Double Star of the Month:
Alpha Canum Venaticorum
AKA: Cor Caroli; STF 1692.
Position: 12 hrs 56 min 01.7 sec +38 degrees 19:min 06 sec.
Due south at 22:22 (BST) on 15 May 2021.
Image credit: Jeremy Perez (http://www.perezmedia.net/beltofvenus)
Used with permission
Having given you a fairly difficult double last month, May’s double star is both easy to find and easy to split. The bright 2.8 magnitude Alpha Canum Venaticorum lies roughly halfway between Phad in the bowl of the Plough and Arcturus in a field otherwise devoid of bright stars. It also lies between Alkaid the star at the end of the handle of the Plough and Denebola in the tail of Leo. The primary is magnitude 2.9 and the secondary is a much dimmer 5.5. However the relatively wide separation of 19.5 arcseconds makes this an easy double to split. The primary is spectral type A0 and is the archetype of an unusual type of variable star. It is both large and strongly magnetised and produces very large starspots. These large starspots are the reason for its slight variation between magnitudes 2.85 and 3. The secondary is just an ordinary F type star. One might expect it to be a blue and yellow double, but in practice there is little agreement about their colours, although the main star is usually considered to be white. Personally I see them as white and cobalt blue.
The most curious aspect of this double is its popular (and now official) name Cor Caroli (heart of Charles). This name was given to it by Sir Charles Scarborough, physician to Charles II who claimed that it shone particularly brightly on Charles II’s return to England in 1660. But does the name refer to Charles II or his father who was beheaded in 1649? Some writers still state that this name refers to the son, but it seems clear that it was intended to honour Charles I, who was considered by the Royalists to be a martyr. Ian Ridpath has pointed out that the first reference to the name on a star chart (in 1673) specifically refers to the heart of Charles the martyred king.