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Sky and Telescope Online Gallery July 15, 2008

While not a cluster, Albireo (Beta Cygni) is one of the true showpieces of the night sky. It's contrasting golden-red and blue colors are a beautiful site in the amateur telescope.

Star colors are usually subtle, ranging from a warm orange red to a hint of blue on white depending on the viewer's eyes. But put a star of one color next to one of another, and the eye seems to exaggerate both, delighting the follower of double-star astronomy. Waxing romantic, astronomers have called the pair topaz and sapphire. With a separation of 34 seconds of arc, the pair is easily seen at low telescopic power.

Albireo beautifully shows how an apparently single star as viewed through the telescope can actually be double, such "binary" stars appearing all over the sky. Somewhere around half, or even more, of the local stars are actually members of some kind of double or multiple system, the stars in orbit about each other. The stars that make Albireo, about 380 light years away, are quite far apart however, and if actually attached gravitationally have an extremely long orbit with a period of at least 75,000 years. Albireo is actually triple. The brighter yellow-colored member, Albireo A, is a much closer double made of a third magnitude (3.3) class K (K3) stable helium-fusing bright giant and a hotter but dimmer (magnitude 5.5) class B (B9) hydrogen-fusing dwarf, the two stars not readily separable in the telescope. The K giant has a temperature of around 4400 Kelvin, a luminosity 950 times that of the Sun, a radius 50 times solar, and a hefty mass of about 5 solar, while the close companion comes in at 11,000 Kelvin, 100 solar luminosities, and 3.2 solar masses. On average separated by about 40 Astronomical Units, they take almost 100 years to go about each other on a highly eccentric orbit. The visually-seen blue star, Albireo B, is similar to Albireo A's companion, and is a class B (B8) dwarf with a temperature of 12,100 Kelvin, a luminosity of 190 Suns, and a mass of 3.3 solar. It distinguishes itself by being a very rapid rotator with an equatorial velocity of at least 250 kilometers per second and a rotation period less than 0.6 days. As is so often the case among such fast-spinning stars, Albireo B is a "B-emission star" that is losing matter and is surrounded by a disk of gas of its own making. From Albireo B, Albireo A would appear as brilliant orbiting orange and blue points about half a degree apart, the K giant shining with the light of 35 full Moons, the close class B companion at about half of that. (Jim Kaler - STARS)

July 1, 2008