• Because disks of gas are so ubiquitous in the universe, appearing around newly born stars, in binary star systems and at the centers of galaxies, astrophysicists are keenly interested in learning their dynamics.
• To explain the radiation from the disks, scientists have long assumed they must be turbulent. Physicists believe that a phenomenon called magnetorotational instability is causing the turbulence.
• Researchers are now exploring how this phenomenon works in different kinds of accretion disks.

Look up at the sky on a clear night and try to find some of the planets that are visible to the naked eye--Mercury, Venus, Mars, Jupiter or Saturn. If you locate three or more, you will see that they all appear to line up within a fairly narrow band that forms a great circle around the sky. This band includes the ecliptic, the path of the sun's apparent movement through the constellations of the zodiac over the course of a year. And if you now focus on the fuzzy trail called the Milky Way, you will notice that it traces a different great circle across the sky.

These observed geometric facts are not accidental. The planets of our solar system, including Earth, revolve around the sun in the same direction and (apart from Pluto) in nearly the same plane. This arrangement is strong evidence that the planets formed from a pancake-like disk of material (mostly gas and dust) that orbited the early sun. Similarly, the appearance of the Milky Way--which is a hazy agglomeration of the light from many billions of stars--shows that our galaxy is also disklike in shape. Because our solar system is situated within this disk, our galaxy appears to encircle us.

Structures in the shape of disks are common in the universe on a vast variety of scales. Saturn's rings are a graceful local example, but not the only one; all the giant planets in our solar system have rings. Disks have also been observed around many young stars; astronomers often call them protoplanetary disks because they appear to be similar to the one that must have formed our own solar system. In some binary star systems, gas escapes from one star and is captured by the gravity of the other to form a disk. Inside the disk, the gas slowly works its way down to the stellar surface in a tight, spiraling motion like a whirlpool. Such structures, called accretion disks, are also thought to exist around supermassive black holes (which can weigh as much as a billion suns) at the centers of galaxies. The largest disks are spiral galaxies such as our own Milky Way, which stretches more than 100,000 light-years across.

Given the ubiquity of disks in the universe, understanding how they work is an important problem in astrophysics. Astronomers believe that accretion disks around supermassive black holes may have influenced the way galaxies formed and evolved. And exploring the dynamics of accretion disks around young stars may shed some light on the early history of our own solar system. Thanks to new theoretical insights and modern computer simulations, scientists have recently discovered an explanation for the roiling turbulence of accretion disks that makes them powerful energy sources. But other phenomena, such as the jets of particles that often stream from the disks, remain a mystery. Researchers still have much to study in the billions of whirling disks that populate our universe.

A DISK'S ROTATION holds it up against gravity. Picture yourself on a merry-go-round that is spinning dangerously fast. If you do not hold on tightly to one of the painted horses, you will be flung in a straight line tangent to the merry-go-round's circle. The tension in your arm provides exactly the force needed to cause your body to move in a circle and therefore to stay on the ride. The rotation of the merry-go-round prevents you from moving inward; you would need to make a tremendous effort to pull yourself toward the center. In the same way, the rotation of the material in a disk prevents it from collapsing inward under the force of gravity.

Rotating objects are endowed with angular momentum, a quantity that is proportional to the object's rotation rate and the distribution of mass around the axis of rotation. (The farther the mass is from the axis, the greater the object's angular momentum.) Angular momentum is central to our understanding of the behavior of rotating systems because, like energy, it is conserved: it can be neither created nor destroyed. A twirling ice skater, for example, can spin faster by pulling in her arms. Because her angular momentum must remain constant, the movement of mass closer to the rotation axis of her body is offset by an increase in her spin rate.

The conservation of angular momentum explains why disks are so prevalent in the universe. Consider a cloud of gas that is contracting inward under the attractive power of its own gravity. Almost everything in the universe rotates at some level, so suppose this cloud has some angular momentum. As it contracts, the principle of angular momentum conservation forces it to rotate faster. Material in the equatorial region of the cloud--that is, in the plane perpendicular to the rotation axis--moves inward more and more slowly as rotation starts to balance the pull of gravity. Material along the rotation axis falls vertically toward the equatorial plane much faster. The resulting object is a rotationally supported disk.

Scientists believe this process explains how protoplanetary disks form around young stars and perhaps how gas disks coalesce around black holes at the centers of galaxies. Whether an entire galaxy becomes a disk is a timing issue: spiral galaxies emerge from gas that becomes rotationally supported before patches of the gas contract into stars. If stars are born in the gas before the galactic cloud becomes rotationally supported, the stars will maintain their individual orbits around the galactic center, creating an elliptical galaxy. In general, galaxies do not form in isolation, and galactic collisions and mergers complicate matters considerably. At least some elliptical galaxies, as well as the bulges and halos of spiral galaxies, may have arisen from such collisions.

