Morse and other astronomers have used computer models of shock waves to make detailed predictions of the diagnostics. By comparing their predictions with the actual light as measured with the Hubble Space Telescope, Morse has concluded that the jets are 100 times denser than previously estimated. In other words, the jets 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 so, 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,” Morse says. In addition, John Bally, an astronomer at the University of Colorado in Boulder, has found a superjet—a string of HH objects 23 light-years long—that he thinks may be 100,000 years old. Morse believes these findings establish a definite link between jets and outflows.
The gas, rather than simply falling in, spirals around the star, ?spinning faster and faster as it approaches the center.
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 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, spirals around the star, spinning faster and faster as it approaches the center. This realization presented yet another theoretical difficulty. Astronomers wondered, what makes the cloud slow its spin enough to allow the matter to drop into the new star?
The conservation of angular momentum, the 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 that the scale is more dramatic. “Skaters change their size by about a factor of two,” says Lee Hartmann, an astronomer at the University of Michigan, “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 sufficient to cancel out the inward pull of gravity. The gas stops falling toward the star. This equilibrium of gravity and angular momentum seemingly presents a fundamental physical barrier that should freeze the development of a new star, preventing further infall of gas before the star could ever be born.
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 National Optical Astronomy Observatory. Because of this gradual accretion 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 to shuck off huge quantities of matter. If a skater spinning his partner suddenly lets go, for instance, the partner gets thrown away, carrying with her most 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: magnetic fields. These, 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. Giant molecular clouds and the dark clouds contained in them also possess powerful magnetic fields. Could those fields power? the jets and outflows too?
The most promising of the 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: If you drew lines indicating the orientation of the magnetic field, they would all run parallel. The rotation of the gas in the disk reinforces this 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, its centrifugal force increases so it begins to overcome the gravitational pull of the young star at its center, and 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.
While this model contains some significant uncertainties, its aesthetic appeal has won it many converts among astronomers. By ridding the disks of angular momentum, the winds solve two problems: 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, Königl says, “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 20-20 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 seem to 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 active galaxies and stretching millions of light-years into space; in fact, the disk-wind theory was developed to explain these extragalactic jets. The similarity between the structures of newborn stars and the eruptions of quasars offers astronomers a golden opportunity to study the 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. In the end, star birth, which started off as a dark mystery, may well end up casting light on some of the most dramatic, violent, and poorly understood events in the cosmos.