MENKENT (Theta Centauri). Centaurus is
dominated by its two brightest stars, Rigil Kentaurus (Alpha Centauri, third brightest
star in the sky and the nearest star to the Earth) and first
magnitude Hadar (Beta Centauri). Though
these two shine brilliantly to the lucky residents of the southern
hemisphere, neither is visible from mid-northern latitudes, so if
those that live there wish to know Centaurus, they must begin with
the third brightest star in the constellation, Menkent, to which is
assigned the rather lowly letter Greek letter Theta
(Gamma Centauri
rather oddly coming in second). The name comes from an Arabic word
for "shoulder" (of the Centaur), to which is attached the Latin
abbreviation for "Kentaurus" for Centaur, tying Menkent back to the
constellation's luminary, Rigil Kentaurus. Menkent, at mid-second
magnitude (2.06) is but four percent fainter to the eye than Polaris. Much closer than Polaris,
however, only 61 light years away, it is intrinsically much less
luminous. At the warm end of class K (K0), the star is a near-
clone of the northern hemisphere's Pollux, just fainter to the eye than Pollux
because it is 80 percent more distant. Menkent, with no known or
even suspected companions, is about as pure a sample of its class
you can come by. From its 4780 Kelvin surface, this yellow-orange
star radiates at a luminosity 60 times that of the Sun, the star's radius 11 times solar. Well
along in its evolution, Menkent is now fusing helium into carbon
and oxygen in its deep core. The star's only offbeat property is
its rather high "proper motion," its speed across the line of
sight. Approaching us at only one kilometer per second, it is
speeding past us at 65 kilometers per second, about twice "normal,"
suggesting that the star really belongs to the outer part of the
Galaxy's disk and is only visiting the solar neighborhood.