Accretion disks also form in binary star systems when one of the stars (for example, a compact, dense white dwarf) gravitationally pulls gas off its companion (usually a larger, less compact star). This gas has considerable angular momentum from the original orbital motion of the two stars around their common center of mass, so it typically cannot fall directly inward toward the white dwarf. Instead the gas ends up forming a disk around the dwarf.

Just as Mercury has a much shorter year than Earth--a mere 88 days--the material in the inner parts of a disk invariably takes less time to complete one orbit than does material in the outer parts. This gradient in orbital periods causes shear: bits of material at slightly different distances from the center of the disk slide past one another. If some form of friction is present in the disk material, it tries to slow down the more rapidly orbiting inner regions and speed up the more slowly orbiting outer regions. Angular momentum is therefore transported from the inner to the outer regions of the disk. As a consequence, material in the inner regions loses rotational support against gravity and falls inward. The overall result is a gradual spiraling of matter toward the central star or black hole.

As material spirals down to the innermost orbit of an accretion disk, it must give up gravitational potential energy. Some of the potential energy goes into giving the material the faster orbital speed it gains as it falls inward; the rest is dissipated into heat or other forms of energy by the friction itself. Thus, the material in the disk can become very hot, emitting copious amounts of visible, ultraviolet and x-ray radiation. The energy release can make accretion disks formidable power sources.

This phenomenon is what first alerted astronomers to the existence of black holes. Black holes themselves cannot emit light, but the accretion disks around them can. (This general statement ignores the theorized Hawking radiation, an emission that would be undetectable for all but the smallest black holes and that has not yet been observed anywhere in the universe.) According to Einstein's general theory of relativity, the energy released by an accretion disk around a black hole should be equivalent to roughly 10 percent of the material's rest-mass energy (which is equal to its mass times the speed of light squared). This amount is spectacularly high, more than 10 times as great as the energy that would be released if the material underwent thermonuclear reactions, such as occur in stars or hydrogen bombs. And yet this prediction agrees with observations of the radiation from quasars, highly luminous objects that are believed to be powered by accretion disks around supermassive black holes in the centers of early galaxies. When one calculates the total energy radiated over time by all the quasars in a given region of space, it turns out to be about 10 percent of the mass of all the supermassive black holes currently observed in an equivalent region times the speed of light squared.

BUT WHAT IS THE NATURE of the friction inside the accretion disks that touches off this enormous energy release? One possibility is that the particles that make up the material in the disk undergo collisions in which they exchange small amounts of energy and angular momentum. This mechanism operates in Saturn's rings: as the pebbles, rocks and boulders that make up the rings collide, their energy is lost as heat, and angular momentum is transferred outward. Ordinary fluids act in much the same way; in fact, Saturn's rings can be thought of as a viscous fluid in which the colliding molecules are actually rocks! The collisions give the rings a tendency to spread radially, but Saturn's moons act as reservoirs of angular momentum that keep the rings confined.

Unfortunately, this simple process cannot explain the activity of many other types of accretion disks. In the accretion disks in binary systems or at the centers of galaxies, particle collisions would produce an inflow of mass that is too small by many orders of magnitude to produce the brilliant luminosity of these disks. Another possibility is that large-scale spiral waves in the disk, similar to the spiral arms observed in galaxies, hasten the inflow of matter. Just as sound waves transport energy through the air, spiral waves can transport both energy and angular momentum outward and facilitate the accretion of material inward. And astronomers, in fact, have seen evidence of spiral-wave patterns in accretion disks in some binary systems. But the spiral waves in these systems do not appear to be large enough to produce the rate of matter inflow needed to explain the observed radiation from the disks.

Many astrophysicists believe, however, that the most widespread mechanism for friction inside accretion disks is turbulence, which would accelerate the inflow of matter by generating violent, large-scale collisions. When water flows in a pipe, the viscosity of the liquid causes the flow speed to be highest at the center of the pipe and lowest near the pipe's inner surface. If the water is forced to move faster, this velocity shear becomes larger and eventually destabilizes the flow, making it turbulent and chaotic. Because accretion disks also contain flows with very high rates of shear, scientists proposed in the 1970s that the disks must also be highly turbulent. But when researchers tried to demonstrate this phenomenon using the basic equations of fluid flow and computer simulations, they found no indication that turbulence would develop in an accretion disk.

The reason for this negative result is still controversial. It could be that the computer simulations are somehow faulty, but it is also possible that the analogy with pipe flow is simply incorrect and that rotating systems like accretion disks are intrinsically different. Investigators have conducted laboratory experiments to search for turbulence in flows that resemble accretion disks, but again the results have been debated. Although turbulence is sometimes seen in these experiments, it may arise from effects that would not be present in a real accretion disk.

