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.