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MENKAR

(Alpha Ceti). Cetus, the Sea Monster (or Whale), appears to move backwards across the sky, his tail (illuminated by Deneb Kaitos) rising first, Menkar, at the leading (eastern) edge of his head, rising last. Though Menkar represents the jaws of the great beast, it inappropriately received a name that from Arabic means "the nose" or "nostrils." Just across the line from second magnitude to third (2.53), Menkar is the second brightest star in the constellation, first place going to solid second magnitude Deneb Kaitos. Nevertheless, Bayer gave Menkar the Alpha designation and Deneb Kaitos Beta quite clearly as a result of the positioning of the two stars. This brightness ranking of course excludes Cetus's great variable Mira, which on rare occasion can exceed the brightness of either of them. Menkar is a cool red (class M) giant with a temperature of only about 3700 Kelvin. From its distance of 220 light years, we find a visual luminosity, as it appears to the eye, 380 times that of the Sun. However, because Menkar is so cool, it radiates much of its light in the infrared where we cannot see it. If that is taken into account, Menkar becomes some five times brighter, its total energy release 1800 times solar. It is a true giant, temperature and luminosity combining to give a radius 84 times that of the Sun. Its size and relative proximity also allow measure of angular diameter (0.0116 seconds of arc), which yields a slightly smaller (but still satisfyingly close) physical radius 77 times solar, about the size of Mercury's orbit. The star has a variety of characteristics that rather set it apart. It is a light irregular variable, changing its brightness erratically by about six percent. (It is technically known as an "Lb," or "giant irregular," variable). Menkar is also notably deficient in carbon, having less than 20% the amount found in the Sun. More interesting perhaps is that it is also observed as a source of radio waves, the radiation coming from a cool wind that blows from the star's surface. Though its exact evolutionary status is uncertain, it is surely dying. The best estimate is that it began life with about three times the mass of the Sun, has quit fusing hydrogen to helium in its core (as the Sun is doing now), has already passed the stage in which it fuses the helium to carbon, and is now entering the stage in which the carbon core contracts. Menkar is then on its way to becoming another great unstable variable like its constellation-mate Mira, its fate to lose its outer hydrogen envelope and turn into a fairly massive white dwarf similar to the companion to Sirius.