ALPHARD (Alpha Hydrae). Not all that well known among bright stars, but surprisingly prominent, Alphard both dominates and lies at the heart of the relatively dim constellation Hydra, the Water Serpent (the longest constellation of the sky), hence clearly deserves to be Bayer's Alpha star. Right in the middle of the range of second magnitude (1.98), Alphard is made more noticeable by lying within a fairly blank region of sky to the southwest of brighter Regulus. Appropriately, Alphard's Arabic name means "the solitary one." A dark sky shows the star glowing a pale orange color, indicative of a common class K (K2) giant star, though one on the bright side. An accurate distance of 180 light years (with an uncertainty of just 2) and an assessment of infrared radiation from its 4260 Kelvin surface allows us to calculate a luminosity 946 times that of the Sun, which with temperature implies a radius of 56.6 times solar, a quarter of an Astronomical Unit, 60 percent the size of Mercury's orbit. The measured angular diameter and distance give a radius of 57.8 times that of the Sun, just two percent greater, showing the parameters of our lonely star to be remarkably accurate. The theory of stellar structure and evolution suggests that Alphard is quietly fusing helium into carbon and oxygen in its deep core, and that it is a "clump star," one of very similar other properties, but with a fairly high mass of around 4.5 times that of the Sun. It probably started life as a hot star in the middle of class B, by itself implying a one-time fairly fast rotation speed. But when stars evolve as giants they spin more slowly, a consequence of the conservation of angular momentum (the same reason a skater spins faster as she brings in her arms). Current measures suggest that Alphard has a projected equatorial rotation velocity of 3.3 kilometers per second, meaning that the rotation period could be as long as 2.4 years (as opposed to the near-monthly solar rotation period). Alphard also distinguishes itself by being a mild form of "barium star," in which barium and other elements that are formed by the slow capture of neutrons are enhanced. Barium stars are thought to be close doubles. When Alphard was young, it had a more massive companion that died first, and in the process contaminated it with the by-products of nuclear fusion that had been shoveled to the top. The companion has now died as a dim white dwarf, and the star that had been contaminated is now itself dying, presenting us with evidence for what once happened. After sloughing off its outer layers, which will briefly make a surrounding planetary nebula, it too will become a white dwarf, with a mass of around 0.8 solar masses. The key is in the word "mild," as most such are probably are not real barium stars at all. In any case, there is no evidence for a tiny close companion. There are, however, two distant faint neighbors. Alphard C, now 4.3 minutes of arc away, is moving too fast to be joined, and is clearly a line of sight coincidence. But ninth magnitude (9.7) magnitude Alphard B, at 282 seconds separation, is more or less tracking bright Alphard A. If really tied together, "B" is probably a low mass K0 dwarf (which as a result of contrast effects in the nineteenth century was considered to be "pale green") at least 15,700 AU away that takes at least 870,000 years to make a circuit. Don't wait up.

Written byJim Kaler 4/3/98; rewritten 6/16/15. Return to STARS.