Mu Cephei

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Mu Cephei
Observational Data
Designation Garnet's Star
HD 206936
Mu. Cep
Right ascension 21h 43m 30.4611s[1][2]
Declination +58° 46′ 48.1602″[1][2]
Constellation Cepheus
Type of object Supergiant star
Magnitude Apparent mag: +4.23[3]
Absolute Mag: -6.81 / -7.07[3]
Distance from Earth 5,930.24 ly[3]
Radial velocity 23.00 ± 0.58 km/s[1][4]
Proper motion RA: 5.36 mas/yr[1][2]
Dec: -3.51 mas/yr[1][2]
Parallax 0.55 ± 0.2 mas[1][2]

Mu Cephei (Mu. Cep, Erakis, HD 206936) is a supergiant star in the constellation of Cepheus.[5] The star is known for being the reddest star visible to the unaided eye and was first observed by Johann Bayer who gave it the Bayer designation "Mu Cephei." The star can be seen high in the sky in summer and lies in the nebula IC 1396, making it an excellent target for astrophotographers. Although it can be seen with the unaided eye, it is better observed with binoculars. It is also a good target to observe in skies affected by light pollution.[6]


Mu Cephei was first observed in 1600 by Johann Bayer.[3] It was later observed by William Herschel in 1783, who noted the star possessed "a very fine deep garnet color." For this reason the star is sometimes called the "Garnet star" or "Herschel's Garnet star." The name is the for the star. John Russell Hind observed the star in 1848 and discovered that is was a variable star, meaning its brightness changes over time.

Properties and Structure

The star is situated 5,930.24 light years away, though some sources suggest it is closer at around only 2,400 ly.[3][7] The star is a class M bright supergiant with a spectral class of M2Ia and one of the largest stars ever observed. The radius of Mu Cephei is 1,650 times greater than the Sun, meaning it is roughly 7.7 times larger than the orbit of the Earth around the Sun.[7] Mu Cephei has a mass of 19 solar masses and a luminosity of 600,000 solar luminosities.[6] This high luminosity means it is not only one of the brightest stars visible in the night sky but also the galaxy. The star is believed to have a surface temperature of 3,690±50 kelvin.[8]

The star deep red color is partly due to a cloud of interstellar dust located in between the star and Earth.[9] The dust absorbs bluer light more strongly than redder light, meaning there is light that reaches us on Earth than blue, and so the star appears red. If the dust was not present, the star would appear significantly brighter with an apparent magnitude of +1.97 rather than the observed +4.23.[9] The red color is also due to the star itself emitting more light in the red region of the spectrum than the blue. A third factor is the Purkinje effect, an optical illusion where the longer someone looks a red star, the brighter it appears to become.

As a variable star, its apparent magnitude varies from a minimum of +4.142 to a maximum of +3.622 and it varies in a semi-regular cycle.[3] Mu Cephei varies over a 755 day (2.5 year) cycle on average though other cycles ranging from 100 days to 12.8 years are known.[6] It is classified as a Semiregular late-M type variable star. It was the first of this type of variable star to be discovered.[7]

Mu Cephei is known to produce a strong stellar wind and is enclosed in a dusty shell that envelopes the star up to a distance of 15,000 Astronomical units.[9] This shell contributes to some of the reddening.


  1. 1.0 1.1 1.2 1.3 1.4 1.5 Mu Cephei. Simbad Astronomical Database. Retrieved on December 28, 2019.
  2. 2.0 2.1 2.2 2.3 2.4 van Leeuwen, Floor (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics 474 (2): 653-664. doi:10.1051/0004-6361:20078357. Bibcode2007A&A...474..653V.  arXiv:0708.1752
  3. 3.0 3.1 3.2 3.3 3.4 3.5 Herchel's Garnet Star (Mu Cephei) Star Facts. (February 12, 2017). Retrieved on December 28, 2019.
  4. Mermilliod, J. C.; Mayor, M.; Udry, S. (2008). "Red giants in open clusters. XIV. Mean radial velocities for 1309 stars and 166 open clusters". Astronomy and Astrophysics 485 (1): 303-314. doi:10.1051/0004-6361:200809664. Bibcode2008A&A...485..303M. 
  5. Kate Davis. Mu Cephei. American Association of Variable Star Observers. Retrieved on December 28, 2019.
  6. 6.0 6.1 6.2 David Dickinson (July 16, 2013). Seeing Red: Hunting Herschel’s Garnet Star. universetoday. Retrieved on December 28, 2019.
  7. 7.0 7.1 7.2 Cepheus Constellation. Retrieved on December 28, 2019.
  8. Mu Cephei - Herschel's Garnet Star. Retrieved on December 28, 2019.
  9. 9.0 9.1 9.2 Jim Kaler (September 20, 2017). Garnet star. Stars. Retrieved on December 28, 2019.