Asteroseismology
The
Study of Stellar Oscillations
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Understanding stars is central to much of
modern astrophysics. Stars are the fundamental entities providing
light and energy in the universe and they have produced most of the
elements (except hydrogen and helium) from which the Earth is made.
In this respect, they are the very source of life on Earth. Stars
also provide vital information about the history and the structure
of the universe, being the only objects for which we can determine
reliable ages.
But we are still very far from a detailed
physical understanding of stars, as much of our knowledge is based
on limited measurements of the light emitted from the stellar
surfaces from which we rely on theoretical models to derive their
internal properties.
Although the light from the
stars (including the Sun), is created deep within the stellar
interior, where the nuclear reactions takes place, its way out of
the dense central regions is very long, as it is constantly
scattered on the particles in the stellar plasma. It only reaches
the surface and escapes the star, after a trip of a few million
years. It then carries information about the outer regions from
which is emitted, and not about the inner regions, where it was
created. Still, the information contained in the starlight can be
compared with the theoretical stellar models, but this indirect
process is somewhat similar to trying to understand the human body
by looking at the skin only. But pulsating stars offer more
possibilities.
Pulsating
stars
Pulsating stars are stars which size,
brightness and temperature vary periodically with time, due to some
internal physical processes. As it was already mentioned in "DASC
and Kepler", these stars can be categorized according to the manner
in which they oscillate, and the stars within the same class of
pulsating stars are also physically similar. With the previous
discussion on the stellar structure and evolution in mind, we can
now show this in more detail.
Pulsations are found in groups
of stars all across the Hertzsprung-Russell diagram (or HR diagram).
The figure below shows the positions of different groups of
pulsating stars in the HR diagram. Most of these groups are named
after the first star of each class where pulsations were detected.
For instance, the β Cepheid stars are named after the second
brightest (hence Greek β, the second letter in the Greek alphabet)
in the constellation Cepheus, in which variability has been known to
astronomers for more than 100 years.
Move the mouse over the
different regions of pulsating stars to see an example of the
typical pulsations observed in the stars belonging to the
group.
For each type of stars, two figures are shown; the
upper diagram shows how the stellar brightness changes with time
during one day, the lower one shows the results of a mathematical
analysis called Fourier analysis, which is used to separate the
individual oscillation frequencies - or tones - which are present in
the complicated light curve.
This technique for analyzing
stellar oscillations in order to extract the frequencies will be
described further in the next section "Measuring Stellar
Oscillations".
The Hertzsprung-Russell
diagram ...
... (also referred to as
the HR diagram or HRD) shows the relationship between
the luminosity and the surface temperature of stars. The
diagram, which was first created nearly 100 years ago by
Ejnar Hertzsprung and Henry Norris Russell, improved
significantly the understanding of stellar evolution, or
the 'lives of stars'. In the diagram, hot, luminous
stars are found to the upper left, while cool, dim stars
are found in the lower right part of the diagram. As
illustrated in the Section "Sun-like Stars" a star moves
in this diagram as it evolves and hence changes its
surface temperature and luminosity. Thus, plotting
values of temperature and luminosity for many stars as
we measure them at present allows us to determine, for
instance, whether a given star is in the main sequence
phase of its life, or if it has evolved away from the
main sequence to become a red
giant.
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| What
this diagram actually shows is that the onset of pulsations in a
star is connected to its physical properties - to its luminosity or
mass, and to its evolutionary stage or age. These, together,
determine the position of the star in the HR diagram.
As the
star evolves along its evolutionary track (see the section on
stellar evolution), it may pass through one of the marked areas in
the figure, and become unstable towards pulsations. This is due to
some internal excitation mechanism that can operate in stars in this
exact region of the HR diagram, and which can cause the star to
pulsate. It then becomes member of the corresponding class of
variable stars. Each group typically contains from a few tens to a
few hundred stars in which the type of pulsations has been detected.
These numbers are expected to be significantly increased by the
Kepler mission, as many new variable stars will be detected from the
extensive data sets on hundred thousands of stars.
As the
star continues to evolve with time, it will eventually leave the
unstable region again and stop pulsating. What happens here is that
due to changes within the star - such as, for example, a change in
density because the entire star expands - the excitation mechanism
is no longer efficient and can no longer make the star
pulsate. This already tells us a lot about the structure of stars
across the HR diagram, as our theoretical models must be able to
reproduce the specific type of pulsations found in each one of these
specific regions in the HR diagram.
We can now, however, do
more than this. Thanks to a fast technological development in
instrumentation we can now do ultra-precise and extensive
measurements of these "star quakes" in the individual stars, opening
up a door to the stellar interior, enabling us to apply the
technique of asteroseismology, and actually use the stellar
oscillations to look beyond the stellar surface.
