Star of the Month
Mu Cephei
AKA: Herschel’s Garnet Star.
Position: 21 hrs 43 min 30 sec 58 degrees 46 min 48 sec
Due south at 23:03 (BST) on 15 September 2020
Mu Cephei
Image: Simbad (http://simbad.u-strasbg.fr/simbad/)
Mu Cephei is big, being about 1000 times larger than the Sun; it is bright, its absolute magnitude is -7.6 making it 100,000 times brighter than the Sun; and it is red with a B-V value of 2.35 (which is much less than R Leporis however). It is not surprising to discover that it is a red supergiant or even a red hypergiant. From previous stars of the month, there are two things that we know about red supergiants: they are variable and they are on the way out. Regarding its variability, it is very much like its close relative Betelgeuse: its variability is rather erratic and it has a short period of about 860 days and a long period of 4,400 days which is twelve years. Its magnitude ranges (or more accurately staggers) between 3.4 and 5.1 which in terms of observing in Havering is the difference between seeing it with the naked eye fairly easily and hardly seeing it at all. Cepheus lies between Deneb and Polaris and has the shape of a triangle sitting on a square. If you look at the bottom two stars of the square, Alpha Cephei (Alderamin) and Zeta Cephei, Mu Cephei lies between them (but closer to Zeta) and below the square. An alternative method to locate it is to take the line (actually a slight curve) between Beta, Gamma and Alpha Cygni (the upright of the Northern Cross) and continue it upwards; Mu Cephei is the next bright star. It should stand out because of its redness. Its fame really lies in the description of the star as the Garnet Star by William Herschel. He probably first noted its colour when making double observations near it in September 1782 (it is not itself a double star). He used the term garnet frequently when describing double stars and this colour was used by later Victorian observers as well, but only this star was The Garnet Star. Garnet is basically deep red (after the gemstone of the same name which in fact can have several colours), but it is not much used by modern observers, perhaps because of the decline of costume jewellery. However as we have noted in passing it is not especially red. The variability of Mu Cephei was first observed by John Russell Hind (of R Leporis fame) in 1848. We all know that red supergiants end with a bang, but what kind of bang in the case of Mu Cephei? Because it is so large, it may become a Wolf-Rayet star which will create a rare Ib or Ic supernova, with possible fireworks such as a gamma ray burst. This may happen next week or it may be in the next million years, who can say? In the meantime look up and admire the Garnet Star while it is still around.