1 AND 4 CEN (1 and 4 Centauri), another two-for one special. The stars of Centaurus (the Centaur) have an odd naming history. The letters look as if Bayer had done his usual job in his great atlas, the Uranometria, Greek letters first then lower then upper case Roman. However, he could not see most of the constellation, nor for that matter could Tycho Brahe, on whose observations the atlas is based. Instead the letters were re-applied using Greek then upper and lower case Roman (reversed from Bayer's scheme) by Nicolas de Lacaille, who explored the southern heavens. Working from England, Flamsteed could see no further down than 35 or so degrees south of the celestial equator, so he measured the positions of only four stars near the constellation's northern boundary with Hydra (the stars just south of Hydra's tail). These four were then later numbered from east to west. Of them 3 Centauri (actually 3 Cen A, a young B5 dwarf) is the best known, really famed, as it's one of the very few stars whose spectrum reveals the presence of the light isotope of helium, He-3 (of which there is practically none on Earth; all our helium comes from the decay of uranium and thorium and is He-4). 2 Cen is a fine red, slightly-variable, class M5.5 giant. So now we round off the quartet with 1 Centauri, a class F (F3) subgiant, and 4 Cen, classed a B4 subgiant but (see below) really a dwarf, all giving us a nice range of color. With a well-known temperature of 6790 Kelvin and distance of 63.8 light years (give or take just 0.3), 1 Cen radiates almost all its energy in the visual spectrum, shining with a luminosity of 6.0 Suns, which leads to a radius 1.8 times solar. A projected equatorial rotation velocity of 75 kilometers per second gives a rotation period under 1.2 days (the star just slightly warmer than the "rotation break" at F5 above which stars lose their outer convective layers and speed up; or rather don't slow down). Theory then reveals a mass of 1.4 Suns and shows the star indeed to be either a subgiant (whose core is exhausted of hydrogen fuel)or an older dwarf soon to become one, the age just under three billion years. By contrast, blue-white 4 Cen's temperature of about 16,400 Kelvin is not well-determined. Factoring in a lot of ultraviolet radiation and the distance of 637 light years (give or take 87), 4 Cen's total luminosity is around 1225 times that of the Sun, its radius 4.3 times solar. A rotation speed of at least 27 kilometers per second gives a rotation period less than 8.1 days. Since there are no obvious abundance anomalies caused by element separation in a quiet atmosphere, the star is probably rotating much faster (so as to keep things stirred up) with its rotation pole pointed more or less at us. With a mass of 5.5 Suns, 4 Cen appears to be an older dwarf closing in on its dwarf lifetime of 65 million years. Both 1 and 4 Cen have spectroscopically-detected companions with respective periods of 9.945 and 6.927 days, which, assuming they are of low mass, give from Kepler's laws a separation from their parent stars of 0.30 and 0.12 Astronomical Units. 1 Cen may be a Delta Scuti type of variable with a short period of 0.02 days, while 4 Cen has a line-of-sight 15th magnitude neighbor currently 15 seconds of arc away. Both 1 and 4 Cen will eventually slough off their outer layers and die as white dwarfs with respective masses of 0.6 and 0.9 times that of the Sun. If nothing else, the set of four stars shows the wondrous variety of the starry sky. (Thanks to Jerry Diekmann, who suggested these stars.)
Written byJim Kaler 4/22/16. Return to STARS.