In astrophysics, asteroseismology is the science of determining the interior structure of
stars based upon their pulsation behavior. While physically more complicated, the behavior of pulsating stars is analogous to the
sounds emitted by a guitar strings of different lengths, compositions, and tensions, or
by bells of different sizes and weights. Asteroseismology is like
trying to figure out the structure of a musical instrument based on the sounds
it makes. It is very similar to seismology of the Earth -- using earthquakes to study the interior structure of our planet -- except that the Sun and stars are entirely gaseous objects.
Variable stars that pulsate have what are called
"normal modes" of oscillation. These normal modes can be
3D-symmetric, "radial" modes where the star expands and contracts
with the frequency of the mode. Or they may be "non-radial" or
"sectoral" modes where different sections of the star move
in and out in a regular fashion. The modes observed in a given star depend
upon two things:
- The interior properties of the star
- The spherical harmonic indices (l,m,n) of
the pulsation mode.
Early theoretical work was conducted on the pulsations of
variable stars by astrophysicists and mathematicians like
Sir Arthur Eddington, Paul Ledoux, and Chaim Pekeris in the first half
of the twentieth century. They studied the behavior of simple
stellar models (like spherical polytropes)
perturbed into one of these normal modes (like
striking a bell). Later, other astrophysicists expanded upon these ideas
to include more realistic physics, like better
equations of state, and the effects of
rotation and convection.
Our star, The Sun is known to pulsate in thousands
of independent pulsation modes, most with frequencies of a few millihertz
(or periods of about five minutes). By studying these pulsations, we
have learned a lot about the structure of the Sun to very high precision.
Several other kinds of stars are also known to pulsate, including
white dwarfs and delta Scuti stars.
The purpose of asteroseismology (beyond
fulfilling our curiosity) is to
study the interiors of stars, even stars that are thousands of light years
away. Since the pulsations are dependent upon the interior conditions, they
can tell us about how old the star is, what its chemical
composition is, and the dynamical behavior of the interior. Studying
individual pulsating stars can teach us things about stars in general, and
by extension, about the universe as a whole.
I note that the spelling is asteroseismology, rather than
astroseismology. The former is the IAU-defined standard, because it
implies the study of an individual star (aster is Greek for "star").