ALBIREO (Beta Cygni). One of the great small-telescope showpieces
of the sky, Albireo, the third-magnitude (3.0) Beta star of Cygnus, the Swan, is a magnificent visual
double whose components
(magnitudes 3.3 and 5.5) have contrasting golden and blue colors.
Though given the second letter of the Greek
alphabet, the star actually comes in at number five in brightness,
beaten out by Deneb (Alpha Cygni), Sadr (Gamma), Gienah
(Epsilon), and Delta. 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. The name has a magnificently
confused and mistranslated origin, and means nothing at all with
regard to its position at the head of Cygnus the Swan. 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.