Mu Cephei (Latinized from μ Cephei, abbreviated Mu Cep or μ Cep), also known as Herschel's Garnet Star, Erakis, or HD 206936, is a red supergiant or hypergiant[4][7] star in the constellation Cepheus.

It is a 4th magnitude star easily visible to the naked eye under good observing conditions.

[19][20] An alternative name, Erakis, used in Antonín Bečvář's star catalogue, is probably due to confusion with Mu Draconis, which was previously called al-Rāqis [arˈraːqis] in Arabic.

[21] In 1848, English astronomer John Russell Hind discovered that Mu Cephei was variable.

The Hipparcos satellite was used to measure a parallax of 0.55±0.20 mas, which corresponds to an estimated distance of 1,800 parsecs.

A determination of the distance based upon a size comparison with Betelgeuse gives an estimate of 390±140 parsecs.

[27][28] The star is surrounded by a spherical shell of ejected material that extends outward to an angular distance of 6″ with an expansion velocity of 10 km s−1.

[4] The bolometric luminosity, summed over all wavelengths, is calculated from integrating the spectral energy distribution (SED) to be 269,000 L☉, making μ Cephei one of the most luminous red supergiants in the Milky Way.

Calculation of the luminosity from a visual and infrared colour relation give 340,000 L☉ and a corresponding radius of 1,420 R☉.

[11] The initial mass of Mu Cephei has been estimated from its position relative to theoretical stellar evolutionary tracks to be between 15 M☉ and 25 M☉.