Mu Cephei

Did You Know?

• Mu Cephei is surrounded by a spherical shell of ejected material extending 6″ outward, with an expansion velocity of 10 km/s, indicating an age of about 2,000–3,000 years.
• Closer to the star, the ejected material shows a pronounced asymmetry that may be shaped as a torus.
• Infrared observations suggest a wide ring of dust and water with an inner radius about twice the star's radius, extending to about four times the star's radius.

History

William Herschel noted the deep red color of Mu Cephei, describing it as a very fine deep garnet colour like the periodical star ο Ceti, leading to its common name Herschel's Garnet Star. Giuseppe Piazzi called it Garnet sidus in his catalogue. The name Erakis, used in Antonín Bečvář's star catalogue, likely arose from confusion with Mu Draconis. The IAU Working Group on Star Names approved the name Garnet Star for Mu Cephei on 19 September 2024, and it is now entered in the IAU Catalog of Star Names. John Russell Hind discovered its variability in 1848, quickly confirmed by Friedrich Wilhelm Argelander, with almost continual records kept since 1881. The angular diameter has been measured interferometrically, though one measurement later turned out to be the surrounding molecular layer rather than the star itself. In 1943 it became the standard star for M2 Ia, updated in 1980 to M2-Ia.

Distance

The distance to Mu Cephei is not well known. Hipparcos measured a parallax close to the margin of error, giving an estimated distance. A size comparison with Betelgeuse yields another estimate. Calculation from angular diameter, surface brightness, and luminosity leads to a distance, while averaging distances of nearby luminous stars with reliable Gaia Data Release 2 parallaxes gives a different value. A spectroscopic determination of luminosity combined with interferometric size favors yet another distance.

Properties

Mu Cephei is a very luminous red supergiant, among the largest stars visible to the naked eye and one of the largest known cool supergiants. It is a runaway star with a peculiar velocity and has been described as a hypergiant. Its bolometric luminosity, summed over all wavelengths, makes it one of the most luminous red supergiants in the Milky Way. Effective temperature from colour index relations implies a radius, and other recent publications give similar temperatures. A 2010 estimate based on effective temperature and luminosity gave a radius, while a 2019 paper using a specific distance gave a lower luminosity and radius consistent with Betelgeuse. A 2026 paper estimated a radius based on a distance and angular diameter. Its initial mass is estimated between 15 and 25 solar masses, and it currently loses mass at a rate of 1.3 × 10^−5 solar masses per year.

Supernova

Mu Cephei is in its final life stage, now converting helium into carbon rather than hydrogen into helium. Once its core turns to iron, it will collapse and trigger a supernova, leaving behind a gas cloud and a compact remnant—likely a black hole given the star's mass. Because Mu Cephei is massive enough, it may shift from a red supergiant back into a blue supergiant, luminous blue variable, or Wolf-Rayet star before the collapse. If it remains a post-red supergiant, the supernova will be type IIn or II-b; if it becomes a Wolf-Rayet star, the explosion will be type Ib or Ic.