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ERAKIS (The 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 northern autumn early winter and scan around within southern Cepheus, the King, husband of Cassiopeia, father of Andromeda. There you may discover for yourself a distinctively reddish star, one more obviously colored than the others. Only mid-fourth magnitude, 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 low temperature (abut 3500 Kelvin) supergiant, it must be 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 association with other stars is around 2700 light years. Even at that distance, Mu Cephei is big enough that astronomers have been able to measure its angular diameter at around 0.02 seconds of arc, making it 15 astronomical units across. If it replaced the Sun, it would extend midway between the orbits of Jupiter and Saturn. Its distance and apparent brightness suggest an extraordinary luminosity a quarter million or more times that of the Sun(!), from which we derive a similar radius. Given that stars this big have ill-defined edges, it could be even larger. Yet it does not actually set the record, which for now belongs to a constellation- mate, dimmer (by almost a magnitude) VV Cephei, which is an eclipsing double whose eclipses tell us of a star that would fill Saturn's orbit. 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 somewhat irregular manner over periods of 2 to 2.5 years, the average

The Garnet Star is seen here to vary by nearly a magnitude over the nearly 14 years of observation. The scale on the bottom is the "Julian Date" of 2440000 plus the number that appears, where the Julian Date is the number of days since January 1, 4713 BC of the Julian Calendar and is commonly used for variable phenomena in astronomy. JD 2446500 corresponds to March 11, 1986. The left-hand scale expresses the difference between the apparent visual magnitude of "Garnet" Mu Cephei and a nearby comparison star. (From an article in the Publications of the Astronomical Society of the Pacific by J. R. Percy, J. B. Wilson, and G. W. Henry.)

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 wind. 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 20 to 25 solar masses almost certainly 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.)