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

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.

Mu Cephei - Herschel's Garnet Star (ESO Digitized Survey)

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

Name Mu Cephei (µ Cep)
HD 206936
HIP 107259
Constellation Cepheus
RA (J2000) 21h 43m 30.46s
Dec (J2000) +58d 46m 48.17s
Apparent Magnitude (Average)4.08 (v)
Apparent Magnitude (Range)3.4 -> 5.1
Absolute Magnitude -7.0
Period (years)approx. 2.0 -> 2.5
Distance (light-years) 1870 ± 323
Mass (Solar)15
Radius (Solar)1650
Luminosity (Solar)60000
Temperature (K)3690 ± 50
Spectral type M2 Ia
Other DesignationsHerschel's Garnet Star, Erakis, HR 8316, BD+58 2316, SAO 33693

Sky Highlights - March 2017

Comet
Comet 41P/Tuttle-Giacobini-Kresak now visible with binoculars as it heads towards perihelion

Mercury
Mercury heading towards greatest elongation east

Minor Planet
Vesta now visible with binoculars and small telescopes.

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for March 2017

Northern Hemisphere
Evening
West:- Venus (mag. -4.8 to -4.1 - first half of month), Mars (mag. +1.3 to +1.5), Uranus (mag. +5.9), Mercury (mag. -1.5 to -0.4 - second half of month)
Midnight
Southeast:- Jupiter (mag. -2.3 to -2.5)
Morning
Southwest:- Jupiter
Southeast:- Saturn (mag. +0.5)

Southern Hemisphere
Evening
West:- Venus (first half of month), Mars, Uranus
Midnight
North:- Jupiter
East:- Saturn
Morning
West:- Jupiter
Northeast:- Saturn
East:- Neptune (mag. +8.0 - second half of month)

Deep Sky
Naked eye / binoculars:-
Melotte 111 - Mel 111 - The Coma Star Cluster (Open Cluster)
Messier 44 - M44 - The Praesepe (Open Cluster)

Telescopes:-
Messier 67 - M67 - Open Cluster
Messier 51 - M51 - The Whirlpool Galaxy (Spiral Galaxy)
Messier 97 - M97 - The Owl Nebula (Planetary Nebula)
Messier 101 - M101 - The Pinwheel Galaxy (Spiral Galaxy)
Messier 65 – M65 – Spiral Galaxy
Messier 66 - M66 - Intermediate Spiral Galaxy
Messier 95 - M95 - Barred Spiral Galaxy
Messier 96 - M96 - Intermediate Spiral Galaxy
NGC 4244 - Spiral Galaxy
NGC 4565 - Needle Galaxy - Spiral Galaxy

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