or the internal pressure might ebb as heat or magnetic fields dissipate.
Charles Lada of the Harvard-Smithsonian Center for Astrophysics (CfA), João Alves of the European Southern Observatory (ESO) and their co-workers have argued for the slow dissipation of thermal support. By mapping molecular clouds at millimeter and submillimeter wavelengths, which straddle the radio and infrared bands, they have been able to identify a large number of relatively quiescent, isolated cores in nearby clouds. Some show evidence of slow inward motions and may be on their way to making stars. An excellent example is Barnard 335, located in the constellation Aquila. Its density structure is just what would be expected if the cloud’s thermal pressure were nearly in equilibrium with external pressure. An infrared source in the center may be an early-stage protostar, suggesting that the balance recently tilted in favor of collapse.
Other studies find evidence for external triggering. Thomas Preibisch of the Max Planck Institute for Radio Astronomy in Bonn and his collaborators have showed that widely distributed stars in the Upper Scorpius region all formed nearly in unison. It would be quite a coincidence for the internal pressure of different cores to dissipate at the same time. A likelier explanation is that a shock wave set off by a supernova swept through the region and induced the cores to collapse. The evidence is ambiguous, however, because massive stars disrupt their birthplaces, making it difficult to reconstruct the conditions under which they formed. Another limitation has been the difficulty of seeing lower-mass stars (which are dimmer) to confirm that they, too, formed in synchrony.
Spitzer has made progress on these questions. Lori Allen of the National Optical Astronomy Observatory, Xavier P. Koenig of the CfA and their collaborators have discovered a striking example of external triggering in a region of the galaxy known as W5. Their image shows young protostars embedded in dense pockets of gas that have been compressed by radiation from an earlier generation of stars. Because compression is a rapid process, these widely scattered objects must have formed almost simultaneously. In short, the triggering of star formation is not an either-or situation, as once thought. It is case of “all of the above.”
Life in a Stellar Nursery Leaving aside the above deficiencies, the standard model explains observations of isolated star-forming cores fairly well. But many, perhaps most, stars form in clusters, and the model does not account for how this congested environment affects their birth. In recent years researchers have developed two competing theories to fill in this gap. The great advance in the computing power available for simulations has been crucial in honing these theories. Observations, notably by Spitzer, are helping astronomers to decide between them.
In one, interactions between adjacent cores become important. In the extreme version, many very small protostars form, move rapidly through the cloud and compete to accrete the remaining gas. Some grow much bigger than others, and the losers may be ejected from the cluster altogether, creating a class of stellar runts that roam the galaxy. This picture, called competitive accretion, has been championed by Ian Bonnell of the University of St. Andrews, Matthew Bate of the University of Exeter, and others.
In the alternative model, the main external influence is not interactions among cores but turbulence within the gas. The turbulence helps to trigger collapse, and the size distribution of stars reflects the spectrum of turbulent motions rather than a later competition for material. This turbulent-core model has been developed by Christopher McKee of the University of California, Berkeley, Mark Krumholz of the University of California, Santa Cruz, and others.
Observations seem to favor the turbulent-core model [see “The Mystery of Brown Dwarf Origins,” by Subhanjoy Mohanty and Ray Jayawardhana; Scientific American, January 2006], but the competitive-accretion model may be important in regions of particularly high stellar density. One very interesting case is the famous Christmas Tree Cluster (NGC 2264) in the constellation Monoceros. In visible light, this region shows a number of bright stars and an abundance of dust and gas—hallmarks of star formation. Spitzer observations have revealed a dense embedded cluster with stars in various stages of development. This cluster provides a snapshot of precisely those stages when either turbulence or competitive accretion would leave its mark.
The youngest stars, identified as those with the largest proportion of emission at long wavelengths, are clumped in a tight group. Paula S. Teixeira, now at ESO, and her collaborators have shown that they are spaced roughly every 0.3 light-year. This regular pattern is just what would be expected if dense cores were gravitationally collapsing out of the general molecular cloud, suggesting that the initial conditions in the cloud are what determine the road to collapse. And yet, even though the observations support the turbulent model, the images have good enough resolution to tell that some of the supposed protostars are not single objects but compact groups of objects. One consists of 10 sources within a 0.1-light-year radius. These objects have such a high density that competitive accretion must be taking place, at least on a small scale.
Therefore, as with triggering mechanisms, the effect of the stellar environment is not an either-or choice. Both turbulence and competitive accretion can operate, depending on the situation. Nature seems to take advantage of every possible way to make a star.
Supersize This Star Massive stars are rare and short-lived, but they play a very important role in the evolution of galaxies. They inject energy into the interstellar medium via both radiation and mass outflows and, at the end of their lives, can explode as supernovae, returning matter enriched in heavy elements. The Milky Way is riddled with bubbles and supernova remnants created by such stars. Yet the standard theory has trouble explaining their formation. Once a protostar reaches a threshold of about 20 solar masses, the pressure exerted by its radiation should overpower gravity and prevent it from growing any bigger. In addition to the radiation pressure, the winds that so massive a star generates disperse its natal cloud, further limiting its growth as well as interfering with the formation of nearby stars.
Recent theoretical work by Krumholz and his collaborators offers one way out of this problem. Their three-dimensional simulations show stellar growth in all its unexpected intricacy. The inflow of material can become quite nonuniform; dense regions alternate with bubbles where the starlight streams out. Therefore, the radiation pressure may not pose an obstacle to continued growth after all. The dense infalling material also readily forms companion stars, explaining why massive stars are seldom alone. Observers are now looking for confirmation using Spitzer surveys of massive star-forming regions. But verifying the model will be tricky. The rarity and short lives of these stars make them hard to catch in the act of forming.
Fortunately, new facilities will soon help with this and the other questions posed by star formation. Herschel and SOFIA, a Boeing 747 that flies above 99 percent of the obscuring water vapor of Earth’s atmosphere, will observe the far-infrared and submillimeter wavelengths where star formation is easiest to see. They have the spatial and spectral resolution needed to map the velocity pattern in interstellar clouds. At longer wavelengths, the Atacama Large Millimeter Array (ALMA), now under construction in the Chilean Andes, will allow mapping of individual protostars in exquisite detail.
With new observations, astronomers hope to trace the complete life cycle of the interstellar medium from atomic clouds to molecular clouds to prestellar cores to stars and ultimately back into diffuse gas. They also hope to observe star-forming disks with enough angular resolution to be able to trace the infall of material from the cloud, as well to compare the effects of different environments on stellar birth.
The answers will ripple out into other domains of astrophysics. Everything we see—galaxies, interstellar clouds, stars, planets, people—has been made possible by star formation. Our current theory of star formation is not a bad one, but its gaps leave us unable to explain many of the most important aspects of today’s universe. And in those gaps we see that star formation is a richer process than anyone ever predicted.
Note: This story was originally printed with the title “Cloudy with a Chance of Stars”
