Spica, also called Alpha Virginis, is a star located in the constellation Virgo, about 10 degrees south of the ecliptic. The star is the brightest star of the Virgo constellation and the 15th brightest star overall in the night sky. It is 260 light years distant from our Solar System. Like Regulus, Spica is regularly occulted by the Moon and less frequently so by the various planets of the Solar System. The next occultation of Spica by a planet will be September 2, 2197, by Venus.
Spica in Culture and History
The name Spica itself comes from the Latin phrase spica virginis, meaning "Virgo's ear of wheat (or grain)". The Greeks called the star Στάχυς, meaning "ear spike". In Medieval Europe, the star was referred to as Azimech, from the Arabic word السماك الأعزل al-simāk al-a‘zal, meaning "the defenseless", or as Alarph "the grape gatherer". Astrologers at that time viewed Spica as a Behenian fixed star, associated with sage and emeralds. For the Chinese, the star is Jiao Xiu 1 (角宿一), in the Chinese constellation Jiao Xiu (角宿), the latter translates as "Horn".
Spica was no doubt known to man since before recorded history. The first known reference to the star is a temple in Thebes that was constructed for the Egyptian goddess Menat (later Hathor), was oriented towards Spica as it set in the sky, and was believed built around 3200 BC. Later, other temples around the Mediterranean were built with the same orientation, including one in 2000 BC at Tell al Amarna, as well as Olympia in 1445 BC, and Athens in 1130 BC, among others. Later, Timochares studied Spica and Regulus around 300 BC, and using the information from these observations some 150 years later, Hipparchus was able to discover the precession of the equinoxes. Later, during the Renaissance, Copernicus used Spica for his own observations while researching the Earth's precession.
Spica is actually a binary star system, with a combined apparent magnitude of 1.04. The primary star is a blue-white giant or subgiant star of spectral type B1 III-IV, while its companion is a blue-white main sequence dwarf of spectral type B2 V. As the two stars are only 0.12 AU apart, they complete an orbit of each other every 4.0145 days. It was commonly believed the variation of the apparent brightness of the two stars was caused by one regularly eclipsing the other from our vantage point on Earth. The variation is actually caused by the tidal distortion the stars afflict on each other, altering the apparent diameters to an observer on Earth as they orbit, which varies the apparent magnitude by around 0.03. In addition, the primary star is a Beta Cephei variable star, and pulsates in brightness with a regular variation of 0.015 magnitudes every 4.2 hours.
The brighter primary has 10.5 times to Sun's mass and 7 times its diameter, with a surface temperature at 22,400 K. Because of the star's large mass, it will most likely end its life as a supernova. The companion star 4 times the Sun's diameter and six times its mass, with a cooler surface temperature of 18,500 K. Combined, the two stars have 2200 times the Sun's visible luminosity. However, because most of the radiation from the two stars are in the ultraviolet, the primary alone actually has a total luminously 12,100 times that of our Sun, while the companion is 1,500 times as bright in total luminosity.
- Allen, Richard Hinckley (2003). Star Names and Their Meanings. Kessinger Publishing. pp. 468. ISBN 0766140288
- Evans, James (1998). The History and Practice of Ancient Astronomy. Oxford University Press. pp. 259. ISBN 0195095391