HOMAM (Zeta Pegasi). The name really belongs to a pair of stars,
Zeta and
Xi Pegasi (in Pegasus, the
Flying Horse), which together to the ancient Arabs became known
(translated from an obscure phrase) as the "lucky stars of the
hero," presumably of the hero Perseus. The name then devolved
specifically to third magnitude (3.40) Zeta Pegasi, leaving poor
fourth magnitude Xi Peg (which lies just to the northeast of Zeta)
nameless. Homam, a name never really used (so we'll go with Zeta),
is a class B (B8) dwarf (but see below) that lies 209 light years
away. From that and an accurate temperature of 11,190 Kelvin
(which allows the calculation of ultraviolet light), we find a
luminosity 227 times that of the Sun and a radius of 4.0 solar.
Like many stars of classes A and B, it is a fast rotator, spinning
at an equatorial speed of at least 140 kilometers per second, 70
times that of the Sun, which gives it a
rotation period of under 1.4 days (as opposed to the 25 day solar
period). Luminosity and temperature then tell of a mass between
3.3 and 3.5 times that of the Sun depending on the exact state of
evolution, which in any case is more that of a subgiant rather than
a dwarf, a star that is giving up core hydrogen fusion (sort of
consistent with an alternative classification as a B8.5 giant). The age is around 260
million years. Another classification has Zeta as a B8 "emission"
dwarf, suggestive of an equatorial disk related to the high
rotation. Typical of local class B stars, its metal abundance is
a bit low relative to that of the Sun, some 40 percent solar
compared to hydrogen. There is no X-ray emission. Showing how far
we have advanced in precision of measurement, Zeta Peg seems to be
a member of the class of "slowly pulsating B stars" (whose
prototype is 53 Persei), varying by a mere
0.00049 magnitudes over a 22.95 hour period. (Most stars would be
variable if examined with enough accuracy.) Zeta appears at first
to have two companions. Zeta
Peg B is a 12th magnitude (11.6) star at a current separation of 59
seconds of arc. Given that in 1879 the separation was 64 seconds,
"B" is clearly a line-of-sight coincidence (the difference being
much to great for an orbiting star). Eleventh magnitude (11.0)
Zeta Peg C, however, seems real, as over more than a century the
separation has stayed constant at 177 seconds of arc. If it is a
real companion at the same distance as Zeta A, Zeta C is a K6 dwarf
with a mass around 0.6 solar. At a minimum orbital distance of
11,000 Astronomical Units, the orbital period would be at least
600,000 years, making "A" and "C" a "fragile binary." From Zeta B,
Zeta A would shine with the light of rather well under that of the
full Moon, while from Zeta A, Zeta B would be about as bright as
our Venus.
Written by Jim Kaler 11/16/07. Return to STARS.