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.