Mu Cephei (μ Cep, μ Cephei), also known as Herschel's Garnet Star, is a red supergiant star in the constellation Cepheus. It is one of the largest and most luminous stars known in the Milky Way. It appears garnet red and is given the spectral class of M2 Ia. Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified. A very luminous red supergiant, Mu Cephei is the largest star visible to the naked eye, and one of the largest known. It is best seen from the northern hemisphere from August to January. This is a runaway star with a peculiar velocity of 80.7 ± 17.7 km/s. The distance to Mu Cephei is not very well known. The Hipparcos satellite was used to measure a parallax of 0.55 ± 0.20 milliarcseconds, which corresponds to an estimated distance of 1,333–2,857 parsecs. However, this value is close to the margin of error. A determination of the distance based upon a size comparison with Betelgeuse gives an estimate of 390 ± 140 parsecs, so it is clear that Mu Cephei is either a much larger star than Betelgeuse or much closer (and smaller and less luminous) than expected. The star is approximately 1000 times larger than our Sun's solar radius, and were it placed in the Sun's position, its radius would reach between the orbit of Jupiter and Saturn. Mu Cephei could fit almost 1 billion Suns into its volume. Mu Cephei is a variable star and the prototype of the class of the Mu Cephei variables. Its apparent brightness varies without recognizable pattern between magnitude +3.62 and +5 in a period of 2 to 2.5 years. Mu Cephei is visually nearly 100,000 times brighter than the Sun, with an absolute visible magnitude of Mv = −7.6. Combining its absolute visible brightness, its infrared radiation, and correcting for its interstellar extinction gives a luminosity of around 350,000 solar luminosities (bolometric magnitude about −9.1), making it one of the most luminous stars known. Mu Cephei is nearing death. It has begun to fuse helium into carbon, whereas a main sequence star fuses hydrogen into helium. The helium-carbon cycle shows that Mu Cephei is in the last phase of its life and may explode as a supernova 'soon' in astronomical terms, although this might not be for some millions of years. When a supergiant star becomes a supernova, it is destroyed, leaving behind a vast gaseous cloud and a small, dense remnant, which for a star as massive as Mu Cephei may be a black hole. Mu Cephei is currently an unstable star, showing irregular variations in light output, temperature, and size. Emissions from the star suggest the presence of a wide ring of dust and water with outer radius four times that of the star (i.e., 2,600 Solar radii) and inner boundary twice the radius of the star (1,300 Solar radii). Placed in the position of our Sun, its disk would span between 6 astronomical units (within Jupiter's orbital zone) and 12 astronomical units (beyond Saturn's orbit). 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. This indicates an age of about 2000–3000 years for the shell. Closer to the star, this material shows a pronounced asymmetry, which may be shaped as a torus. The star currently has a mass loss rate of a few times 10−7 solar masses per year.