DASC and
Kepler
A new
Challenge
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Kepler is a NASA
satellite that will be launched in February 2009. On board the
satellite is a photometer capable of continuously measuring the
brightness of about 170000 stars in or near the constellation of the
Swan (Cygnus).
None of these thousands of stars is
visible to the naked eye, but by using even a relatively small
telescope in space we will be able to measure even the slightest
changes in brightness in any of these many stars.
Worlds Away
The aim of the
mission is to find planets the size of the Earth, orbiting other
stars in the Milky Way - in orbits that allow for liquid water to be
present on the planet's surface. If the planet is too close to its
star, the surface will be too hot, if the planet is too distant, any
water on the surface will be frozen.
 The gas giant Jupiter in
comparison to Earth
| Just as
the Earth is orbiting our Sun at a distance allowing liquid water to
be present, earth-like planets are assumed to be present around
other stars as well. But they are very difficult to find, because
they are so small (compared to the stars), and so far away. Larger
planets, on the other hand, are easier to find. At present, more
than 250 planets orbiting other stars are known. Most of these are
of the size of Jupiter, thus much larger than the Earth, while the
smallest of the known exo-planets, as they are called, are about 5-7
times more massive than the Earth.
The first exo-planet was
found in 1995, and the fact that over 250 more have followed since
then, with new ones found every month, suggests that many of the
stars in the Milky Way actually have planets. Thus, planets the size
of Earth may also be common - but we just can't detect them with our
present technology.
However, if a small planet in its orbit
around a distant star passes in the line-of-sight between us and the
star, the planet will cause a slight dimming of the brightness of
that star. This is called a planet transit and the method has
already now been used from the ground to detect more than a handful
of the know exo-planets.
All the known transiting
planets are relatively large. The change in the star's brightness is
proportional to the ratio of the planets surface area to the surface
area of the star, and in order for us to detect it from the ground,
the brightness change has to be relatively large. This is because of
disturbances from the Earth's atmosphere which set a lower limit as
to how precisely we can measure stellar brightness from the ground.
A small, earth-sized planet passing in front of it's distant star,
as seen from the Earth, will cause such a slight dimming of the
starlight that it will be impossible to observe it from
ground.
But the disturbing effects of the atmosphere are
obviously not a problem when observing from space. Therefore,
launching even a relatively small, 1-m sized telescope into space
makes observations, which cannot be obtained even with the largest
telescopes from ground, possible - observations precise enough to
detect earth-like planets around other stars.
Looking inside the
Stars
However, this technique of continuously
monitoring the brightness of thousands of stars to a very high
degree of precision brings about other scientific possibilities,
too. There are other mechanisms that cause variations in a star's
brightness, mechanisms which are connected to the star
itself. These can be processes on the surface of the star -
starspots, for instance - or, more importantly in the present
context, processes in the interior of the star, which cause the star
to vibrate, or rather oscillate.
Such stellar oscillations,
or starquakes, introduce small, periodic variations in the
brightness of stars. The oscillations, where the gas in the star is
continuously compressed and decompressed, actually corresponds to
sound waves here on Earth. They are well-defined tones, excited by
physical processes inside the star and, generally speaking, one can
say that large stars oscillate in low tones, smaller stars in
higher.
There are big differences in how different stars
oscillate. Some stars oscillate in only a few tones, others in
quite a lot, in some stars the variations in brightness are large,
in other very small. It is actually possible to categorize pulsating
stars from the way their brightness changes and it turns out that
stars, oscillating in about the same way are also physically quite
similar - having about the same mass and age. The situation is the
same for humans: our voices all sound more or less alike, but each
voice is, in fact, unique. Likewise, no two stars oscillate in
precisely the same way.
At the Danish AsteroSeismology
Centre (DASC) we are working on understanding the life
and inner structure of stars, using stellar oscillations. Much like
geologists use earthquakes to infer the interior structure of the
Earth, astronomers use starquakes to determine the interior
structure of stars. This branch of science is called
asteroseismology and this is the subject of these pages.
DASC is involved in the Kepler project, as we
are coordinating the use of the Kepler data to learn about
the inner structure of stars, using asteroseismology. On the
following pages you can read much more about how asteroseismology
works, and why Kepler will provide a fantastic dataset for
looking inside the stars with this technique. When you have studied
these pages, follow the link to the Kepler homepage to learn
more about how exactly Kepler will detect earth-like planets
in orbits around other stars.
The Kepler Homepage
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