At magnitude 4.32, Alpha Comae Berenices is not quite the brightest star in Coma Berenices (Berenices Hair) - it is just fainter than β Com. α and β Com are both off to the eastern side of the constellation, away form the beautiful cluster which is the prominent portion of Coma Berenices.
The name "Diadem," meaning a jeweled crown in the hair, is of modern and unknown origin, and is not commonly used.
Components
α Com is a close double; its two ordinary class F5 hydrogen-fusing stars are almost identical to each other, with apparent magnitudes of 5.05 and 5.08. At maximum separation, they are 0.7" apart, while at closest passage (as during the year 2000) they are effectively inseparable.
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 26 years. The orbital tilt - a mere tenth of a degree to our line of sight - is probably enough to keep the stars from eclipsing each other, however. Averaging 12 AU apart, they come as close as 6 AU and as far apart as 19 AU.
The closeness of the pair makes distance measure by parallax from space nearly impossible. The old ground-based result of 65 light years, however, gives the stars just the luminosities expected of F5 dwarfs, so it must be very close to correct. With temperatures of 6500 K, α Com's stars are both 2.5 times more luminous than the Sun, and have masses about 25% greater.
The stars' luminosities and temperatures suggest that they are still quite young. At least one is magnetically active, like the Sun, with a rotation period of only 3 days; the fast spin generates its magnetic field. Their characteristics are close enough that they might even evolve together to produce a rare double red giant - and finally an orbiting pair of identical lower-mass white dwarfs.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]