Stellar oscillations, like those observed in the Sun, manifest themselves by periodic variations in the surface temperature and overall brightness of the star.
The amplitude of these variations is very small. In temperature the variations are a tiny fraction of a degree and for light intensity the variations are a few parts per million. Such tiny changes are exceedingly difficult to measure from the Earth, mainly because of disturbances from the Earth’s atmosphere.
However, the oscillations also cause periodic motions of the surface of the star, and although these motions are almost negligible compared to the size of the star, they can in fact be measured. This requires the most sophisticated methods and instrumentation currently available.
The right Technique
The technique making these extremely sensitive observations possible is spectroscopy, where the light from the star is dispersed according to wavelength, using the same principles as when the light from the Sun becomes dispersed by raindrops and forms a rainbow.
The result of this dispersion is a spectrum of the star, which carries an enormous amount of information about, for instance, the chemical composition of the star, its rotation rate and, most importantly for SONG, the relative movement of the stellar surface with respect to the Earth. This measurement is referred to as the radial velocity of the star.
The Radial Velocity Measurement
The radial velocity measurement will be dominated by the overall motion of the star either away from or towards the Earth. But as the stellar oscillations cause the very surface of the star constantly to expand and contract, the information about these oscillations will also be included in the radial velocity observations as tiny velocity variations over time.
When for example the radial velocity of a star is measured every minute over a period of time, the resulting data will contain the information about the stellar oscillation periods and the amplitudes. And this is the seismic information that can be compared to theoretical stellar models, i.e. this is the information needed to do asteroseismology.
The radial velocity technique is by far the best method for measuring stellar oscillations from the ground, as it is much less affected by the Earth’s atmosphere than any other method.
Naturally this is the technique which will be used by SONG.
The other main technique to observe stellar oscillations is photometry, which measures changes in the stellar brightness.
For observations of solar-like oscillations the disturbing effects of the atmosphere can only be avoided from space by using satellite telescopes.
In fact the radial velocity method from ground is superior even to such measurements.
This is due to the fact that other surface phenomena on the stars create a so-called stellar background noise, and the contrast between the oscillations and the noise is higher for radial velocity measurements than for brightness measurements, even if the latter are done from space.
Detecting Planets
Nearly all the known exoplanets have been discovered using the radial-velocity technique. In this case, the gravitational tug from the planet as it orbits the star will cause the star to wobble a little bit, introducing slight but observable shifts in its radial velocity.
Another promising detection method exists which relies on measuring the partial occultation of the parent star by a planet, causing a reduction in the observed light intensity. This method is referred to as the transit method.
However, for detecting earth-sized planets it is mandatory to use space-based instruments, in order to obtain the necessary precision.
The top figure shows actual Radial Velocity measurements of the star Beta Hyi, obtained with HARPS on the 3.6m telescope at ESO, La Silla, Chile.The figure below was made by using the above data, adding an artificial signal of a planet, which has 20 times the Earth’s mass and an orbital period of 2 days. It is possible to detect much smaller planets, even if they are much less visible on the measurement data by using mathematical models. |
Two satellite missions, COROT (France) and Kepler (NASA), will both be launched in the next few years, with the purpose to monitor the light intensity of thousands of stars over a long period of time (several years), in order to search for earth-like planets with the transit method.
Using the radial-velocity technique from the ground, SONG will also be able to detect earth-sized planets, if their orbital periods are less than one week, i.e. if they are in a close orbit around the parent stars. If this is not the case, the gravitational effect of the planet on the star's radial velocity will be too small to be observable.
It is currently not known how high the stellar background noise is for different types of stars, simply because the very precise radial velocity measurements have only become possible in the very recent years.
However, if the estimates of the stellar background noise based on observations of the Sun hold, SONG may be able to detect planets as small as Mars (mass only 1/10 the mass of the Earth) in a close orbit.
Larger planets, on the other hand, will be easily detectable with SONG even in much wider orbits.
Although the goals of SONG and of Kepler/COROT are similar with respect to planet detection, there are important differences. Whereas SONG will observe about one hundred very bright stars, Kepler and COROT will observe many thousand faint stars, hereby making the three projects complementary.