Garnet Star (Mu Cephei)
Garnet Star (Mu Cephei) is a star in the constellation Cepheus, magnitude 4.1.
Herschel’s Garnet Star: one of the largest stars visible to the naked eye, glowing like a dark red gem.
William Herschel saw in it ‘a very fine deep garnet colour’, and the name stuck: the IAU made Garnet Star official in 2024. Few stars visible to the naked eye are larger. Recent estimates put it at roughly 760 to over 1,200 times the Sun’s radius; set in the Sun’s place, it would swallow the orbit of Mars and the asteroid belt.
This red supergiant shines more than 100,000 times as brightly as the Sun, and it wanders erratically between magnitudes 3.4 and 5.1, with rough rhythms of about 860 and 4,400 days. John Russell Hind noticed the changes in 1848, and observers have tracked this variable star almost without a break since 1881. It is also a runaway, ploughing through space at about 80 kilometres a second.
Its end is near, as stars go. It has begun fusing helium into carbon, and it is wrapped in a shell of gas it threw off some 2,000 to 3,000 years ago. When its core finally collapses it will explode as a supernova, most likely leaving behind a black hole.
Garnet Star (Mu Cephei) is 1,400 light-years away: the light we see from it set off around AD 630, 90 years after the Hagia Sophia was completed in Constantinople.
Did you know?
- A yardstick for red supergiants. Since 1943 the Garnet Star has been one of the ‘dagger stars’ — standards for classifying other stars. Today it defines the M2- Ia class of red supergiants.
- Erakis, by mistake. Some catalogues call it Erakis, but that name probably comes from a mix-up with Mu Draconis, once known in Arabic as al-Rāqis.
- A ring of dust and water. Infrared observations suggest a wide ring of dust and water around the star, beginning at about twice the star’s own radius.
A deep orange-red 4th-magnitude star a little south of the midpoint between Alderamin and Zeta Cephei, on the edge of the faint nebula IC 1396. Binoculars show its colour best — compare it with white Nu Cephei, two degrees to the north.
When and where to see it
It is best placed in the evening sky around September, when it crosses the meridian at about 10 pm local time (around midnight in August). It can be seen from everywhere north of 31°S and never sets north of 31°N.
At its highest it stands 81° up from 50°N, 66° up from 35°N, 31° up from the equator, below the horizon from 35°S.
Facts
- Type
- Star
- Constellation
- Cepheus
- Designations
- Mu Cephei, Herschel's Garnet Star, Erakis, μ Cep, HR 8316, HD 206936, HIP 107259
- Magnitude
- 4.1
- Position
- RA 21h 43.5m, Dec +58° 47′ (J2000)
- Spectral type
- M2-Ia
- Colour
- deep red (B−V 2.35)
- Brightness
- varies from magnitude 3.4 to 5.1
- Distance
- 1,400 light-years
- Discovered
- John Russell Hind, 1848, Discoverer of its variability; the star itself is visible to the naked eye
Open AstroPilot to see when Garnet Star (Mu Cephei) is highest tonight from your location, how easy it is with your telescope or binoculars, and an eyepiece view.