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.