EPS MON (Epsilon Monocerotis). Good celestial sights can hide away in faint constellations. Such is the case for Epsilon Monocerotis (Flamsteed 8 Mon), in far northwestern Monoceros, the Unicorn, the star not even part of the figure's classical (such as it is) outline. Not far off, at a distance of 128 light years, fourth magnitude (4.31) Eps Mon is a fine, easily-split double star that consists of a fourth magnitude (4.44) class A (A5) subgiant (but see below) coupled to a seventh magnitude (6.72) F5 hydrogen-fusing dwarf, the pair currently separated by an angle of 12.1 seconds of arc. Smythe and Chambers call them "golden yellow and lilac," their proximity and brightness difference fooling the eye a bit. The star is also the gateway to the famed Rosette Nebula, a degree- wide, ring-shaped cloud of interstellar gas and dust 5400 light years away that lies just two degrees east and a bit north of the star.
Rosette The Rosette Nebula, just to the east of Epsilon Mon and one of the most famed of diffuse nebulae, is illuminated (ionized) by hot stars within a whole young cluster at its hollowed-out center. The inner portion is seen here. Note the contrast between the hot blue stars and the red color of the nebula, which is caused by emission from hydrogen. Similar to the Orion Nebula, but much bigger, at a distance of around 5400 light years, it's over 50 light years across. Buried within, and in the foreground, are long black streamers and clouds of dusty interstellar gas that are associated with new star formation. [NASA, the DSS-II and GSC-II Consortia (with images from the Palomar Observatory-STScI Digital Sky Survey of the northern sky, based on scans of the Second Palomar Sky Survey copyright 1993-1999 by the California Institute of Technology.)]
Stellar parameters are not well studied. While Eps Mon A's temperature has been measured at 8000 Kelvin, the value is old, 8300 K more appropriate for an A5 star, 6600 K best for Eps Mon B. With little infrared or ultraviolet light to worry about, the luminosities come in at 20 and 2.5 times that of the Sun, radii at 2.2 and 1.2, and masses (from the theory of stellar structure and evolution) at 1.9 and 1.25 solar. Luminosity and temperature show that Epsilon A is really a dwarf about halfway through its hydrogen-fusing lifetime of 1.4 billion years (Eps B lasting much longer, another 3 billion). The brighter component is a rapid spinner, with a projected equatorial velocity of 137 kilometers per second, which gives a rotation period of under 19 hours. Typical of its lower mass, the secondary takes longer, under 2.4 days. The pair is at least 500 Astronomical Units apart, which leads to an orbital period of at least 6000 years. From Eps Mon A, B would shine with the light of 5 full Moons, while from B, A would be 8 times brighter. But there is more! We have a triple. Spectroscopy reveals that Eps Mon A has a lesser companion in a 331 day orbit. Guessing its mass at half that of the Sun leads to a separation of just 1 1/4 AU. Off in the distance, 1.5 minutes of arc away, is Eps Mon C, whose motion shows it clearly to lie just along the line of sight.
Written by Jim Kaler 3/13/09. Return to STARS.