Star of the Month
Y Canum Venaticorum
AKA: La Superba; HD 110914.
Position: 12 hrs 45 min 07.8 sec 45 degrees 26 min 25 sec
Due south at 23.39 (BST) on 15 April 2021
Y Canum Venaticorum
Image: Simbad (http://simbad.u-strasbg.fr/simbad/)
If you have been following the Star of the Month regularly, you will immediately realise that Y CVn to use its short name is a variable because of the initial letter in its name. You might also surmise that it is a red giant and a carbon star. You would, of course, be right on all three counts. Y CVn is a semi-regular variable with a variation of about a magnitude over a cycle of roughly 160 days, and like other red giants (such as Betelgeuse) it has another underlying cycle of around 190 days. As a red giant it is now in its dotage and it is probably on the brink of giving up altogether and becoming a planetary nebula – indeed it is already surrounded by a 2.5 light year wide shell of ejected material. The core of the star will become a white dwarf. Furthermore it is one of the coolest stars known, the surface temperature being only 2760K. It belongs to a rare class of carbon stars called J stars which contain unusually large amounts of C-13 rather than the usual C-12. It is very red, but it is not (as sometimes claimed) the reddest star in the sky. R Leporis (Hind’s Crimson Star, Star of the Month for January 2020) is much redder. Y CVn has a B-V value of 2.54 (similar to the Garnet Star, Mu Cephei, Star of the Month for September 2020) whereas R Leporis has a B-V of 5.7. Y CVn was given its poetic name, La Superba, by the Jesuit astronomer and pioneer of stellar spectroscopy Angelo Secchi, probably in 1872. As Secchi doubtlessly knew, La Superba was the traditional epithet of the port (and state) of Genoa, just as Venice was La Serenissima. As far as I am aware, no-one has called a star “La Serenissima”! La Superba has recently become the IAU’s official name for this star. It is not easy to find in the sky unfortunately. La Superba’s maximum is about magnitude 4.9 and its minimum is 7.3, so in Havering it is below naked eye brightness. It is about one third of the way along a line between Alpha Canum Venaticorum (Cor Caroli) and Delta Ursae Majoris (Megrez), the star which joins the bowl of the Plough to the handle. You can however spot it by its redness in a field empty of bright stars.