ALTERF (Lambda Leonis). At the low end of fourth magnitude (4.31),
rather faint to have a proper name, Alterf probably stood out to
the ancients for its position in the front of the Lion's head. The
name, from the Arabic "al-tarf," means "the glance," and refers to
one of the eyes of the much larger Arabic Lion, which stretched
from the Greek Gemini into Virgo. It fits well with the smaller
Greek version of Leo, however, the
historic name hanging on. It's hardly ever used though, the star
always now referred to by its Greek letter
name, Lambda Leonis. (Do not confuse with Cancer's "Al Tarf" -- Beta
Cancri -- which, according to Allen, can also mean "extremity," or
"end.") Find Lambda Leo just to the west of the top of Leo's "Sickle." A coolish (3950 Kelvin) class K
(K5) giant, the star has two
claims to some prominence. While many stars are recognized for
their extreme properties, others for their oddness, Lambda Leo is
best known as a "calibrator" star, one whose properties are stable
and enough well known to act as a foundation with which to examine
other stars. It is used both as an infrared calibrator and one
with which to test measures of angular diameter. Of more interest,
though, is Alterf's rather vague evolutionary status. It's almost
a clone of much brighter Aldebaran in
Taurus, though at a distance of 329
light years (give or take 5), five times farther away and therefore
some 25 times fainter to the eye. Allowing for a significant
amount of infrared radiation, the star shines with the light of 460
Suns, which with temperature gives it a
substantial radius 46 times solar, or just over half the size of
Mercury's orbit, the iron content (relative to hydrogen) about 3/4
that of the Sun. As expected for such a large star, it takes a
long time to complete a spin, a projected rotation speed of 6
kilometers per second giving a rotation period that could be as
long as a year. The mass and evolutionary status are both
uncertain, as stars of different masses within this giant state all
rather look alike. It probably falls between 1.5 and 2.5 times
that of the Sun. Most class K giants are quietly fusing their
internal helium into carbon and oxygen. Alterf, however, seems to
be more a star in transition, either brightening with a dead helium
core, fading some after just having fired its helium, or more
interesting yet, brightening for its second time with a dead carbon
and oxygen core after helium fusion is over (the choices depending
on mass, this last possibility the higher-mass option). If so,
Lambda Leo is on its way to becoming a long-period variable like Mira, and will quickly (on an astronomical
timescale) get a lot brighter. We simply do not know, as the
precision of both observation and theory are insufficient to
discriminate the various options. So far as we know, Alterf is
single, with no companion to
watch its progress.
Written by Jim Kaler 4/16/10. Return to STARS.