ZETA LEP (Zeta Leporis). Immediately below Orion, the Hunter, lies a smaller, fainter figure that looks something like a distorted box kite, Lepus, the Hare, Orion's prey. Rather dim it may be, but two of its stars still have proper names, Arneb (Alpha Leporis) and Nihal (Beta Lep), which stack atop each other south of Orion's Belt. Not so Zeta Lep, which lies just northeast of Arneb. At fourth magnitude (3.55) ranking fifth in the constellation (after the above two, then Epsilon and Mu Lep), it carries no proper name, just the one from Bayer and the usual host of catalogue monikers. It does, however, have some outstanding characteristics that we will get to below. But first, it's a common (and so far as we know, single) class A (A3) hydrogen-fusing dwarf 70.5 light years away. Temperature measures seem to be a problem, with a high spread running from 8360 Kelvin to 9910, though the average of 8940 is about what is expected for an A3 dwarf. Given that (to allow for a bit of ultraviolet radiation), the luminosity comes out at 15 times that of the Sun, which leads to a radius just 1.60 times solar and (from the theory of stellar structure and evolution) a mass of 1.9 Suns. Direct measure of angular diameter coupled with distance gives a similar radius of 1.55 solar, which is satisfyingly close to that found from luminosity and temperature. Now to things of note. The first one is the star's youth. At the extreme interpretation, it is very young, just born, though one source brings it in at an age of 230 million years, still young, about 15 percent of the way through its core-hydrogen-fusing lifetime. The next notable feature is Zeta's spin. It's fast, with a projected equatorial rotation speed of 220 kilometers per second, 110 times that of the Sun. An estimate for axial tilt gives a true rotation speed up to 300 km/s, which leads to a rotation period of about a quarter of a day. (That is near the extreme, but not unknown for the class; Vega's is 270 km/s). The rotation flattens the star at the poles (and bulges it at the equator), which leads to temperature problems, as the poles will then have a significantly higher temperature than the equator. Which then do you choose, and how do you average? The real winner, though, is Zeta Lep's complex surrounding dusty debris disk, known through its infrared radiation and that makes it a "Vega-type" star. There seem to be multiple rings that surround it at least out to three Astronomical Units, which is about the size of our Sun's asteroid belt. Where there is such a disk, are there buried planets, as we find orbiting Beta Pictoris and Fomalhaut, which are also class A stars? So far none has been found, but the odds seem high that Zeta Lep may someday belong to the ranks of those with their own "solar systems."
Written by Jim Kaler 2/19/10. Return to STARS.