Nevertheless, astrophysicists have persisted in their belief that accretion disks are indeed turbulent. Proceeding under this assumption, most researchers have adopted a crude mathematical guess for the effects of disk turbulence that was introduced in 1973 by Soviet physicists Nikolai Shakura and Rashid Sunyaev. By cutting the Gordian knot in this way, astrophysicists have been able to build theoretical models of accretion disks to compare with observations of actual disks. This research program has achieved several successes over the years. For example, some accretion disks in binary star systems occasionally undergo large, temporary increases in luminosity. (Dwarf novae, explosions of light from the accretion disk around a white dwarf in a binary system, are examples of this phenomenon.) Scientists have convincingly demonstrated that these transient increases are triggered by an instability in the disk that causes material to flow rapidly inward.

Despite these accomplishments, however, the Shakura-Sunyaev approach has really just concealed our ignorance. Discrepancies between model predictions and observational data might arise simply because the widely accepted guess for disk turbulence is wrong. In addition, the turbulence might have observable consequences besides providing angular momentum transport in the disk, but researchers are unable to predict these consequences without understanding the process behind them.

FORTUNATELY, A SPECTACULAR breakthrough in the turbulence problem came in 1991. Steven Balbus and John Hawley of the University of Virginia realized that if the material in the accretion disk was highly electrically conductive and magnetized, even if only weakly, then the magnetic field would produce a fluid instability in the disk. The instability would invariably cause a turbulent flow that would transport angular momentum outward and dissipate gravitational binding energy. This effect, which was called the magnetorotational instability (MRI), is now believed to play a central role in the way many accretion disks operate.

Magnetic field lines in a highly conductive medium must move with the medium's underlying flow. Where the material goes, so goes the field. Magnetic field lines also exert forces on the medium. In particular, just like elastic strings, the field lines exert tension forces when they are bent or curved.

To understand the effect of magnetic field lines, imagine two particles in orbit about Earth that are tethered together by an elastic coil. If the particles are momentarily pulled apart (with one particle moved a bit closer to Earth and the other a bit farther away), most people would assume that the resulting tension in the coil would cause them to snap back to their original configuration. If the tension is sufficiently weak, however, its presence can actually drive the particles farther apart. The particle moved inward must speed up in its orbit to conserve angular momentum, and the particle perturbed outward must slow down for the same reason. The stretched coil acts to slow the faster, inner particle and speed the slower, outer particle. Deprived of some of its kinetic energy, the inner particle falls farther inward (and paradoxically speeds up in its orbit), while the velocity boost flings the outer particle farther out (where it moves even more slowly). In effect, the coil is transferring angular momentum from the inner to the outer particle. In an accretion disk composed of charged particles, magnetic field lines work in exactly the same way.

It is easy to see how this instability would lead to turbulent flow. Consider another analogy: a circular racetrack with the cars in the inner lanes moving faster than the cars in the outer lanes. Suppose someone hooked chains between cars in different lanes. The cars in the inner lanes would lose angular momentum as they are dragged backward, while the cars in the outer lanes would gain angular momentum as they are flung forward. The result would be considerable chaos and mayhem, just like the turbulence that develops in an accretion disk.

The discovery of the MRI has revolutionized our understanding of accretion disks. The situation is rather similar to that prevailing in the early 20th century when astronomers first realized that the primary energy source for stars was nuclear fusion reactions occurring in the stellar core. Now astrophysicists have deduced the mechanism that powers even greater energy sources such as quasars and active galactic nuclei (highly energetic galactic cores that are also thought to be fueled by matter falling into supermassive black holes). Researchers are currently exploring how MRI turbulence works in different physical situations and how that might explain the observed deviations in behavior among various types of accretion disks.

For example, some scientists are interested in whether and how MRI turbulence acts in protoplanetary disks. Such disks form a much cooler environment than those around white dwarfs, neutron stars and black holes because of the much smaller gravity of the central star. As a result, the disks are composed largely of electrically neutral dust and gas rather than ionized plasma. Whether magnetic fields can affect the flow of such material is far from clear.

My group and others are trying to figure out how MRI works in hot, opaque accretion disks around black holes. The turbulence in these disks can be effectively supersonic, forming and reforming shock waves of charged particles just as supersonic aircraft produce sonic booms. Because these motions can kick photons to high energies, and because the photons can move more easily through the relatively transparent regions between shock waves, MRI turbulence can produce characteristic patterns of radiation that astronomers should be able to observe from black hole systems.

