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's exceptionally striking when compared to nearby white stars. In fact, Mu Cephei is an extremely luminous red supergiant and one of the reddest known stars of all. It may be the largest star visible to the naked eye with an estimated radius of 1.15 billion kilometres (710 million miles) or 1,650 times that of the Sun.
Mu Cephei is located in the far northern constellation of Cepheus. With a declination of +58 degrees, the Garnet star is well placed for Northern Hemisphere based observers and is circumpolar from latitudes greater than 32N. In major cities such as London, Paris, Moscow and New York it never sets. For sky watchers south of 32S, this star never rises at all.
In 1848, English astronomer John Russell Hind discovered that Mu Cephei is a variable star, which was later 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 brightness of magnitude +5.1, it's more challenging to spot. The average magnitude is +4.1. Since 1881, the variability of Mu Cephei has been continuously monitored.
Mu Cephei has a spectral type of M2Ia. As with other red supergiants accurately determining its distance 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 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 a size comparison with a similar but closer star, such as Betelgeuse. Using this technique, Perrin et al in 2005 estimated the distance of Mu Cephei to be 1,272 ± 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 1,870 ± 323 light-years (573 ± 99 parsecs).
Assuming a distance of 1,870 light-years, Mu Cephei is incredibly large and if located at the centre of the 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. With an average magnitude of +5.0, it's a magnitude fainter than Mu Cephei.
In astronomical terms, Mu Cephei doesn't have long to live. It has almost certainly stopped internal hydrogen fusion, and is likely fusing helium into carbon. Whatever the current state, the Garnet star will almost certainly go bang sometime in the "near" future and destroy itself in a massive supernova explosion.
Mu Cephei Data Table
|Name||Mu Cephei (μ Cep)|
|RA (J2000)||21h 43m 30.46s|
|DEC (J2000)||+58d 46m 48.17s|
|Apparent Magnitude (Average)||+4.08|
|Apparent Magnitude (Range)||+3.4 -> +5.1|
|Period (years)||approx. 2.0 -> 2.5|
|Distance (light-years)||1,870 ± 323|
|Temperature (K)||3,690 ± 50|
|Spectral type||M2 Ia|
|Other Designations||Herschel's Garnet Star, Erakis, HR 8316, BD+58 2316, SAO 33693|