Alpha Virginis - Spica

Spica or Alpha Virginis, at magnitude 0.98, is the brightest star in Virgo, and the 15th brightest star in the night sky. Spica and Arcturus are the two brightest stars in the spring sky. An easy way to find Spica is to follow the arc of the handle of the Big Dipper to Arcturus, and then continue on the same distance to Spica ("arc to Arcturus, then spike on to Spica").

Located close to the ecliptic, Spica can be occulted by the Moon and sometimes by planets. The last planetary occultation of Spica occurred when Venus passed in front of the star on November 10, 1783. The next occultation will occur September 2, 2197, when Venus again passes in front of Spica. The Sun passes a little more than 2° north of Spica around October 16 every year, rendering the star a fall harvest symbol that is reflected in its name.

Name and Historical Observation

The name Spica derives from Latin "spica virginis", meaning "Virgo's ear of grain" (usually wheat). In Chinese astronomy, the star is known as Jiao Xiu 1 in Jiao Xiu, one of the Chinese constellations. In Hindu astronomy, Spica corresponds to the Nakshatra Chitra. Medieval names include Azimech, from Arabic "as-simak al-a'zal" meaning "the Undefended", and Alarph, Arabic for "the Grape Gatherer".

Spica is believed to be the star that provided Hipparchus with the data which enabled him to discover precession of the equinoxes. A temple to Menat (an early Hathor) at Thebes was oriented with reference to Spica when it was constructed in 3200 BC and, over time, precession resulted in a slow but noticeable change in the location of Spica relative to the temple. Nicolaus Copernicus made many observations of Spica with his home-made triquetrum for his researches on precession.

Properties

Spica is about 250 light-years away, and has a visual luminosity about 2300 times the Sun's. Spica actually consists of two stars, so close together that they orbit each other with a period of only 4.0142 days. Its brightness varies from magnitude +0.92 to +1.04; this slight dip in brightness is barely noticeable visually, and was once thought to be caused by a grazing eclipse. The variation is actually caused by the fact that the stars are not quite spherical. They are so close together - only 0.12 AU apart - that they distort each other tidally, and present different apparent diameters as they orbit one another. This makes Spica the brightest of the rotating ellipsoidal variables.

Both components are hot, blue stars of spectral classes B1 III-IV and B2 V, with surface temperatures of 22,400 and 18,500 K. Their luminosities are 13,400 and 1,700 times the Sun's, including ultraviolet radiation; their radii are 7 and 4 times the Sun's; and they contain 11 and 7 solar masses, respectively. The brighter component seems to be near the point of exhausting the supply of hydrogen in its core. It is a β Cepheid variable star that superimposes another variation of 0.015 magnitudes with a much shorter period of 0.17 days. Strong X-ray emission has been recorded, at least some of which seems to be produced when the stellar winds that flow from the brilliant components collides together violently.

Additional evidence suggests that Spica may in fact be a multiple star system, containing two other fainter components.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]