Stars are born in darkness, and in secret. they form deep within clouds of interstellar gas and dust so dense and opaque that no visible light can escape, and so their births have been kept well hidden from our earthbound telescopes. Unable to pierce the black veil, astronomers have had to content themselves with constructing, from physical principles, their own scenario for how a star takes shape. The chief actor in their makeshift tale is gravity: a clump of relatively dense gas in the center of a cloud drags in on top of itself a bit more of the cloud, making the center a bit more dense. More and more gas falls into the center, which grows and grows until it reaches a certain critical mass and collapses under its own enormous weight.
This appealingly straightforward infall theory has endured for decades with remarkably little variation. For the past several years, however, unprecedented observations from a new generation of telescopes-- most significantly, the Hubble Space Telescope--have made the tale seem increasingly simplistic. For the first time, astronomers are getting glimpses of gas clouds in the process of becoming new stars. What they are finding is far more complicated, and picturesque, than they had expected. Not only does gas fall inward, but vast quantities of gas and dust also stream outward, away from the nascent star. Narrow jets of gas apparently race from the stellar cradle at extremely high speeds and stretch several light-years out into space. Even more remarkably, stars in the throes of birth also seem to exhale giant peanut-shaped bubbles of gas, called outflows, 100 times more massive than our sun.
These spectacular outpourings of gas are among the most extraordinary surprises of modern astronomy. The realization that jets and outflows might occur as part of the normal process of star formation just blew everybody away, says Jon Morse, an astronomer at the University of Colorado in Boulder. It also presented astronomers and physicists with a puzzling question: Why do newly forming stars simultaneously take matter in and spew it out?
Astronomers weren’t looking for these phenomena when they discovered them, of course; they were looking instead, as they had been looking for many years, for confirmation that stars were indeed born from infalling gas, as theory maintained. But the problems in obtaining such observational support were daunting. Aside from the opacity of the dark clouds that provide the raw stellar material, the birth of a star creates quite a mess for astronomers to sort through. The dark clouds almost never exist on their own but are usually just a small part of an even larger, turbulent complex of gas and dust known as a giant molecular cloud. These vast structures span hundreds, even thousands, of light-years. Throughout such a cloud, stars are continually forming, often handfuls at a time in loosely packed groups. This abundance of activity produces a chaos of wisps, clumps, and knots of gas and dust that makes life very difficult for an observer on Earth.
Back in the 1940s, astronomers George Herbig and Guillermo Haro were systematically sorting through this mess for some sign of infalling gas when a particular type of debris caught their attention. In 1951 they saw bright knots of gas, knots emitting so much visible light that they didn’t know what to make of them. For decades, astronomers watched these structures closely, keeping track of their positions in the sky. It wasn’t until 1979 that Herbig and a few colleagues announced the dramatic conclusion they had drawn from their observations, one that made the standard infall theory of star formation seem too pat. The knots of gas were traveling at more than 100 miles a second--far too fast to be propelled by the gravitational collapse of a dark cloud. Even more to the point, they were moving in the wrong direction--outward, away from the dark cloud, rather than inward.
For the next several decades Herbig, Haro, and other astronomers struggled unsuccessfully to explain just what these bright knots of gas-- now called HH objects--were. That discovery, however, had to await advances in electronics that would boost the sensitivity of optical telescopes. Finally, in the late 1980s, new observations revealed the HH objects in far greater detail. They appeared not so much as knots of gas but as large, distinct blobs that trailed away from the dark clouds like beads on a necklace. Threading these blobs, the telescopes revealed, were smaller, pencil-thin jets of bright gas being driven out of the clouds.
The appearance of the jets constituted an even bigger conundrum for astronomers than the original sighting of the HH objects, because their geometry was so peculiar. What physics could possibly cause a thin stream of gas to come squirting out from a basically spherical cloud of collapsing gas? The jets were also apparently propelled with such force that they stretched more than a light-year out from the cloud while remaining impossibly narrow. In scale, they were akin to a tight stream of water shot five miles into the sky by a garden hose.
While astronomers pondered the jets, another discovery further deepened the paradox. Astronomers had been working throughout the 1970s on new radio telescopes that operated on millimeter-length waves. Since these telescopes would be able to pierce the dark clouds that had kept stellar birth hidden from direct observation, astronomers hoped that they would reveal infalling gas and confirm the timeworn infall theory. Instead they almost immediately uncovered vast and extraordinarily massive outpourings of gas around embryonic stars, ballooning out on opposite sides of the newly forming stars and stretching much farther away from them than the jets. Although these bipolar outflows were moving much more slowly than the HH objects--about ten miles per second--they were ten times more massive than the jets, with a volume many times larger. They were also much colder and much older. By measuring the velocity of the gas and its distance from the dark cloud from which it originated, estimating its age was a simple matter. And whereas the jets appeared to have been around for only 1,000 years or so, the oldest outflows had been streaming from the nascent stars for more than 100,000 years. Since stars take only 100,000 to one million years to form, the outflows seemed to play an integral role in the stars’ formation--though precisely what that role was, nobody could say.
