BETA HOR (Beta Horologii). Running east of, and sort of parallel to, the southern stretch of River Eridanus, but going past Achernar, ticks Horologium, the Clock, a modern constellation that extends as far south as 67 degrees below the celestial equator. As obscure as the constellation are its individual stars, the brightest named one (Alpha Horologii) just fourth magnitude, most of the rest all fifth. The singular exception to this lack of notoriety is Iota Hor, which has an orbiting planet. Even poor Beta, a white class A (A3-5) giant (but see below) makes little splash, as at mid-fifth magnitude (4.99) it does not even rank second, that "honor," such as it is, marginally reserved for Delta, Beta Hor instead sitting in third chair. All it seems to have going for it is being the constellation's southern-most of the Greek-named stars. And even that is compromised, as it is not at all clear which star "Beta" was originally meant to have referred to: if any, as the letter seems at first to have been assigned to a star that does not actually exist. In terms of study and examination, Beta Hor does not do much better, the star being cited in just 20 studies over a 150 year period. Sirius, by contrast, racks up more than 1000 citations. Though there is no temperature measure, the class suggests around 8400 Kelvin. From a distance of 295 light years (give or take 5), Beta Hor shines with a total light of 65 Suns, nearly all of it in the visual part of the spectrum, that and temperature revealing a radius of 3.8 times solar, not much for a supposed "giant." Instead, the theory of stellar structure shows that it is not yet a giant, though it is at least close to the point of finishing up the fusion of hydrogen (to helium) in its core, and could better be classed as a subgiant with a mass of 2.5 Suns or so and an age of 600 million years. The uncertainty in spectral class is probably due to the star's unusual metallic composition, which is common for hydrogen-fusing stars in this range of temperatures. Metallic-line ("line" referring to absorptions in its spectrum) stars are caused by separation of chemical elements, some falling under the influence of gravity, others thrust upward through severe absorption of stellar radiation. Such stars usually occur among those of slower rotation, in which the spectra-producing atmospheres are undisturbed. Beta Hor rotates with a projected equatorial rotation speed of 84 kilometers per second, giving it a rotation period of less than 2.3 days, which is more or less within range. Beta Hor, however, has never been included in any kind of study to explore its actual chemistry, nor are there modern measures of rotation speed, which give us two more items in the list of the star's neglect and obscurity. Its one redeeming quality may be that it's the Beta star, and that is not much given the equal obscurity of its parent constellation.
Written by Jim Kaler 1/21/11. Return to STARS.