GAMMA DEL (Gamma Delphini). Among the favorite stars in the sky
are the modest doubles, those that consist of two stars that are
easily split with a small telescope, favorites being Albireo, Mizar,
and Almach (Gamma Andromedae). While
Mizar's pair is white, the others appear as nicely colored. Among
the lesser known beautiful pairs is the Gamma star of Delphinus, the Dolphini, which consists of a fifth
magnitude (5.14) class F (F7) dwarf called Gamma-1 (as it is the
western of the two) coupled with a fourth magnitude (4.27) class K
(K1) subgiant called Gamma-2, the two now 9.2 seconds of arc apart.
Together, they make a combined bright fourth magnitude (3.87)
naked-eye star that places Gamma Del third in brightness within the
constellation. When seen individually, star colors are subtle.
Put stars together into a pair, however, and the eye sees the
colors notably enhanced, the Albireo duo appearing a shimmering
orange and blue. While the fainter of the Gamma Del pair (Gamma-1)
is really white, the brighter is a pale yellow orange. As a
result, the fainter has been described as yellow, green, or blue.
Even "reddish-yellow" and "greyish-lilac" have been ascribed to the
two. They are a true physical pair that lies 104 light years away.
Physically, Gamma-1 and Gamma-2 have respective temperatures of
6060 and 4700 Kelvin, luminosities of 7.5 and 26 times solar, and
radii of 2.5 and 7.5 solar. The cooler, Gamma-2, is the brighter
not only because its mass of 1.7 times that of the Sun is somewhat higher than Gamma-1's, but it
has also recently given up hydrogen fusion in its core and is now
in the process of expanding and slowly brightening with a dead
helium core as it works toward becoming a helium-fusing giant.
Gamma-1, with a mass of 1.5 solar, is a little behind Gamma 2 in
its life-cycle. Though still fusing hydrogen in its core, it does
not have long to go before it follows its mate in becoming a giant,
the pair about two billion years old. Over the past 200 years we
have watched them move enough around each other to enable an
approximate orbit to be calculated. They revolve with a period of
3200 years at an average distance of 330 Astronomical Units. A
high eccentricity makes them as much as 600 AU apart, and brings
them as close as 40. On the average, from each one, the other
would appear as bright as 100 of our full Moons.