To a scientist, nothing is more troubling--or exciting--than a diversity of phenomena without a unifying cause. Most astronomers felt viscerally that some common physical explanation underlay both the jets and the outflows, but the situation clearly demanded new ideas. The most natural idea was also the most obvious--that the jets were somehow powering the much larger, and much more distant, bipolar outflows. As the jets plowed through the cloud surrounding the fetal star, perhaps they swept up molecules of gas, or pulled the gas along, piling it up into the vast peanut-shaped outflows. A big problem with this theory was that the jets seemed so young. It was very puzzling, says Morse. What was more confusing was that even if the age dilemma could be resolved, the jets did not seem to carry nearly enough matter or force to create the massive bipolar outflows.
Despite its flaws, this theory still seemed more plausible than any of the alternatives, so astronomers set out to test it. Their next step, therefore, was to find another way of measuring the age and size of the jets. Another way of saying this is that astronomers needed to make better use of the dim light that managed to cross the thousands of light- years of space to their telescopes.
Researchers thought that the answer might come from an analysis of that light in terms of quantum mechanics and the physics of shock waves. A shock wave is nature’s way of slamming on the brakes. It occurs whenever a supersonic stream of fluid, such as hot gas from a newly forming star, strikes an obstacle, such as older, slower-moving gas, in its path. As the fast-moving atoms of the stream slam into the slower-moving atoms of the obstacle, they dissipate most of their kinetic energy as heat. As a result, the stream undergoes a violent deceleration, and temperatures rise accordingly. However, not all that energy is given off as heat. Some of it is absorbed by the colliding atoms, then reradiated in the form of photons, or particles of light. The rules of quantum mechanics give atoms discrete ways to absorb energy in collisions or lose it to photons. Thus astronomers can interrogate the photons to reveal the basic physical properties, like velocity and density, of gas passing through shock waves in the jets. Astronomers call the light emitted by shocks diagnostics. Just as a doctor has to infer conditions inside a patient from a description of his or her symptoms, astronomers use the information locked in the light they gather to infer conditions in the star formation outflows thousands of light-years away.
Morse and other astronomers have used computer models of shock waves to make detailed predictions of the shock diagnostics. By comparing their predictions with the actual light as measured most recently with the Hubble telescope, Morse has concluded that the jets are 100 times denser than previous estimates. In other words, the jets actually spout the equivalent of one Jupiter’s worth of matter into space every day. If this is true, then the jets indeed emit enough mass with enough power to have created the gigantic outflows that project far beyond them into space.
Furthermore, Morse’s observations suggest that the giant outflows form when the faster material from the jets catches up with older, slower material in its path. If he is correct, it would mean that the outflows consist in part of older material that was originally part of the jets, making the jets much older than previously thought. Having older material already out there means that the jets go much farther out than what we see in their brightest structures, says Morse. John Bally, an astronomer at the University of Colorado in Boulder, recently found a superjet--a string of HH objects 23 light-years long--that he thinks may be 100,000 years old. Morse believes that these findings establish a definite link between jets and outflows.
Showing the relationship between jets and outflows is only the first step toward a new understanding of how stars form. The ultimate goal is to explain what role these phenomena play in the life of the embryonic stars themselves. Before scientists could begin to postulate a mechanism, however, they first needed to come to an understanding of precisely where the old infall model of star formation, in which gravity plays the starring role, needed fixing.
Even though astronomers had failed repeatedly in their efforts to find incontrovertible evidence of a collapsing gas cloud on its way to becoming a star, they nevertheless made great strides in refining their ideas about what might happen when a gas cloud collapses. First of all, they realized that the dark cloud feeding the star should have been set spinning by small random motions left over from its formation in the giant molecular cloud of which it was a part. As a result, the gas, rather than simply falling straight in and onto the protostar at the center of the cloud, actually spirals around the star, spinning faster and faster as it falls toward the center. This, however, presents astronomers with yet another theoretical difficulty: What makes the cloud slow its spin enough to allow the matter to drop into the new star?
The conservation of angular momentum, a well-known physical law that explains why ice-skaters spin faster as they pull their arms in toward their torsos, holds equally true for spinning gas clouds, except the change in speed is much more dramatic. Skaters change their size by about a factor of two, says Lee Hartmann, an astronomer at the Harvard-Smithsonian Center for Astrophysics, but these clouds shrink by a factor of a million. As the cloud contracts and its spin accelerates, eventually it whips around so fast that its outward centrifugal force should be of sufficient strength to cancel out the inward pull of gravity. The gas stops falling toward the star. This equilibrium of gravity and angular momentum is what keeps the planets locked in orbit about our sun, but it presents a fundamental physical barrier that should freeze the development of a new star.