GIVEN THAT MANY accretion disks are thought to contain very turbulent flows, it is hardly surprising that observations show a high degree of variability in their output of radiation. The variations are usually random and chaotic, but there is occasionally order within the chaos. Intriguing, inexplicable patterns in the light output occur over and over again, and oscillations with reasonably well defined frequencies are sometimes evident. The Rossi X-ray Timing Explorer, a satellite that can measure rapid changes in x-ray brightness, has significantly aided the study Of oscillations in accretion disks around neutron stars and stellar-mass black holes (those with a mass four to 15 times as great as the sun's).

Astrophysicists do not know what causes these variability patterns or the oscillation frequencies. An exciting possibility, proposed by Robert Wagoner of Stanford University and others, is that the oscillations reflect discrete modes of vibration of the disk, very much like the harmonics of a violin string. And just as the notes produced by a violin string can reveal the string's tension and mass, the observed frequencies of an accretion disk might be able to tell us about the disk's structure and the spacetime around the neutron star or black hole.

Although much of the gravitational binding energy released by the spiraling material in accretion disks ends up in the form of radiation, sometimes the energy also drives winds and jets of particles from the disk. Astronomers are intently exploring how such outflows are generated and what determines the partitioning of accretion power into radiative and kinetic luminosity. In all likelihood, different types of disks have different mechanisms for expelling particles. In some cases, the outflows probably wield a controlling influence on the accretion disk, because they carry not only mass and energy outward but perhaps significant amounts of angular momentum as well.

One possible driving mechanism for some types of outflows is pressure from the photons that are produced by the accretion disk. Even though photons have zero rest mass, they still carry momentum. When photons scatter off material, they exchange momentum with the particles they hit and thereby exert a force on them. (This is the principle behind solar sails.) Ultraviolet photons radiating from young massive stars are known to drive particle winds outward by scattering off the atoms and ions surrounding the star. In the same way, ultraviolet photons from accretion disks around white dwarfs and in active galactic nuclei or quasars may also accelerate winds from the disk.

Some systems, such as young stars and certain classes of active galactic nuclei, produce very fast, narrow jets of particles extending up to several light-years in the case of young stars and to more than several million light-years for active galactic nuclei. The fact that these jets remain collimated in a narrow beam over such great distances suggests that magnetic fields may be involved. (Astronomers have also inferred the presence of such fields in active galactic nuclei from their effects on the polarization of radio waves.) Because the accretion disk itself is believed to be magnetized, the rotation of the disk can twist the magnetic field lines into a helix. Tension in the field lines that spiral around a jet of particles can help confine it. Back in the 1980s, Roger Blandford and David Payne of the California Institute of Technology suggested that the rotation of the disk may also help fling material outward along the field lines, providing the initial acceleration and mass loading for the jet. Unfortunately, we do not yet know how to relate the inward accretion flow in the disk, with its complex MRI turbulence, to the apparently more ordered field structure in a jet outflow. But the rapid progress we are making in studying the magnetic fields in accretion disks may help us crack these kinds of Problems.

Astrophysicists have spent decades trying to figure out how accretion disks work, and now we believe we have a basic understanding of these systems. As we investigate how magnetic turbulence operates in different environments, we hope to someday comprehend the remarkable variety of phenomena these spinning disks exhibit. It was an accretion disk that gave birth to our solar system, so unraveling the dynamics of these fascinating objects may ultimately help explain how we came to be.

Gravity's Fatal Attraction: Black Holes in the Universe. M. Begelman and M. J. Rees. W. H. Freeman and Company, 1998.

Accretion Power in Astrophysics. Third edition. Juhan Frank, Andrew King and Derek Raine. Cambridge University Press, 2002.

Accretion Processes in Star Formation. Lee Hartmann. Cambridge University Press, 2004.

More information about accretion disks, black holes and other astrophysical objects can be found at http://imagine.gsfc.nasa.gov/

PHOTO (COLOR): ACCRETION DISK surrounds a black hole in this artist's rendering of an x-ray binary system. The powerful gravity of the black hole pulls gas off the companion star, a red giant. The disk of gas emits copious amounts of x-ray radiation as it spirals' inward toward the black hole. The disk also generates jets of particles that shoot from the innermost region.

PHOTO (COLOR): ODD PATTERNS appear in the x-ray radiation from accretion disks such as GRS 1915+105, which surrounds a black hole in a binary system about 4.0,000 light-years from Earth. Physicists do not know what causes these oscillations.

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By Omer Blaes

OMER BLAES has long been intrigued by the dynamics of accretion disks. A professor of physics at the University of California, Santa Barbara, he earned his Ph.D. in 1985 from the International School for Advanced Studies in Trieste, Italy. He then did postdoctoral research at the California Institute of Technology and the Canadian Institute for Theoretical Astrophysics in Toronto. Blaes is a theorist who works in the area of high-energy astrophysics; in addition to accretion disks, he is particularly interested in the physics of compact objects such as black holes, neutron stars and white dwarfs.