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.
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