Beta LMi

BETA LMI (Beta Leonis Minoris). Most constellations follow the broad general rule that the stars are given Greek letters more or less in order of brightness, though the exceptions are legion. In a surprising number of cases, "Alpha," for example, does not lead the pack. Among the odder instances is that of Leo Minor, the Smaller Lion. It not only has no Alpha star, but only one with a Greek letter assigned to it, Beta (and precious few Flamsteed numbers), the probable result of both faintness and shifting constellation boundaries. And Beta LMi is not even the luminary, that honor going to third magnitude 46 Leonis Minoris, which oddly carries a proper name, Praecipua. At least Beta LMi is consistent in being the constellation's second brightest star. Its glory lies not in its brilliance, but in its duplicity. It's a fine double, though one whose components are so close (a few tenths of a second of arc) that they are inseparable through the small telescope and require sophisticated interferometry.

At left is an interferometric image of the binary star Beta Leonis Minoris. Beta LMi A is at center; Beta LMi B is the small blip a mere 0.06 seconds of arc to the upper right of Beta A. At right is the apparent orbit of Beta B with Beta A at the apparent focus (the cross toward lower left). In reality, the two orbit each other about a center of mass that lies on a line between them. The orbit is seen as projected against the plane of the sky. (Left: W. M. Keck Observatory from K. Matthews, A. M. Ghez, A. J. Weinberger, and G. Neugebauer in Pub. of the Astr. Soc. of the Pacific, vol 108, 615, 1996; right: From G. Gontcharov and O. V. Kiyaeva in Astronomy and Astrophysics, vol. 391, 647, 2002.)

The fourth magnitude (4.40) primary star, Beta LMi A, is a class G (G8) core-helium-fusing giant. With a temperature of 5075 Kelvin, it shines with the light of 36 Suns, leading to a rather small radius for a giant of 7.8 solar and a modest mass of around twice solar or perhaps a bit less. The lesser sixth magnitude (6.12) companion (Beta LMi B) is nominally a class F (F8) hydrogen-fusing dwarf with a temperature estimated at around 6200 Kelvin, a luminosity just 5.8 solar, a radius double solar, and a mass only (at most) 35 percent greater than that of our own star, showing how sensitive dwarf luminosity is to mass. Evolution, however, plays a role. Temperature and luminosity show that the star is really a brightened subgiant at or near the end of its hydrogen-fusing life. The pair orbit each other every 38.62 years at an average distance of 16.25 Astronomical Units, a high eccentricity taking them as far apart as 27 AU and as close as 5.4 AU. While planets are known among binary stars, these are probably too close together to have allowed them (and indeed none is known). If there were one orbiting Beta B, Beta A would not be very overwhelming, and would on average shine only 15 percent as brightly as the Sun does in our sky and appear at only half our Sun's angular size. Like many other binary systems, the pair beautifully illustrates stellar evolution in action, the more massive of the pair (which began life as a mid-class-A dwarf) being the first to evolve to gianthood. The secondary will eventually follow, quite likely leading to a double white dwarf.

Written by Jim Kaler 3/30/07. Return to STARS.