10 MON (10 Monocerotis). The eye is not exactly drawn to Monoceros, the celestial Unicorn. Made of the relatively faint stars to the east of Orion, Monoceros is a "modern" constellation invented in the seventeenth century, when astronomers began filling in the blanks that lie between the traditional sky figures, in addition to making more constellations out of the riches of the deep southern hemisphere. Alpha and Beta Mon, at fourth magnitude, almost tie for luminary. Though dim, Monoceros lies within the Milky Way, and has considerable appeal as home to the Rosette Nebula, the complex system of S Monocerotis, Plaskett's Star, and a variety of other celestial sights. Among them is 10 Mon (the number from the eighteenth century Flamsteed catalogue), a fifth magnitude class B (B2) dwarf two and a quarter degrees almost due north of Beta Mon that lies smack on the open cluster NGC 2232.

10 Mon The bright star, fifth magnitude 10 Monocerotis, is the luminary of the open cluster NGC 2232 in Monoceros, which lies just over two degrees north of Beta Mon. With an angular diameter of about half a degree, at the star's distance of 1420 light years, the understudied young cluster (25 to 30 million years old) is 10 light years across. (STScI Digitized Sky Survey)

That does not mean the star is actually a part of the cluster. Coincidences abound. The most glaring example is Aldebaran, which sits in front of the Hyades but is less than half the cluster's distance of 152 light years. 10 Mon, however, 1090 light years away (give or take 115), fits well with the cluster's estimated distance of 1060-1170 light years. Moreover the motions are similar, the star thus given a 99 percent chance of belonging to the cluster and thus of being the luminary as well. Factoring in a quarter magnitude of interstellar dimming and the ultraviolet light from a 19,070 Kelvin surface, we arrive at a luminosity of 5500 times that of the Sun, which in turn gives a radius of 6.8 times solar. Given a projected equatorial rotation velocity of 75 kilometers per second, the star must take under 6.8 days to make as full turn, slow for a class B star, suggesting that it is rotating rather pole-on. The theory of stellar structure and evolution gives a mass of 8 Suns if the star is just giving up hydrogen fusion, or 7 1/4 if it already has done so, in any case yielding an age of 28 to 35 million years. That fits well with an age for the cluster of 25 to 30 million years derived elsewhere. The star is enriched in helium (by about 50 percent), the cluster in iron (by a factor of 1.6 to 2), which presumably applies to the star as well. Two possible tenth magnitude companions seem to hang around, 10 Mon B (magnitude 9.57) 77 seconds of arc away and 10 Mon C (9.85, 78 seconds), though at physical separations of at least 26,000 Astronomical Units away from 10 Mon A, it's hard to see how any membership could survive within cluster's internal gravitational environment. If they are attached they would be on the warm side of class A and take well over a million years to orbit. Odds are they are merely in the same line of sight. In any case, 10 Mon is right on the cusp of the limit above which stars explode as supernovae. If under it, the beacon of NGC 2232 will die as a massive white dwarf near the limit of 1.4 solar masses.
Written byJim Kaler 4/22/16. Return to STARS.