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.