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.