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.