54 LEO (54 Leonis). The most northerly of a triangle of fainter stars to the northwest of Leo's hindquarters (which includes 60 and 72 Leo), Flamsteed's 54 Leonis lies six degrees northwest of Zosma (Delta Leo) and almost on the border with Leo Minor (the lesser Lion). As a pretty fourth magnitude (4.3) visual double (as seen through the telescope), it is rather special, the pair consisting of white class A (A1 and A2) dwarfs six seconds of arc apart. Writing from the nineteenth century, Smythe and Chambers say: "A neat double star just over the Lion's back, where it is preserved from the Lesser Lion by one of the map maker's nooks...A 4 1/2, white; B 7, grey. This is a beautiful object." Study of the star goes way back. The first measures of separation were made as long ago as 1777, none other than William Herschel recording one in 1781.

Yet the pair still presents a considerable mystery. The distance is essentially unknown as the duplicity evidently messes up the parallax. The average absolute visual brightnesses for the spectral classes compared to the apparent magnitudes give 180 light years for brighter (magnitude 4.50) 54 Leo A, but nearly double that for sixth magnitude (not seventh as Smythe and Chambers had it) 54 Leo B, showing that something is wrong someplace, that the stars are not "average." Giving brighter "A" double weight, the estimated distances for each yield a mean of about 230 light years. Taking the mean distance as some sort of reality and adopting temperatures of 9430 and 9100 Kelvin (the latter from the class), we get respective luminosities of 70 and 13 times that of the Sun, radii of 3 and 1.5 solar, and masses of 2.5 and 2.0 solar. But don't take the numbers very seriously. If the mean distance is close to correct, the stars are at least 440 Astronomical Units apart and take at least 4500 years to circle each other. What IS known is that both are fast rotators, "A" spinning with a projected equatorial velocity of 180 kilometers per second, "B" at a significantly higher 250 km/s, which give respective rotation periods of under 0.8 and 0.3 days. As a result, the chemical abundances are not confused as a result of gravitational settling and radiative lofting, the spins keeping things well stirred (allowing us perhaps to believe a measured iron abundance for "A" of half solar). We think of the naked eye stars as being so bright as to be well understood. If nothing else (aside from pleasing the eye), 54 Leonis shows us differently.
Written by Jim Kaler 6/01/12. Return to STARS.