process works over a couple of
million years, eventually trans-
forming a diffuse cloud of starless
cores into a f lattened disk of
protostars.A widening gyre
Where there is gravity and mat-
ter, there will also be accretion.
Infalling matter forms a swirling
accretion disk. This gravitational
gyre forms because the infall-
ing material — like everything
else in the universe — had some
motion and angular momentum
before becoming caught up by an
object’s gravity. The laws of phys-
ics state that angular momentum
must be conserved — so, to fall
into a star, black hole, or other
object, material must lose its
angular momentum first. It can-
not be simply sucked toward the
core along a straight line. Instead,
it forms a f lattened structure
called an accretion disk.
The closer the material is car-
ried to the center, the faster it
spins. (Physicists often use the
metaphor of a figure skater to
demonstrate this effect; when the
skater pulls their arms in, they
spin faster.) There’s just a small
problem: For material to actually
fall onto the core, it must slow
down and eventually come to astop. But how can this happen
when the closer it gets, the faster
it moves? Why doesn’t it just swirl
around forever? What dissipates
the angular momentum, allowing
gravity to win the tug-of-war?
There are two prevailing theo-
ries. “The old idea is that disks
are turbulent, and this turbulence
generates a kind of viscosity,”
or friction within a fluid, says
Ilaria Pascucci, an astrophysicist
and planetary scientist at the
University of Arizona in Tucson.
In this scenario, the disk is full of
eddies, which means the gas par-
ticles don’t orbit smoothly. As
inner material accelerates, it drags
the material outside of it along for
the ride, like a jar of molasses
being stirred. “The viscosity
redistributes angular momentum
outward, enabling disk gas close
in to accrete,” says Pascucci.
This turbulence-viscosity
model of disk accretion was first
suggested around 40 years ago.
But astronomers have never really
been able to make the numbers
add up. The models required
disks to be stickier and more
viscous than turbulence could
probably account for.
Then, in the last decade, an
overlooked characteristic of
accretion disks — magnetic fields— started to show promise.
“What if the angular momentum
is not redistributed in the disk,
but extracted through winds?”
Pascucci says.
Star-forming regions have
magnetic fields running through
them. While they do not affect
neutral gas, particles that have
been heated and ionized have an
electric charge and will tend to
follow these magnetic field lines.
As the large-scale clouds collapse
under their own gravity, these
magnetic field lines also become
twisted and tangled. And if mag-
netic field lines are somehow bent
outward, anchored to plasma that
remains outside the collapsing
cloud, plasma that is zipping
along the magnetic field might
be able to overcome gravity and
accelerate away from the disk.
Astronomers call these outf lows
magnetohydrodynamic (MHD)
winds, and they could carry away
angular momentum. This would
enable the leftover disk material
to fall onto the forming protostar.
Both observations and
simulations seem to point towardAt just 450 light-
years away, the
Taurus Molecular
Cloud is an ideal place
to search for accretion
disks. Two examples
are the young stars
HL Tauri (bright blue,
at upper center left)
and V1213 Tauri
(lower right). The
latter is hidden by an
accretion disk, though
the star partly
illuminates the disk
above and below it.
The visible disk and
the jets comprise the
object HH 30. ESA/HUBBLE
AND NASA; ACKNOWLEDGEMENT:
JUDY SCHMIDT