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Spica (α Vir, α Virginis, Alpha Virginis) is the brightest star in the constellation Virgo, and the 15th brightest star in the nighttime sky. It is 260 light years distant from Earth. A blue giant, it is a variable of the Beta Cephei type.


Observation history

Spica is believed to be the star that provided Hipparchus with the data which enabled him to discover precession of the equinoxes.[9] 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.[10] Nicolaus Copernicus made many observations of Spica with his home-made triquetrum for his researches on precession.[11][12]

As one of the nearest massive binary star systems to the Sun, Spica has been the subject of many observational studies.[13]


Spica is a close binary star whose components orbit about each other every four days. They remain sufficiently close together that they can not be resolved as individual stars through a telescope. The changes in the orbital motion of this pair results in a Doppler shift in the absorption lines of their respective spectra, making them a double-lined spectroscopic binary.[6] Initially, the orbital parameters for this system were inferred using spectroscopic measurements. Between 1966 and 1970, the Narrabri interferometer was used to observe the pair, and to directly measure the orbital characteristics and the angular diameter of the primary. The latter was determined to be (0.90 ± 0.04) × 10−3 arcseconds, while the angular size of the semimajor axis of the orbit was found to be only slightly larger at (1.54 ± 0.05) × 10−3 arcseconds.[5]

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