7 MON (7 Monocerotis). Near the extreme southeastern corner of the modern constellation of Monoceros (the Unicorn), making a flat triangle with Gamma and Beta Mon to the north, lies fifth magnitude (5.27) 7 Monocerotis of the Flamsteed catalogue. As are so many of the faint stars seen near and east of Orion, 7 Mon is a hot, blue class B (B2.5) luminous dwarf. The star is faint partly because of its great distance of 872 light years (give or take 68), which is farther than either the other two of the little triangle. With the star not far off the Milky Way, we might expect a good deal of dimming by interstellar dust. But the line of sight is pretty clear, the star dimmed by a mere 0.16 magnitude There seems to be but one temperature measurement of 16,338 Kelvin (low for the class, which averages closer to 20,000 Kelvin). Including a rather large correction for ultraviolet radiation, 7 Mon shines with the light of 1905 Suns, which leads to a radius of 5.46 solar radii. Application of the theory of stellar structure and evolution tells of a mass 6 times that of the Sun and that the star is nearing the end of its 6.3 Megayear hydrogen-fusing main sequence dwarf lifetime, after which it will swell to become a giant star, produce a planetary nebula from the ejection of its outer layers, and die as a white dwarf of about 0.95 solar masses. It's not massive enough to blow up as a supernova. The star is listed as a candidate for Beta-Cephei-type oscillations, though none has as yet been found, nor is there any indication of infrared radiation that might imply a surrounding dusty disk. The projected equatorial rotation speed is not well known, with values ranging from 95 to 152 kilometers per second, which give respective rotation periods under 1.8 and 2.9 days. 7 Mon has a companion separated from it by about a tenth of a second of arc, which implies a minimum actual separation of 87 Astronomical Units and, given a low mass for the companion, a minimum orbital period of 100,000 or so years, but nothing whatever is known about it.
Written byJim Kaler 03/24/17. Return to STARS.