ETA LUP (ETA LUPI). Lupus, the Wolf,
lying in wait in the Milky Way to the
southwest of Scorpius, is filled with
little-known treasures, not the least of
which is its many bright, blue, hot stars. Among them is the third
magnitude (3.35) class B (B2.5) subgiant (but as seen below is
really a young dwarf) Eta Lupi, which is going through life
accompanied by two companions, making it a triple
system, one member of which is very distant from Eta itself. Such
stars, Eta included, have a fine spectral background with which to
study the local interstellar gas. From a distance of 441 light
years (plus or minus 10), a temperature of 21,800 Kelvin (to
account for a lot of ultraviolet
light), and a six percent correction for dimming by
interstellar dust, we calculate a large luminosity of 4570 times
that of the Sun. A radius of 4.7 solar
combined with a projected equatorial rotation speed of 210
kilometers per second gives a rotation period of under 1.1 days.
In spite of the rapid spin, there is no evidence for a surrounding
disk. Application of theory then gives a mass of 8.5 times that of
the Sun, which puts Eta Lupi at or near the limit of 8-10 Suns
beyond which stars explode as supernovae. Most likely it
will die as massive white
dwarf near the white-dwarf limit of 1.4 solar masses (beyond
which their electron-support mechanism fails). Theory also reveals
that the star is not a subgiant at all (such mis-classifications in
this temperature realm quite common), but a fairly young dwarf
maybe a third of the way through its hydrogen-fusing lifetime of
only 28 million years. Like many of its kind(possibly including Beta Lupi), Eta is part of a broad "association" of gravitationally
unbound stars that were born at roughly the same time from the same
interstellar cloud complex, this one the "Upper Centaurus-Lupus"
grouping, whose central distance of 460 light years fits quite
well. On a much smaller scale, Eta Lup has a pair of orbiting
companions, the brighter and innermost (magnitude 7.5 Eta Lupi B)
of which was first measured at 15 seconds of arc from "A" (best
value 14.4) by none other than John Herschel. From its brightness
it's a two-solar-mass class A4 dwarf with a luminosity of around 15
Suns. The minimum separation of 1950 Astronomical Units from Eta-A
along with
Kepler's Laws give an orbital period of at least 26,000 years.
Much more distant, at 135 seconds of arc, is tenth magnitude Eta
Lup D, which for more than a century has been a steadfast
travelling companion and is most likely fragilely bound to Eta-AB.
With a separation of at least 18,000 AU, it must take a minimum of
three-fourths of a million years to make a complete circuit. From
its brightness, it seems to be a solar mass star. Were there
inhabited planets in orbit around Eta-D,
their residents would see Eta Lupi A as a brilliant blue point
shining with the light of dozens of full Moons, while next to it
would be a white point with the brightness of a dozen times
Venus's, the two no more than six degrees apart. What happened to
Eta Lupi C? At tenth magnitude, and moving rapidly relative to
Eta-A, it's clearly just a line-of-sight coincidence.
Written by Jim Kaler 6/24/11. Return to STARS.