The
principles in asteroseismology are the same as those geophysicists
use to infer the internal structure of the Earth: by using
vibrations of the Earth's crust, either brought about naturally by
earthquakes or with explosives, in combination with mathematical and
physical models, very detailed investigations of the structure of
the Earth's interior can be carried out.
Two diagrams showing asteroseismic measurements of the
nearby star Alpha Centauri A (4.3 light years away), obtained
by the SONG group, compared with data for the Sun.
As
the sizes and internal properties of these two stars are
different from each other, their oscillations also show
different patterns on the two diagrams below.

| The study of the stellar structure and evolution through
asteroseismology is likewise an interplay between complicated
theoretical calculations and ultra-precise observations of stellar
oscillations, carried out with the best telescopes and instruments
available, at the best astronomical sites in the world.
The
background for asteroseismology is, however, found in the
Sun.
Helioseismology
The Sun
is the best-studied star in the sky. This is because we receive far
more light from it than we do from the distant stars, which makes it
much easier to collect precise data. Furthermore, it is close enough
that we can resolve its surface, which is not the case for the other
stars.
There are several telescope networks, set up all
around the globe, with the sole purpose of observing the Sun. In
each network, 6-8 telescopes are strategically positioned at
different longitudes, allowing for precise, continuous observations
of the solar surface. At the same time, several satellites observe
the Sun, all of which has been taking place for the last several
years.
From these measurements, it has been found that the
Sun is pulsating simultaneously in millions of different tones. The
typical oscillation periods for the individual tones are about 5
minutes, which is far too long for our ears to hear - if we could
stand on the surface of the Sun and listen to its ringing. All these
tones mean that the overall brightness of the Sun varies in a very
complicated manner. But because of very extensive datasets,
collected with the telescope networks and the satellites, the
individual tones have been determined to high precision. These many
tones, or frequencies, can accordingly be used to determine the
internal properties of the Sun and be compared with very complicated
mathematical and physical computer models of the structure of the
Sun, which in turn can be improved and developed, in order better to
match the observations.
In this way, a very detailed
knowledge of the interior structure of the Sun has emerged, and we
have obtained a deep understanding of how a star like the Sun
works.
Asteroseismology
However,
the problem with helioseismology is that we are only investigating
one single star; the Sun. But are younger or older, or more or
less massive stars, similar in structure to the Sun?
 Click
on the image to listen to some star sounds
| To answer this question, we must
observe other stars as well. And the answer to the question is,
perhaps not surprisingly, that although the basic principles are the
same, there are quite significant differences in structure between
stars, in particular between stars of different mass. Stars of about
the same mass and age, on the other hand, are quite alike. This
means that by doing seismic studies of a number of stars with
different properties (mass, age), a more detailed picture of the
inner structure of stars can be obtained, and we can investigate how
stars evolve. We can, for instance, study stars that are similar in
mass to the Sun, but older or younger, and obtain knowledge of both
the past and the future of the Sun.
However, although very
exciting results are being obtained at present, asteroseismology is
still well behind helioseismology. This is because we need to study
many stars, which takes time. And again because we receive much less
light from the stars. Furthermore, we cannot resolve their surfaces,
as we can with the Sun. And since the stellar oscillations manifest
themselves in variations in the overall stellar brightness, as well
as in complicated local variations across the stellar surface, we
simply have less information to work with, as compared to the Sun.
This makes it very demanding to obtain sufficient data for
determining the tones precisely, which is necessary for doing
asteroseismology and to compare observationally determined
frequencies with theoretical stellar models.
In the
next section, we describe how observations of stellar oscillations
are being done, and why a space telescope such as Kepler, offers
fantastic possibilities for asteroseismology.
Click here, if you want to read
more about asteroseismology.
A short history of helio- and
asteroseismology
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1961
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First evidence for periodic variation in the surface
velocity of selected areas of the Sun. |
1979
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Solar full-disk observations reveal global
oscillations. |
1981
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The BiSON network starts limited operations. This
network is still operating. |
1986
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The IRIS 7-station network starts operation. Operation
ends 2001. |
1991
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The first evidence for solar-like oscillations in
another star (Procyon) is published. Controversial at the
time, but later confirmed. |
1995
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The GONG network for solar oscillation observations
starts full operations. Operations are still
ongoing. |
1995
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The first detection of individual oscillation modes is
published for the star Eta Bootis. Controversial at the time,
but later (2003) confirmed. |
2001
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First clear detection of excess power in another star
(Beta Hyi). |
2001
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First definite solar-like oscillation measurements in
Alpha Cen A |
2004
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First detection of l=3 modes and measurement of mode
lifetime in another star (Alpha Cen A). |
2005
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Most precise measurements of stellar radial velocities
are made with the ESO VLT and the UVES spectrograph (Alpha Cen
B). |
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