SPICA (Alpha Virginis). Spica, the luminary of Virgo, becomes prominent in the southeast in northern spring evenings, and can easily be found by following the curve of the Big Dipper's handle through Arcturus and then on down. Though a large constellation, Virgo, the Virgin, does not have much of any prominent stellar pattern, relying on Spica to tell us where it is. The star lies about 10 degrees south of the celestial equator, and practically on the ecliptic, the path of the Sun, and is regularly occulted, or covered over, by the Moon. The Sun passes Spica in the fall, rendering the star a harvest symbol that is reflected in its name, from Latin meaning "ear of wheat," the name actually going back to much more ancient times. Though at a distance of 250 light years (second Hipparcos reduction), Spica is still first magnitude (1.04), showing its absolute brilliance, the star visually 1900 times more luminous than the Sun. The apparent brightness is deceptive, however, as Spica actually consists of two stars very close together (a mere 0.12 Astronomical Units apart) that orbit each other in slightly elliptical paths with a period of only 4.0145 four days, which makes them difficult to study individually. Both are blue class B (B1 and B4) hydrogen-fusing dwarfs (the brighter nearing the end of its stable lifetime), making Spica one of the hottest of the first magnitude stars. The high temperature produces a great deal of radiation in the ultraviolet, which renders Spica vastly brighter than visually indicated. The brighter primary star has a temperature 22,400 Kelvin, a true luminosity of 12,100 Suns (after taking ultraviolet radiation into account), a radius 7 solar, (25 percent the separation between the two stars) and a mass 10.5 times solar, which may be enough to send it someday into a supernova explosion. The more poorly-known respective parameters for the secondary cooler star are 18,500 Kelvin, 1500 solar luminosities, almost 4 solar radii, and just 6 solar masses. Spica exhibits subtle brightness variations that were once thought to be caused by a grazing eclipse, each star slightly cutting off the light of the other each orbital period. The variation of 0.03 magnitudes is instead actually caused by the fact that the stars tidally distort each other and are not quite spherical, so as they orbit they present changing apparent diameters to the observer. The primary star is also a pulsating Beta Cephei (or Beta Canis Majoris) variable, which superimposes another variation of 0.015 magnitudes with a much shorter period of 0.17 days. The star is a strong source of X- rays, at least some of which seem to be produced when the winds that flow from the companions violently collide. Lunar occultaions yeild evidence that Spica is in fact multiple, with three other fainter components.
Written by Jim Kaler 4/10/98; last updated 7/03/09. Return to STARS.