RIJL AL AWWA (Mu Virginis). Many constellations are hosts to informal figures,
to various "asterisms." Some, like the Big and Little Dippers (which belong to Ursa Major and Minor), Orion's Belt,
and Pegasus's Great Square, are famed and beloved. Many, however,
particularly those belonging to the ancient Arabic culture that has
played such a strong role in the history of our constellations and
star names, are remarkably obscure. To
Virgo belongs one of the latter, one
made of the five stars of the Arabs' "Awwa," or "Barker." To the
northwest of Spica, Al Awwa
relates to a "Kennel," and consists of Porrima (Gamma Virginis), Vindemiatrix (Epsilon), Delta Virginis, Zaniah (Eta), and Zavijava (Beta). Almost as an
afterthought, fourth magnitude (3.88) Mu Virginis (way on the other
side of Spica in eastern Virgo) gets
included as "Rijl al Awwa," the "Foot of the Barker." While
serving as a window to another long-gone culture, it is known today
only by its Greek letter name,
not to mention also by a long list of catalogue names. All that
said, the star itself, while formally listed as a class F (F2) giant, actually has all the
characteristics of a mid-temperature (6730 Kelvin) hydrogen-fusing
dwarf, and quite a close one too, just 60 light years away with an
uncertainty of less than 1 l-y. Mu's luminosity (the vast majority
in the visual spectrum, where
we can actually see it) of 7.5 times that of the Sun leads to a radius of 2.0 times solar (close
to the value of 2.1 solar radii used as a secondary calibrator for
interferometry measures). A projected equatorial rotation velocity
of 54 kilometers per second (27 times that of the Sun, but still
not all that high) yields a rotation period of under 1.9 days.
Theory then gives us a mass of 1.55 Suns and clearly shows Mu
Virginis to be a dwarf about in the middle of its hydrogen-fusing
lifetime of 2.5 billion years. Such "misdiagnoses," warmer dwarfs
classified as giants, are not all that uncommon since for such
stars the differences in the spectra are not all that great. Later
classification indeed put it into the ranks of true dwarfs. The
star's metal content relative to the Sun is slightly low, while the
velocity is a bit high (about double the average). There is some
indication that Mu has a spectroscopic companion with a
roughly-determined period of 358 days, which given low mass would
imply a separation of around three-quarters of an Astronomical
Unit, but it has never been confirmed and may well not exist. So
far as we know, there are no planets, nor
even any indication of a surrounding dusty disk that might imply a
planetary system, Mu Vir then fading into the obscurity implied by
its even more obscure proper name.
Written by Jim Kaler 5/06/11. Return to STARS.