DELTA LUP (Delta Lupi), along with a close neighbor, GG LUPI.
Tucked in under, and just to the west of, the great curve of stars
that makes Scorpius, and to the south of
much dimmer Libra, lies the sparkling
constellation of Lupus the Wolf. While
there is no first magnitude luminary, the figure is filled with
hosts of modestly bright hot blue stars, many of which belong to
the Upper Centaurus-Lupus expanding association (in the jargon of the
trade, "UCL.") Among them is third magnitude (3.22) Delta Lupi, a
hot (22,400 Kelvin) blue class B (B1.5) subgiant that lies on the
western side of the waistband of the "looping" constellation at a substantial distance of
880 light years (with an uncertainty of 95). After correction for
0.13 magnitudes of dimming by interstellar dust (not much given the
distance and proximity to the Milky Way) and
for a LOT of ultraviolet
radiation from the hot surface, we find a notably high luminosity
of 24,200 Suns, which leads to a radius of
just under 11 solar. Typical of such stars, Delta Lup is a fast
spinner, at least 220 kilometers per second at the equator, which
gives a rotation period of under 2.4 days. Theory then indicates
a mass of a dozen Suns, which places it just above the accepted
level at which stars explode as supernovae, and also shows it
to be a true subgiant that has just given up core hydrogen burning
(or is about to do so), the age about 15 million years. The star
(with no known companion) is
firmly listed as a member of the UCL association, but is quite a
bit farther than the association's average distance of 460 light
years. It's also speeding pretty fast across the line of sight, at
a rate relative to the Sun of 40 kilometers per second, well over
double that of the average star. Like many of its kind, Delta Lup
is a Beta Cephei type of variable, one
that changes in visual brightness by a few hundredths of a
magnitude with a very short period of 3.97 hours. Additional
oscillations undoubtedly remain to be discovered.
In a minor role, Delta Lupi is the gateway to a well-regarded eclipsing double star, sixth
magnitude (but at 5.59 just barely) GG Lupi, which lies only half
a degree due west of it. Except for proximity on the sky, though,
the two have nothing to do with each other, as GG is but 546 light
years off, only 60 percent Delta's distance. This blue-white star
actually consists of a pair of cooler class B dwarfs (B7 plus B9,
with temperatures of 14,750 and 11,000 Kelvin). In tight orbit,
the dimmer (39 Suns) cuts off light from the brighter (240 Suns)
every 2.164 days, knocking the combined apparent brightness down by
a few tenths of a magnitude. Analysis of the light curve (apparent
brightness with time) coupled with the orbital speeds gives precise
masses of 4.11 and 2.50 solar masses. Evolutionary theory yields
4.0 and 2.5 solar, remarkably and satisfyingly similar. Though
very close together, just a dozen solar radii (five times the
radius of the brighter star), the two stars are still far enough
apart so that neither of them disturbs the other, which gives the
system great significance in establishing the way in which
luminosity changes with stellar mass. That isolation will change
when the more massive one starts to expand with cessation of core
hydrogen fusion, which will result in growth to the giant state, interaction between
the two, and mass exchange.
Written by Jim Kaler 8/13/10. Return to STARS.