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The Question

(Submitted July 17, 1997)

We would like to find out some information on the star, Alpha Centauri. We are 10 years old, but already have followed up a great interest in astronomy. We would especially like to know what Alpha Centauri is made of, what its measurements are and if it has any special assets.

The Answer

Here are some facts about Alpha Centauri:

Visible only from latitudes south of about 25 degrees N, the star we call Alpha Centauri lies 4.35 light-years from the Sun. But it is actually a triple star system. The two brightest components Alpha Centauri A and B form a binary. They orbit each other in 80 years with a mean separation of 23 astronomical units (1 astronomical unit = 1 AU = distance between the Sun and Earth). The third member of the system Alpha Centauri C lies 13,000 AU from A and B, or 400 times the distance between the Sun and Neptune. This is so far that it is not known whether Alpha Centauri C is really bound to A and B, or if it will have left the system in some million years. Alpha Centauri C lies measurably closer to us than the other two: It is only 4.22 light-years away, and it is the nearest individual star to the Sun. Because of this proximity, Alpha Centauri C is also called Proxima (Centauri).

Alpha Centauri A is a yellow star with a spectral type of G2, the same as the Sun's. Therefore its temperature and color also match those of the Sun. Alpha Centauri B is an orange star with a spectral type of K1. Whereas Alpha Centauri A and B are stars like the Sun, Proxima is a dim red dwarf with a spectral type of M5 - much fainter, cooler, and smaller than the Sun. Proxima is so faint that astronomers did not discover it until 1915.

Want more info?

As regards the size of the system, alpha Cen A is a double star with a period of 79.9 yrs. In terms of how the size of the system compares with ours, that is not a well defined question. If you were to use the size of our solar system as defined by the planetary orbits, and compare this with the size of alpha Cen A as defined by the binary orbit, then out solar system would be larger. The size of an orbit varies as the 2/3 power of the orbital period, so, going out to the orbit of Pluto which has an orbital period around the Sun of 248 yrs, the relative size of the orbit compared to a Cen A is (248/80)^{2/3}, or about twice as big. If, for our solar system, you were to include the orbits of comets, this ratio would be even larger. On the other hand, the a Cen system may also contain smaller bodies such as planets and comets which orbit far out from the central stars, so the sizes of the of our two solar systems may be rather similar.

There is every reason to believe that other stars have planets. It is not clear how stable the planetary orbits would be, however, in a double or triple star system. They could not have nice, nearly circular orbits like the planets in our solar system. To be stable, the orbits would have to be in some kind of "resonance" with the stars, in order to be prevented from being expelled from the system. I would find it difficult myself to accept stable planets in a multiple star system, unless the planets' orbits were large compared to the separation of the stars. In that case, however, the light from the stars would be pretty faint, and the planets might be too cold for life.

Tim Kallman and John Cannizzo
for Ask an Astrophysicist

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