The reddest star in the sky – Mu Cephei, Herschel’s Garnet Star

Mu Cephei

Name:                                                  The Garnet star

Other names:                                      Herschel’s Garnet star

Constellation:                                     Cepheus

Other ID:                                             Erakis, HD206936, HR8316, SAO33693, HIP107259

Magnitude:                                         4.08 (variable)

Absolute magnitude:                          -7.63

Luminosity, Sun = 1:                          370,000 (possibly as much as 600,000, uncertain figure)

R.A. 2000:                                          21hr 43min 30.4609sec

Dec 2000:                                           +58deg 46min 48.166sec

Spectral type:                                     M2Ia supergiant

Temperature:                                      3,690 +/- 50 K

Mass, solar masses:                            15

Radius, solar radii:                             1650

Distance in light-years:                      2,400 (Kaler – uncertain)


This is the reddest star in the entire night sky and at magnitude 4.08 it is naked-eye from reasonably dark sites too.   The incredible output power of the Garnet Star (check that Luminosity figure!) makes it one of the most luminous stars in the whole of our Milky Way.  Mu Cephei varies by almost a full magnitude with a semiregular period between 800 and 1,000 days. In the accompanying image, north is up and Mu Cephei is the bright star towards top left.  As you can see it sits on the outer edge of the massive IC1396 emission nebula.  At bottom far right you can see the famous “Elephant’s Trunk” nebula.  The constellation Cepheus is a very rich region for imaging interesting deep-sky objects.


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