GAMMA PER (Gamma Persei). The naming of stars at times seems to have little to do with their brightnesses. All the first magnitude stars, and most of the second magnitude visible from classical lands, have names (an outstanding exception being Gamma Cassiopeiae), but from there on the naming is erratic. Gamma Persei (though actually fifth brightest in Perseus)carries no known western name even though it lies at mid-third magnitude (2.93). One reference calls it "Algenib," an alternative name for Mirfak (Alpha Persei), but that is surely a mistake. And a pity too, as the star fascinates. Lying about 225 light years away, it is a close (only barely separable) double that consists of a class G (G8, temperature 4900 Kelvin) giant coupled with an ordinary main sequence class A (A2, 9000 Kelvin) dwarf, which together shine 300 times more brightly than the Sun, the yellow giant notably the brighter of the two. Rather overwhelmed in Perseus by Algol, the brightest eclipsing double star in the sky, Gamma Per takes its fame from being the second brightest eclipser, a fact only recently discovered. The star was long known from spectroscopic (Doppler) observations to be a double that takes 14.6 years to orbit. Moreover, the plane of the orbit was found to lie tantalizingly in the line of sight, presenting the small possibility that the two might actually eclipse each other. Diligent observations discovered the eclipse (which produces a 30 percent dip in the light output of the system and is just visible to the naked eye) only in 1990. The event, in which the giant gets in the way of the dwarf, takes somewhat under two weeks. Analysis of the observations shows the average separation of the stars to be 10 Astronomical Units. The orbit, however, is highly elliptical (almost as much as that of Sheratan, Beta Arietis), the stars moving from a maximum separation of 18 AU to a minimum of only 2 AU. The brighter giant has a mass 2.5 times that of the Sun, while the dimmer A star weighs in at 1.9 solar masses. The giant, like the brighter component of Capella, is a helium-fusing giant, while the lower mass star is still -- like the Sun -- fusing hydrogen to helium in its core. The age of the pair seems to be around 1.9 billion years. The chances of getting an eclipse visible on Earth from such a wide separation of the component stars is quite small. Algol eclipses every 2.87 days. Gamma Per's last eclipse was in 2005, but at that time the star was so close to the Sun that it was very difficult to see. The next one will be in 2019.
Written by Jim Kaler 1/05/01. Return to STARS.