ZETA MON (Zeta Monocerotis). Almost more belonging to Hydra (the Water Serpent) than to Monoceros (the Unicorn), just three-quarters of a degree from the Mon-Hya border, Zeta Mon as 29 Mon is also Monoceros's last Flamsteed star. At mid- fourth magnitude (4.02) in an equally dim and hardly recognizable constellation, it remains pretty obscure to astronomers of all stripes. Look for it a bit over 10 degrees southwest of Hydra's rather scary head. A close look, however, reveals quite a surprise, a massive, luminous, class G (G2) supergiant, although of the lesser variety. The star's temperature of 5200 Kelvin is almost 600 K cooler than the Sun because of a higher ionization balance caused by the low density in the extended supergiant atmosphere, which brings it back to class G2 (a dwarf at that temperature would be class K1). It's also a whopping distance off, 1060 light years, with an uncertainty range of 65. In spite of the distance, the amount of dimming by interstellar dust seems small; we adopt 0.32 magnitude. Given all this information, Zeta Mon radiates at a rate of 2535 times that of the Sun, from which we calculate a radius 62 times solar, 0.29 Astronomical Units (three- quarters the size of Mercury's orbit). Were it our Sun, it would appear some 30 degrees across in our sky (though given its luminosity, nobody could be here to see it). An equatorial rotation velocity of at least 6.7 kilometers per second leads to a rotation period that could be as long as 1.25 years. The metal (specifically the iron) abundance is closely solar. Theory next reveals a mass that falls between 5.8 and 6.5 solar masses. If Zeta Mon is still expanding with a dead (shrinking) helium core, it takes on the higher value, while if it has already fired its helium to fuse into carbon and oxygen (which, given the time-scales, is much more likely), it assumes the lower. So far as we know stable, Zeta lies just on the cool edge of a zone of temperature and luminosity within which stars pulsate and change their brightnesses as Cepheid variables (the HR diagram's "Instability Strip"). Zeta Mon began life as a hot B2-B3 dwarf some 50-60 million years ago. Not massive enough to explode, it does not have all that long to go before it ceases core helium fusion, after which it will hugely expand, slough off its outer layers through a powerful wind, and transform itself into a massive white dwarf (the old nuclear-burning core) of near a solar mass in the mold of Sirius B. At first seemingly not alone, Zeta has three "companions" hovering around it, 10th magnitude Zeta B and C respectively 33 and 61 seconds of arc away, and 13th magnitude "D" at 38 seconds. Smythe and Chambers in the nineteenth century called the brighter three "a delicate triple star...light yellow, grey, pale blue." Alas, their motions strongly suggest that all are just in the line of sight, which is not at all surprising for the densely-packed stellar skies of the Unicorn.
Written by Jim Kaler 4/08/11. Return to STARS.