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.