RHO SCO (Rho Scorpii). The rhyming name falls easily from the
tongue. Paying attention to this fourth magnitude (3.88) star in
Scorpius is another matter, however, as it
is more or less lost relative to the luster of the Scorpion's famed
three-star head to the north of it. It's easily found, though, by
following the trio downward from Graffias (Beta Sco) through Dschubba (Delta) and Pi Sco: the next star you come to is Rho.
Follow the line further, and Rho Sco then provides a gateway to one
of the odder stars of the sky, Chi Lupi,
and its world of weird chemical abundances. Though surrounded by
other lustrous stars, Rho Sco still stands out as a massive blue-
white hot (21,150 Kelvin) class B (B2) subdwarf-dwarf, suggesting
that it is near the end of its hydrogen-fusing life: but see below.
In spite of its proximity to the Milky Way,
Rho shines at us with a luminosity of 2540 Suns from a distance of
410 light years through relatively clear space, the star dimmed by
interstellar dust by only 0.13 magnitudes. Luminosity and
temperature then lead to a radius of 3.75 times that of the Sun and
(from theory) to a hefty mass of 7.5 times solar and an age under
half the hydrogen fusing lifetime of 33 million years, clearly
indicating it to be a dwarf. It's just under the lower limit for
supernova explosion, and
will most likely die as a massive white dwarf similar to Sirius B. For a hot class B star, its
projected equatorial rotation speed of 140 kilometers per second is
not all that large, and leads to a rotation period under 1.4 days.
Like many of its kind, Rho Scorpii has a complex family. First,
it's a tight spectroscopic binary
whose companion orbits in 4.003 days, but about which nothing else
is known. Assuming that it has a solar mass (?), the two average
only a tenth of an Astronomical Unit apart (a quarter that of
Mercury's distance from the Sun), an eccentricity of 0.27 carrying
them between 0.075 and 0.13 AU from each other. That such a close
orbit has not been made into a circle by tidal forces is consistent
with youth. Then some 40 seconds of arc away is a 13th magnitude
(12.8) visual companion that
seems to be keeping pace with Rho proper. If not a line-of-sight
coincidence, Rho B lies at least 4900 AU away from Rho A and takes
more than 114,000 years to orbit. On a wider scale, Rho Sco also
belongs to the unbound Upper Scorpius association of O and B stars
along with Rho Ophiuchi,
48 Librae, and
magnificent Antares, with which it once
shared a giant set of associated birthclouds.
Written by Jim Kaler 6/20/08. Return to STARS.