New research reveals the dynamics of the spinning disks of gas that surround young stars and gargantuan black holes
- 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
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
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
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.
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.