72 LEO (72 Leonis). To the north of a line that extends between Leo's "Sickle" and the Lion's triangular hindquarters (that end in Denebola, the Tail), lies a fainter triangle of stars known by their Flamsteed numbers, 60 Leo at the southern apex (just three degrees west of Zosma, Delta Leo), 54 Leo (five degrees more or less north of 60, and our star, 72 Leonis (three degrees north of Delta). While 60 and 54 are made of white class A stars (54 an especially nice visual double), 72 decided to be a bit different, the fifth magnitude (but at 4.63, not by much) class M (M3) giant casting an orange-red glow, one that is rather obvious in binoculars. Somewhat on the luminous side, the star is actually classed in between the ordinary giants and the "bright giants," which are a stepping stone to the fainter supergiants. The farthest of the trio, 72 Leo lies at a rather whopping distance of 960 light years, far enough for a significant uncertainty of 180 light years, which gives us quite a range in parameters. In spite of its distance, the star's angular separation from the Milky Way results in no dimming by interstellar dust. An absolute visual magnitude of -2.77 (the magnitude the star would have at a distance of 32.6 light years) places it where the spectrum said it should be, between the giants and bright giants. A cool temperature of 3734 Kelvin plus distance give the star a rather high luminosity (which includes a lot of invisible infrared radiation) of 4570 times that of the Sun (with a rather large uncertainty that reflects that in distance), which again is consistent with its fairly bright giant class. Luminosity and temperature then conspire to give a radius of 162 times that of the Sun, or 0.75 Astronomical Units, which in our planetary system would take the star to the orbit of Venus. In spite of its distance, the size of 72 Leo's disk is accessible with sophisticated interferometers (which make use of the interfering properties of light waves that come from different parts of the star), from which we find a radius 179 times that of the Sun, just 10 percent larger, which given the various uncertainties, is fine agreement. 72 Leo is a fairly massive star, theory applied to luminosity and temperature giving us around 6 solar masses. It's hard to say just what evolutionary state the star is actually in. It could be brightening with a dead helium core or it could be on its "second ascent" among the giants after fusing its helium to carbon and oxygen, resulting in a dead carbon/oxygen core. Whatever the case, 72 Leo is slightly and irregularly variable (hence the variable star name FN Leo) with a range of magnitudes between 4.51 and 4.64, not surprising given the size of the star and its resulting instability. And whatever the case, 72 (once when new, a hot class B dwarf) will eventually slough off its outer hydrogen shell and finally die as a fairly massive white dwarf approaching a solar mass.
Written by Jim Kaler 5/11/12. Return to STARS.