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RIGEL KENTAURUS
Among the most famed stars of the entire sky, surely rival in renown to Sirius and Polaris even though not visible to much of the world's population, is the "foot of the Centaur," Rigel Kentaurus, "Rigil Kent," the first star of Centaurus, probably much better known as Alpha Centauri or just Alpha Cen. Its fame, indeed that it is the third brightest star in the sky (after Sirius and Canopus), lies not in its extreme characteristics but in its geometry, as it is the closest star to the Sun, lying a mere 4.39 light years away, the distance known to a mere 0.2 percent. Placed well down in the southern hemisphere, in fact the most southerly of naked eye stars, it cannot be seen above about 30 degrees north latitude, making it one of the great luminaries of the southern hemisphere. Alpha Cen deceives the eye. Through but a modest telescope we see it as double. The brighter is a yellow class G dwarf (hydrogen-fusing) star that, with a temperature of 5830 Kelvin (50 degrees hotter than solar), appears almost identical to the Sun, certainly an odd coincidence. The companion, over a magnitude fainter, is a cooler (4680 Kelvin) class K star, the two making an obvious color contrast. The pair orbit each other every 79.9 years. Though they average 24 astronomical units apart (23 percent farther than Uranus is from the Sun), the elliptical orbit sends them from a farthest distance of 36 astronomical units to 11, about Saturn's solar distance. Because of Alpha Cen's proximity, the brighter component is still of the zeroth magnitude, and would by itself still be the sky's third brightest star, the secondary coming in at first magnitude at number 21. The orbit and orbital speeds yield masses of 1.16 solar for the brighter star, 0.97 for the fainter (as expected for ordinary hydrogen fusing stars on the "main sequence"). The respective luminosities of 1.6 and 0.9 times solar coupled with metal contents almost double that of the Sun lead to a calculation of ages of about 2 billion years, less than half the solar age. Alpha Centauri has yet another member, a faint tenth magnitude companion called "Proxima" that is a huge two degrees away from Alpha proper and that orbits with a period of at least a million years. If indeed it does orbit (and that is not certain), it is now on the near side of its path and some 10,000 astronomical units closer than the bright pair, making it actually the closest known star (but since it is part of Alpha, surely it is still fair to call Alpha the closest star). As a mid-class M dwarf star, Proxima is faint indeed, to the eye 19,000 times dimmer than the Sun. From Alpha Cen proper, Proxima would appear as only fifth magnitude, about as bright as the faint stars of the Little Dipper. When infrared radiation produced by its 2800 Kelvin surface is accounted for it is seen to be more luminous, but still only 1/500 as bright as the Sun, the result of a mass only 20% solar. While the chief component is best known as a solar analogue, Proxima is famed as a dwarf class M "flare star," one that suddenly erupts with fearsome violence as a result of the collapse of its complex and unstable magnetic fields. Only from Rigil Kentaurus would our Sun have any kind of magnificence. Since the star is close to us, its inhabitants, were it to have any, would see much the same constellation patterns that we do except that Centaurus would be missing its brightest star and the stars that lie between Cassiopeia and Perseus would be the setting for our first magnitude Sun, which would be the eighth brightest in their sky.