The Garnet Star
The Garnet star (so named by William Herschel from its unusual shade of red), otherwise known as m Cephi, is a red supergiant in the northern constellation of Cepheus. The Garnet star is nearly circumpolar from Houston, Texas located at RA: 21h 43m 30.3 sec DEC: +58° 46' 48", m = + 4.08. It transits the Houston area meridian at around 11:30 PM in September at an altitude of 61°. m Cephi is also classified as an semi irregular variable with a two superimposed periods of 850 and 4400 days with a 1.4 mag amplitude. It’s variability seems to have been first identified by J.R. Hind in 1848. See the light curve plot below from The Observer's Handbook 2004 p. 262 of m Cephi based on 33,576 observations covering a 50 year time period.
m Cephi is one of the reddest stars visible to the naked eye in the northern hemisphere, with a color index of +2.26. Previous distance estimates placed m Cephi between 250 and 350 parsecs (800-1,200 light years) however new data from the HIPPARCOS satellite place it at about 1,600 parsecs (5,200 light years). Prior computations of its distance may have been unreliable due to its location near the edge of an extensive nebulous region known as IC 1396. Possibly some of its light may have been absorbed by the obscuring clouds of IC 1396. Its spectral type is M2 Ia and its computed luminosity is a staggering 42,000 L¤ !! Its estimated diameter is 1,224 D¤ making it perhaps the largest known star. This huge star places its size extending past the orbit of Saturn.
Figure 2. Comparative size of m Cephi to our Solar System.
Supergiant stars have many of the same characteristics: Late spectral type K- M, cool surface temperatures (2000 - 3500° K) and extremely luminous, being many thousands of times more luminous than our Sun. The ten largest known stars are given in the following table (using HIPPARCOS distances):
STAR DIAM, D¤ LUM, L¤ (HIP) SPECTRA
m Cep 1224 41,612 M2 Ia
V509 CAS 910 1,794 G4 Ov
a ORI 800 8,932 M2 Ib
V382 CAR 747 69,707 G0 Ia
r CAS 738 157,646 G2 Oe
a SCO 600 10,167 M1 Ib
y1 AUR 511 11,726 K5 Iab var
o1 Cma 482 8,036 K3 Iab
j CAS 460 4,205 F0 Ia
Another interesting fact reported by Burnham (p. 597) was the detection of strong water vapor bands (steam !!) in the spectra of m Cephi. This unusual find has only been seen in just a handful of stars and remains unexplained. A finder chart for m Cephi is shown below as Figure 3.
Figure 3. Finder chart for m Cephi
--from James Kaler... Star of the week http://www.astro.uiuc.edu/~kaler/sow/garnet.html
THE GARNET STAR. 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 "m 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 3,500 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, m 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 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 m 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.)
REFERENCES:
Burnham, R., 1976, Burnham’s Celestial Handbook, Dover Publications, 1976, p. 596 - 597.
Dibon-Smith, R., 1992, StarList 2000, John Wiley & Sons, Inc., New York, p. 61.
Dibon-Smith, R., 1998, The Flamsteed Collection, Clear Skies Publishing Company, Toronto, Canada, p. 70.
Gupta, R.editor, 2004, Observer's Handbook 2004, University of Toronto Press, p. 261-262.