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.