Taking a turn through the world of astrophysical disks
Right:
Artist's
depiction of an
accretion disk
Thin disks of rotating gas and dust can form around young stars, black
holes, white dwarfs, neutron stars and even planets. Saturn's rings are
an example of such disks, as are the disks that form around active
galactic nuclei (AGN).
As mass flows inward through these accretion disks to fuel
giant energy outputs, many types of waves can form.
Pressure forces and the rotation of the gas itself can
Right:
Image of an
give rise to oscillations. Magnetic fields can cause
accretion disk
disturbances to become unstable, exciting both
around a black
very large and very small waves. Both
hole in
misalignments and the gravity of nearby stars
galaxy NGC 4261
can warp and twist disks, and the
deformations themselves can move inward
as wavelike disturbances.
Spiraling out of control
Wacky Warping
Spiral density waves can be excited by
gravitational perturbations from planets or
nearby companion stars. Spiral waves are
steepening into shock fronts in the face‐
on simulation snapshot to the right,
and interacting unstably with
other waves in the edge‐on
snapshot below.
The cartoon below illustrates the
warping deformations that can
propagate like waves through an
accretion disk.
Circling the planets
Saturn's rings give a nearby example of an accretion
disk.
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Magnetic Mayhem
on
s. . .
Sufficiently ionized accretion disks can
be threaded by magnetic fields,
which exert tension and
pressure forces on the gas
while at the same time
being pushed and pulled
by the flow. Combined
with the rotation of
the fluid, this causes
the gas to become
unstable, resulting in
turbulent transport
and wave excitation
on large and small
scales. The snapshot
to the right is from a
simulation that has
reached this turbulent
state.
Listening to the stars
...
in a
Mechanical
oscillators and
standing waves
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strophysical ob
Stars, disks and organ pipes
are
all
examples
of
mechanical oscillators. Every
oscillator has its own set of
special frequencies that it
resonates at. Larger objects
have lower frequencies, which is
why longer organ pipes produce
lower notes. In addition to their
fundamental frequency, objects can
also resonate at higher frequencies,
called overtones.
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What
makes
stars oscillate?
Stars are balls of gas held
together by gravity.
Many types of waves,
such as sound waves,
can travel through
the gas. These waves
interact to form
standing waves, and
the entire star then
"rings" with a certain
frequency. This is very
similar to the way
organ pipes work.
Some stars, such as
Cepheid variables, are intrinsically
unstable to oscillations. These are
known as classical pulsators, and while
their pulsations can be quite dramatic,
these stars are relatively uncommon. Other
stars, like our Sun, oscillate because they
possess convection zones. The "boiling" motions in
these zones pummel the star and make it ring. Stars
like these are called solar‐like oscillators and are quite
common. Their pulsations are much weaker than Cepheids.
What happens when stars oscillate?
Oscillations often involve the movement of gas
inwards or outwards. Some stars, like white
dwarfs, can also undergo twisting motions.
Most stars are too far away for us to
directly image their oscillations. However,
motions of the gas cause the surface
temperatures to change, producing
brightness fluctuations that can be
measured.
Left:
Right: Brightness changes over
time in a red giant star called
Mira. Pulsating stars similar
to Mira are called Mira
variables.
Examples
of standing
waves in a
half‐open pipe.
Their wavelengths
are determined by the
length of the pipe.
Why study stellar oscillations?
By applying the principles of fluid mechanics, we
can learn about the properties and internal
structure of stars. We can derive information
about their mass, radius, rotation speed,
chemistry and magnetic field.
Above:
Sound waves
bouncing around inside a
star. If the waves constructively interfere,
a pattern called a mode is formed.
Below: The spectrum of modes in our Sun.
Each peak corresponds to one frequency,
or "note" in which the Sun is ringing.