Thanks to James B. Kaler. These contents are the property of the author and are reproduced from original without the author's express consent because of fair use and valid educational purposes.
ALGIEBA
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 Algieba 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, but even a modest telescope under good atmospheric conditions will allow you split the pair, one appearing at bright third magnitude, the other at bright fourth, separated by just under five seconds of arc. The brighter, the more orange of the two, is a class K giant with a temperature of 4400 Kelvin, the fainter a somewhat warmer (4900 Kelvin) class G giant, making it the yellower one. The orbital period is so long, over 500 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 170 Earth-Sun distances apart, four times greater than the distance of Pluto from the Sun. 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. We have not seen enough of the orbit to be able to calculate masses, but comparison with evolutionary calculations suggests that each are about double the solar mass. 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. (Thanks to Jason Pero, who helped research this star.)