ALGIEBA (Gamma Leonis). Double
stars dot the sky, many the favorites of amateurs and
observatory nights, some like those that make Mizar in Ursa Major
plain white, others like Albireo
exhibiting beautiful contrasts. Even when the colors are fairly
similar, the eye enhances them, rendering close pairs like second
magnitude Algieba (Leo's Gamma star) still
quite lovely, some observers seeing them as orange and yellow,
others yellow and greenish. Named after its place in the foreparts
of Leo the Lion, the Arabic name Algieba means "the forehead," and
was originally applied to several of the stars of Leo's famed
"Sickle." The star marks the radiant of the famed Leonid meteor
storm (the debris of Comet Temple-Tuttle), which returns with
varying degrees of success every 33 years, the last in 1998. In
1833, the storm produced a fall at a fantastic rate of 100,000 per
hour. To the eye, Algieba shines at mid-second magnitude (2.21,
the two stars combined), but even a modest telescope under good
atmospheric conditions will allow you split the pair, one appearing
at bright third magnitude (2.61), the other at bright fourth (a
rather uncertain 3.5), separated by just a under five seconds of
arc. The brighter, the more orange of the two, is a class K (K1)
giant with a temperature of 4400 Kelvin, the fainter a somewhat
warmer (4900 Kelvin) class G (G7) giant, making it the yellower
one. Such giant pairings (like Capella) are rather unusual. The
orbital period is so long, over roughly 620 years, that only a
fraction of the full path has been seen since discovery. At the
star's distance of 126 light years, the components are at least 97
Earth-Sun distances apart, over twice the distance of Pluto from
the Sun. A very high eccentricity takes the
two between 15 and 180 AU apart. Both stars are quite luminous,
the brighter 180 times brighter than the Sun, the other 50 times
(when invisible infrared is taken into account), leading to
respective diameters of 23 and 10 times solar. Both are true giants, meaning that they have
stopped fusing hydrogen to helium in their cores and have expanded
to their great proportions. Evolutionary calculations suggest that
each are about double the solar mass, while the still-uncertain
orbit gives a total mass of 2.35 solar. Born from the same
interstellar cloud perhaps two billion years ago, they are each
deficient in metals, their iron contents about a third that of the
Sun. It is hard to tell how far along they might be in their
evolution. They are may both be fusing helium in their cores or
they might be giants in development, with quiet helium cores that
are waiting to fire up, or each may be in a different stage, the
chemical composition at the surface, which is influenced by age,
suggesting the former. The pair moves with rather high speed, 71
kilometers per second relative to the Sun, over four times normal
suggesting that they come from a different part of the Galaxy.
Some six minutes of arc away are two tenth magnitude "companions,"
one of which may actually belong to the system. If so, it is at
least 14,000 AU away and would take roughly a million years to
orbit Algieba proper. From there, the binary would wth the
brightness of a couple full Moons half a degree apart. (Thanks to
Jason Pero, who helped research this star.)