The Oscillations of Centauri B

by Paul Gilster on December 23, 2005

Information on Alpha Centauri comes in all too slowly for Centauri Dreams, but astronomers at the Anglo-Australian Telescope and the European Southern Observatory’s Very Large Telescope in Chile have come to the rescue. They’ve teamed up to observe Centauri B, an orange K1 star slightly cooler and less massive than the Sun. In question was the rate at which the star’s surface is pulsating, which tells us about its temperature and internal composition.

The precision of these observations is remarkable. A moving stellar surface causes slight alterations in the wavelength of light it emits; the study of this Doppler shift supplies information. Centauri B’s surface moves about 300 meters an hour, surely a tiny figure to determine given the 4.3 light-year distance to the target, not to mention the encroaching light of Centauri A, the star’s close companion. And yet the sensitivity of the instruments in question was better than 1.5 cm/s, or less than 0.06 km per hour. It makes sense that both Paul Butler and Geoff Marcy, planet hunters extraordinaire, were on the team studying Centauri B. After all, planet hunters know all about tiny Doppler shifts.

The star is pulsating because gas in its outer layers is in violent enough motion to create low-frequency sound waves that bounce off inner layers of the star. Measuring the light from Centauri B once a minute for seven consecutive nights, the researchers made more than 5000 observations.

From an AAO news release:

A star’s surface can oscillate in many different patterns, or modes, simultaneously. The researchers were able to determine 37 modes of oscillation in alpha Centauri B. They also measured the mode lifetimes (how long the oscillations last), the frequencies of the modes, and their amplitudes (how far the surface of the star moves in and out).

Other stars, including our own Sun, are known to vibrate like Centauri B, but these observations are the most detailed ever made of such oscillations. From the data, we should learn much about Centauri B’s internal composition. The paper is Kjeldsen, Hans et al., “Solar-like Oscillations in α Centauri B” in the December 20 issue of the Astrophysical Journal. An abstract is available here.

Centauri Dreams‘ take: It’s extraordinary to consider that we are finding ways to study the internal dynamics of stars, learning about regions within them that are completely hidden from view. Our Sun’s pulsations occur about every five minutes and are apparently caused by the same phenomenon being studied on Centauri B. People talk about stars ‘ringing’ because sound waves become trapped in an internal region that can act like the cavity of a musical instrument. The waves are actually variations in pressure that move through the gases within the star, where temperature and density change drastically between the surface and the stellar core. Studying these ‘peals’ of sound should make our nearest neighbors just a little less mysterious.