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.