GARNET STAR (Mu Cephei). We tend, rather obviously, to admire the bright first and second magnitude naked-eye stars and to pay little attention to those of fainter rank. But bring a pair of binoculars outdoors in the northern-hemisphere autumn and early winter and scan around within southern Cepheus, the King, husband of Cassiopeia, father of Andromeda. There you will discover a distinctively reddish star, one more obviously colored than the others. Only mid-fourth magnitude (4.08), the star is too faint to have a classical proper name and falls way down the Greek letter scale, Bayer in the 1600s designating it "Mu Cephei." Though sometimes known as "Erakis," it is more familiarly referred to as "Herschel's Garnet Star," the name honoring both the star's deep color and Sir William Herschel, who in 1781 discovered the planet Uranus and who also founded modern observational astronomy with vast numbers of other discoveries that included infrared radiation. Mu Cephei -- the Garnet -- has a magnificence all out of proportion to its seemingly fainter status. As a red class M (M2, but which has also been classed M1) bright supergiant with a low temperature of 3700 Kelvin, it is one of the larger stars visible. Indeed, it is one of the largest and most luminous stars that can be seen not only with the naked eye, but in the entire Galaxy. Its distance, too far for parallax, is uncertain, but from its traditional membership in the Cepheus OB2 association of hot stars is around 2400 light years. Even at that distance, Mu Cephei is big enough that astronomers have been able to measure its angular diameter at 0.021 seconds of arc, giving it a radius 1650 times that of the Sun, 7.7 Astronomical Units. If it replaced the Sun, it would extend midway between the orbits of Jupiter and Saturn. Part of the star's visual color comes from the absorption and reddening of its light by the Milky Way's interstellar dust. Were no dust present, the star would shine at mid-second magnitude (1.97). Mu Cep's distance (corrected apparent brightness) and temperature (so as to allow for a lot of infrared radiation, making Herschel's name very appropriate) give a luminosity 353,000 times that of the Sun and a radius of 1450 Suns. Better agreement is found with the radius measured from angular diameter if the distance is boosted to 2800 light years (an earlier estimate for Cepheus OB2). The luminosity then comes in at 475,000 Suns. (Oddly, one study shows the star NOT to be a member of the association.) As big as it is, the Garnet Star is rivalled by a constellation-mate, dimmer (by almost a magnitude) VV Cephei, which is an eclipsing double whose eclipses tell us of a star with a radius between 7.5 and 8.8 AU. As is the case with most huge supergiants, The Garnet Star cannot quite find a place for itself, and is variable, wobbling in brightness by a over a magnitude in a semi-regular manner with periods of 2 to 2.5 years, the average magnitude varying over a period of a decade or so, the star dipping as faint as fifth magnitude. At the same time it is losing mass through a strong
Mu Cephei varies by nearly a magnitude with a semi-regular period of 800-1000 days. The visual magnitude scale on the left is relative only. The lower axis gives the Julian Day number (2440000+number given), which is a running count since January 1, 4713 BC on the Julian calendar. The dates span March 10, 1986 to November 11, 1999. (From J. R. Percy, J. B. Wilson, and G. W. Henry, Publ. Astr. Pacific vol. 113, p. 983, 2001.)
wind that has enshrouded it in a dusty shell that extends up to 15,000 AU from the star (and which contributes to the reddening). Although we know Mu Cephei has ceased internal hydrogen fusion and is dying, we cannot quite be sure of its internal status. Odds are it is now fusing its core helium into carbon. Whatever the conditions, this great star, which began life containing perhaps 30 or so solar masses, is fated to explode as a grand supernova. Which will go first, Mu or VV? Keep your eye on celestial King Cepheus, and maybe we will see. (Thanks to Jeff, who suggested this star.)
Written by Jim Kaler. Return to STARS.