MARKEB (Kappa Velorum). Markeb, of Vela,
comes (according to Kunitzsch and Smart) from "markab," which
refers to a vehicle of some sort (here Argo, the Ship, of which
Vela represents the sails). It is not to be confused with the star
name "Markab" (Alpha Pegasi), which is a
corruption of "mankib" and refers to the shoulder of the flying
horse. It's no wonder that astronomers long ago began using more
logical Greek letters, numbers, and other stellar
designations from extensive catalogues. Markeb has several
claims to prominence. First, adopting values from the Yale Bright
Star Catalogue, it is the faintest of the second magnitude stars, coming in right at
magnitude 2.50 (91st in brightness, though some other sources place
it as bright as 2.46). Second, Markeb is the faintest member of
the prominent four-star "False Cross."
Lying near a rich part of the Milky
Way, the figure rather nicely resembles Crux, the famed
Southern Cross, and also includes Avior
(Epsilon Carinae), Aspidiske (Iota
Car), and Kappa's brighter constellation-mate, Delta Velorum. Third, while not very
extensively observed, Kappa Vel has some impressive properties.
Shining from a distance of 540 light years, its light dimmed by 0.2
magnitudes by interstellar dust, this hot (estimated at 22,300
Kelvin) blue class B (B2) subgiant-dwarf radiates at rate of 18,400
Suns (most in the ultraviolet), from which
we find a radius of 9.1 times solar. Temperature and luminosity
combine to imply a mass of between 10 and 11 times that of the Sun,
and reveal a star that --consistent with its subgiant status -- is
near the end of its 16-20 million-year hydrogen-fusing life. An
equatorial rotation speed of at least 52 kilometers per second
gives a rotation period less than 8.7 days. Next, as learned from
its spectrum, Markeb has a companion that orbits in just 116.65
days at an average separation of at least half an Astronomical
Unit. Assuming that the companion contains a solar mass, the true
separation is closer to 1.1 AU, implying an orbital tilt to the
plane of the sky of 26 degrees, which in turn suggests (if the
axial tilt is the same as the orbital) a true rotation period of
just 4 days. Markeb, which is rather windy and is losing mass at
a rate of roughly a billionth of a solar mass per year, also
radiates X-rays, which may be coming either from the star itself,
or from magnetic activity in the lesser companion. No one really
knows. The fate of the star is not well determined either, as it
is near the dividing line at which stars become either massive white dwarfs or blow up as
supernovae. With its
simple spectrum, Markeb is particularly important as a background
source of light with which to study the intervening gases of
interstellar space.