Diadem (Alpha (α) Comae Berenices) is the principal star of Coma Berenices, Queen Berenice’s Hair.
The hair in question belonged to Queen Berenice II of Egypt, who pledged it to the gods in exchange for the safe return of her husband, King Ptolemy III Euergetes.
The star is located in the southeast corner of Coma Berenices, 15° to the west of the bright, orange giant, Arcturus (Alpha (α) Bootis).
At mag. 4.3, Diadem is the second-brightest star in Coma Berenices after Beta (β), located 10.4° to the north.
Coma Berenices is characterised by the large, triangular-shaped open cluster, Melotte 111, which is located further to the west. It’s the faint cluster stars that represent the queen’s hair.
Diadem and Beta Comae Berenices are not part of the cluster.
Diadem is derived from the Arabic meaning ‘the braid’. It’s a binary star consisting of two equally bright components of magnitudes +5.1.
The orbital period for the binary is relatively short at 25.87 years. From Earth, the maximum separation we see between both stars is 0.7 arcseconds.
When closest, they appear inseparable, the inclination of their orbital plane being just one-tenth of a degree.
The last close passage occurred in 2001, which means the pair are currently closing for another.
As well as sharing the same brightness, the two binary components of Diadem are F5 dwarfs, both shining with a warm, yellowish hue.
This guide originally appeared in the May 2023 issue of BBC Sky at Night Magazine.