Observational astronomers partially solved this problem by postulating the existence of gaseous disks. The idea was that the centrifugal force created by the cloud’s spin causes the collapsing cloud to assume the shape of a disk. The effect is similar to what happens when a ball of dough is spun into a pizza crust--centrifugal force pushes material at the poles out to the equator. As the disk spins around, the explanation went, gas gradually inches its way toward the center, eventually reaching the inner edge of the disk and dropping onto the hungry star. You shouldn’t think of a star starting at a large radius and contracting, but instead think of these very tiny seeds that are built up by accretion of material that first passes through the disk, says Stephen Strom, an astrophysicist at the University of Massachusetts. Because of this gradual feeding of matter to the star, astronomers call these gaseous Frisbees accretion disks. The disks become reservoirs that hold the cloud’s angular momentum, says astronomer Suzan Edwards of Smith College. As the gas in the disk rotates, it has time to shed its angular momentum and slow down enough for gravity to drag it inward in a long contracting spiral.
Exactly how the disk manages to shed its angular momentum, however, remained a mystery. Friction among the gas atoms in the disk is not enough to dissipate the huge amount of energy stored in the disk’s rotation. The only way nature can reduce the angular momentum of an accretion disk is, of course, to shuck off huge quantities of matter. If a skater spinning his partner suddenly lets go, the partner gets thrown away, carrying with her the lion’s share of the angular momentum. As astronomers mulled over the growing evidence for jets and outflows, they began to suspect that the accretion disks were doing something similar. The major impediment to incorporating this idea into a theory of star formation was the problem of the peculiar geometry of the jets. Shouldn’t matter being thrown off an accretion disk travel outward in all directions along the plane of the disk? The jets and outflows, by contrast, seemed to throw matter up and down along the disk’s axis of rotation. The idea that the jets and outflows came directly from the disk and served to rid it of angular momentum seemed as absurd as a spinning ice-skater letting go of his partner and having her shoot straight up into the air.
Yet that absurdity may disappear when you take into account one more cosmic phenomenon: that of magnetic fields. Magnetic fields, after all, are found almost everywhere in space and are powerful enough to shape much of what goes on there. They create sunspots, control the rippling curtains of Earth’s auroras, and give pulsars their pulse. The giant molecular clouds and the dark clouds contained in them also possess powerful magnetic fields. Perhaps they power the jets and outflows too?
The most promising of the recent magnetic field theories, developed by astrophysicist Arieh Königl of the University of Chicago, postulates a magneto-centrifugal wind that flings matter up from the disk into the jets. Königl starts with the standard assumptions that the dark cloud from which a star is born possesses a magnetic field and that in the immediate vicinity of the new star, the direction of the field is consistent; in other words, if you drew magnetic field lines indicating the direction of the field, they would all run parallel. The rotation of the gas in the disk reinforces and strengthens this magnetic field. The gas in the accretion disk is hot enough for some of its atoms to lose electrons and become ionized--that is, to take on a positive electric charge. At the same time, as the cloud collapses, the magnetic field lines get compressed along with the gas and end up embedded in the disk. They form a sort of hourglass shape, much as stalks of wheat would look if you tied them in the middle.
With this magnetic field in place, the stage is set for matter to come streaming off in jets. As the disk accelerates its spin, and its centrifugal force increases so that it begins to overcome the gravitational pull of the young star at its center, gas molecules near the disk’s surface are flung off. Since charged particles tend to follow magnetic field lines, moving along them in a sort of corkscrew motion, the gas molecules fly not only outward but also upward and downward along the magnetic field lines.
This model contains some significant uncertainties--not least that no one has been able to observe the magnetic field of a real accretion disk. Nevertheless, its aesthetic appeal has won it many converts among astronomers. By ridding the disks of angular momentum, the winds solve two problems at once: they not only power the jets but also slow the rotation of gas in the disk enough to let it make the final hop onto the star. This theory also explains why the jets appear to be beadlike. As the accretion disk spins faster and its centrifugal force stops matter from falling in, a clump of gas gets thrown off the disk and up into the jet. The loss of matter slows the disk down, allowing more matter to move through the disk and toward the center; that transference of mass, like the skater’s arms coming in toward his body, serves to accelerate the disk once more. As this process of fits and starts is repeated, matter is fed into the jets in discrete chunks. In addition, astronomers have conjured some tricky mathematics to show that the magnetic field lines contract and twist as they get farther and farther away from the disk. If so, this would explain why the jets are so tightly focused.
With the disk-wind theory, says Königl, it no longer seems like an accident that we see all those outflows in connection with star formation. The theory gives astronomers the feeling that, if they had just thought hard enough about it beforehand, they could have predicted the existence of outflows even before they observed them. This kind of twenty- twenty hindsight is the mark of a powerfully descriptive theory.
The journey of a new star from the turbulent chaos of the giant molecular clouds to the serene constancy of maturity seems to contain many of the structures found elsewhere in the universe. Accretion disks have been observed around many white dwarfs and neutron stars, and they may also fuel black holes at the center of quasars. Jets, too, can be seen traveling at close to the speed of light from the center of galaxies and stretching millions of light-years into space; in fact, the disk-wind theory was first developed to explain these extra-galactic jets. The similarity between the structures of newborn stars and other processes in the galaxy offers astronomers a golden opportunity to study the physical mechanisms of these more distant phenomena. And the unity that astronomers seek in their theories of star formation may even provide some clues to a grand synthesis of these much larger events. In the end, star birth, which started off a dark mystery, may well end up casting light on poorly understood events throughout the cosmos.
