DIADEM (Alpha Comae Berenices). Not quite the brightest star in Coma Berenices (Berenices Hair), the Alpha star just loses out to Beta. The prominent portion of Coma Berenices is the beautiful Coma Berenices cluster. Diadem (Alpha) and Beta are both off to the eastern side of the constellation, and neither is a part of the cluster itself (which is 4.4 times farther away). The name "Diadem," a jewelled crown in the hair, is of modern and unknown origin, and is never really used for the Alpha star, which is commonly known just as Alpha Comae. Alpha Comae is a close double star, its two class F (F5) dwarfs (ordinary hydrogen fusing stars) almost identical to each other, very much as are the twin class F0 (and just slightly warmer) stars of Porrima (Gamma Virginis). Their apparent magnitudes of 5.07 together make Alpha Comae a fourth magnitude (4.3) star. The orbit of the pair is almost exactly edge-on, causing the two to appear to move back and forth in a straight line over a period of 25.85 years. At maximum separation they are not quite a second of arc apart, while at close passage (which takes place during the year 2001) they are effectively inseparable. The orbital tilt, however, a mere tenth of a degree against the line of sight, is enough to keep the stars from eclipsing each other. Averaging 12 astronomical units apart (a bit farther than Saturn is from the Sun), they come as close as 6 AU and go as far apart as 19 AU. The closeness of the pair makes distance measure by parallax (the apparent shift in position as the Earth orbits the Sun) from space (with the Hipparcos satellite) nearly impossible. The old ground-based result, however, of 60 light years gives the stars just the luminosities expected of F5 dwarfs, so it must be very close to correct. With temperatures of 6500 Kelvin, Diadem's stars are each 2.5 times brighter than the Sun and have masses about 25 percent greater, their luminosities and temperatures suggesting that they are still quite young. One at least is magnetically active like the Sun, with a rotation period of only 3 days (the fast spin generating the magnetic field). They are close enough in characteristics that they might even evolve together to produce a rare double giant star. Their mutual fate is to be an orbiting pair of identical lower-mass white dwarfs.
Written by Jim Kaler. Return to STARS.