A cloud of interstellar gas is rotating slowly around its axis and
contracting because of the attractive pull of its own gravity. As the
cloud collapses, it rotates faster.
The gas in the cloud’s equatorial plane moves inward more slowly
because its rotation starts to balance the gravity. Gas above and
below the plane falls inward much faster.
Gravitational
contraction
Rotation
Faster
contraction
Slower
contraction
galaxy. In general, galaxies do not
tic collisions and mergers compli-
At least some elliptical galaxies, as
of spiral galaxies, may have arisen
m in binary star systems when one
compact, dense white dwarf) grav-
companion (usually a larger, less
considerable angular momentum
otion of the two stars around their
it typically cannot fall directly in-
arf. Instead the gas ends up form-
.
ch shorter year than Earth—a mere
nner parts of a disk invariably takes
bit than does material in the outer
al periods causes shear: bits of ma-
stances from the center of the disk
ox on page 54]. If some form of fric-
erial, it tries to slow down the more
s and speed up the more slowly or-
ar momentum is therefore trans-
outer regions of the disk. As a con-
ner regions loses rotational support
ard. The overall result is a gradual
he central star or black hole.
to the innermost orbit of an accre-
avitational potential energy. Some
into giving the material the faster
ls inward; the rest is dissipated into
y by the friction itself. Thus, the ma-
very hot, emitting copious amounts
ay radiation. The energy release can
dable power sources.
A cloud of interstellar gas is rotating slowly around its axis and
contracting because of the attractive pull of its own gravity. As the
cloud collapses, it rotates faster.
The gas in the cloud’s equatorial plane moves inward more slowly
because its rotation starts to balance the gravity. Gas above and
below the plane falls inward much faster.
Gravitational
contraction
Rotation
Faster
contraction
Slower
contraction
of the stars (for example, a compact, dense white dwarf) grav-
itationally 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 in-
ward toward the white dwarf. Instead the gas ends up form-
ing 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 ma-
terial at slightly different distances from the center of the disk
slide past one another [see box on page 54]. If some form of fric-
tion is present in the disk material, it tries to slow down the more
rapidly orbiting inner regions and speed up the more slowly or-
biting outer regions. Angular momentum is therefore trans-
ported from the inner to the outer regions of the disk. As a con-
sequence, 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 accre-
tion 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 ma-
terial 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 ex-
istence 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
JIAN
A cloud of interstellar gas is rotating slowly around its axis and
contracting because of the attractive pull of its own gravity. As the
cloud collapses, it rotates faster.
The gas in the cloud’s equatorial plane moves inward more slowly
because its rotation starts to balance the gravity. Gas above and
below the plane falls inward much faster.
Rotation
Faster
contraction
Slower
contraction
Blaes, SciAm
Disks are ubiquitous in
the universe
rsnemmen
keyakkie
rabernat
joisino
peterdesmet
kayceesrk
usamik26
signer
asymptotic_minato
viven_inc
konakalab
butchi
aki_iinuma
mthomps
bcantrill
speakerdeck
schacon
62gerente
edds
brad_frost
matthewcrist
mraible
hursman
ddemaree