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.
Written by Jim Kaler. Return to STARS.