RHO VIR (Rho Virginis), plus 27 and 33 VIR! Where else can you get
three nifty stars at one sitting? Near the northwest corner of
sprawling Virgo, the second-farthest
north of its Greek-lettered
stars, and at magnitude 5 (4.88) among the faintest of them, one
wonders why Bayer took any note of Rho Virginis at all, leaving it
for Flamsteed's 30 Vir.
Five degrees west and slightly south of Vindemiatrix (Epsilon Vir), it's
nestled just 0.2 degrees south-southeast of sixth magnitude (6.19)
27 Virginis, the two looking almost like a naked eye double. They are not. Rho
Virginis is a class A0 dwarf
rather like Vega 118 light years away (give
or take 1), while 27 Vir is, at class A7, down the main (dwarf) sequence and more like Altair, but 205 light years away (plus or
minus 4), almost double Rho's distance. At first glance, Rho's chief
attribute (and 27's for that matter) is its marvelous setting against
the huge Virgo Cluster of
galaxies not far southeast of the cluster's center. Around 55
million light years away, the Virgo Cluster spreads 15 degrees across
the sky and contains a couple thousand members, many easily visible
in a home telescope. But Rho Vir quite stands out all by itself.
With a temperature of 8875 Kelvin, the star shines with the light
of 12.1 Suns, which leads to a radius of 1.5
times solar. Theory gives a mass of 1.9 Suns and shows the star to
be quite young and recently formed. It's also a fast spinner, an
equatorial velocity of at least 167 kilometers per second yielding
a rotation period under 0.44 days. Three things make Rho Vir
special. The star appears to be a Delta
Scuti type variable with a variance of around two percent and
simultaneous periods of 0.02 and 0.10 days and probably more.
Better, like Vega, it is surrounded by a debris disk that radiates in the
infrared part of the
spectrum, one that's actually been resolved. Where there's a dusty
circulating disk, there could well be planets,
though none has as yet been found. But of greatest significance,
Rho Vir belongs to the rarefied class of "Lambda Bootis stars" that are highly
depleted in iron (Rho Vir's iron content relative to hydrogen only
a tenth that of the Sun) and other elements. The best explanation
is that the star has accreted interstellar or circumstellar gas
whose iron content had been depleted by deposition onto dust grains.
The grains were then wafted away by the stellar wind leaving the outer
layers of the star bereft of iron, nickel, and silicon, and more.
There's a 20th magnitude companion 13 seconds of arc
away, but it's probably just in the line of sight.
We can't, though, just leave 27 Vir hanging. With a temperature of
7855 Kelvin, it radiates at a rate 10.2 times that of the Sun, has a
radius 1.7 times solar, a mass 1.75 times that of the Sun, and is
relatively older than Rho, which accounts for it being as luminous as
it is relative to its neighbor. It too is a fast spinner, at least
163 kilometers per second at the equator, giving it rotation period
under 0.54 days. Curiously, it is also a Delta Scuti star, oscillating
by a couple percent with periods of 0.056 and 0.042 days. A pair of
"companions" are just in the line of sight.
But wait! There's more! Rho Vir is a guide to yet another unusual
star. Just 1.3 degrees southeast of Rho lies a sixth magnitude (5.67)
K1 giant-subgiant, 33 Virginis. With a distance of 147 light years
(plus or minus 3) and a temperature of 4740 Kelvin, 33 Virginis glows
with the light of 13 Suns. Its radius is 5.3 times solar, not much
for a giant star. But that's because it seems most likely (especially
given spectral characteristics of a subgiant) to be caught in the
act of brightening and swelling with a dead helium core, and has
recently embarked on its way to becoming a true helium-fusing red giant. With a mass of 1.5 Suns, it
was born as a mid-class A dwarf, within the realm of Rho and 27 Vir.
But what makes the star really stand out is its proper motion. It falls within
the special category of "half-second of arc" movement across the line
of sight, the star clipping along at a rate of 0.53 seconds per year.
Over the course of a dedicated observer's lifetime, 33 Vir would change
its position by a full half-minute of arc. From the distance plus a
line of sight (radial) velocity of 52 kilometers per
second, 33 Vir is seen to be sailing through space at 125 kilometers
per second relative to the Sun, ten times normal! Such stars are
usually drifters from different parts of the Galaxy and are just
passing through. Yet the iron abundance, which commonly unusual for
high velocity stars (low for those from the Galactic halo, high for
those from the disk's interior), appears more or less normal. Looking
at them all, the trio makes a fine sight standing against the background
of a magnificent cluster of galaxies.
Written byJim Kaler 6/6/14. Return to STARS.