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Mu Cephei - Herschel's Garnet Star

When Sir William Herschel observed Mu Cephei in 1783 he described it as a most beautiful object of a very fine deep garnet colour that is exceptionally striking when compared to nearby white stars. In fact Mu Cephei is an extremely luminous red supergiant and one of the reddest of all known stars. The star may even be the largest star visible to the naked eye with an estimated radius of 1.15 billion kilometres (710 million miles) or 1650 times that of the Sun.

Location

Mu Cephei is located in the far northern constellation of Cepheus "The King". With a declination of +58 degrees the Garnet star is circumpolar from latitude 32N and is therefore easily visible and sometimes almost directly overhead for many northern hemisphere observers. In major cities such as London, Paris, Moscow and New York it never sets. For sky watchers south of 32S, Mu Cephei never rises.

Mu Cephei - Herschel's Garnet Finder Star Chart - pdf format

Variability

In 1848, English astronomer John Russell Hind discovered that Mu Cephei is a variable star, which was subsequently confirmed by German astronomer Friedrich Wilhelm Argelander. When at peak brightness of magnitude 3.4, the star is easily visible to the naked eye whereas at minimum it is a little more challenging at magnitude 5.1. The average magnitude is about 4.1. Since 1881 the variability of Mu Cephei has been continuously monitored.

Stellar Classification, Distance and Size

Mu Cephei is a red supergiant star of type M2 Ia. As with other red supergiants measuring accurately the distance to Mu Cephei is difficult. The Hipparcos satellite measured a parallax of 0.62 ± 0.52 milliarcseconds, which corresponds to a distance of about 5,258 light-years (1,612 parsecs). However, the margin of error is extremely large and based on the Hipparcos observations, Mu Cephei may be as close as 2,863 light-years or as far as way as 32,638 light-years! An alternative method is to make a size comparison with a similar but much closer star, Betelgeuse for example. Using this technique, Perrin et al in 2005 estimated the distance of Mu Cephei to be 1272 ± 457 light-years (390 ± 140 parsecs). In the same year, a maximum likelihood estimate of the distance using a kinematics study by Famaey et al gave a value of 1870 ± 323 light-years (573 ± 99 parsecs).

Assuming a distance of 1870 light-years, Mu Cephei is so large that if it were at the centre of our solar system it would reach somewhere between the orbits of Jupiter and Saturn. Currently, there are believed to be only a few known stars that are larger than Mu Cephei. These include VY Canis Majoris, KW Sagittarii, KY Cygni, V354 Cephei and VV Cephei. Of these, only VV Cephei is visible to the naked eye, but with an average magnitude of around 5.0 it is usually on average 1 magnitude fainter than Mu Cephei.

Supernova

In astronomical terms, Mu Cephei does not have long to live. The star has almost certainly stopped internal hydrogen fusion and is likely fusing helium in its core into carbon. Whatever the current state the Garnet star will almost certainly go bang and destroy itself in a massive supernova explosion.

Mu Cephei Data Table