Spica
Brightest star in Virgo, a close binary Beta Cephei variable.
Spica, the brightest star in the constellation Virgo, ranks among the top 20 brightest stars visible from Earth. Its Bayer designation is Alpha Virginis, often shortened to Alpha Vir or α Vir. Parallax measurements place it about 250 light-years from the Sun. The system is a spectroscopic binary and a rotating ellipsoidal variable: the two stars orbit so closely that tidal forces stretch them into egg shapes, and they can only be distinguished through their spectral lines. The primary star is a blue giant that also varies in brightness as a Beta Cephei type variable.
Along with Arcturus and either Denebola or Regulus (depending on the source), Spica forms the Spring Triangle asterism. This triangle, in turn, is part of the larger Great Diamond asterism, which includes the star Cor Caroli.
The name Spica comes from the Latin *spīca virginis*, meaning "the virgin's ear of grain," and was once anglicized as Virgin's Spike. In 2016, the International Astronomical Union’s Working Group on Star Names officially approved Spica as the star’s proper name, and it is now listed in the IAU Catalog of Star Names. The Bayer designation α Virginis is also used. In a Sumerian star list from Uruk (dating between 3200 and 1500 BC), Spica—or a group of stars in Virgo—was called *absinnu* or *šer'u*, meaning "The Seed-Furrow." Other traditional names include Azimech (from Arabic for "the unarmed simāk"), Alaraph (Arabic for "the grape-gatherer" or "gleaner"), and Sumbalet (or variants like Sombalet and Sembalet), from the Arabic *sunbulah* for "ear of grain." In Chinese astronomy, Spica and ζ Virginis form an asterism called Horn; Spica itself is known by a corresponding Chinese name. In Hindu astronomy, it corresponds to the Nakshatra Chitrā.
Spica is one of the nearest massive binary systems to the Sun and has been heavily studied. It is thought to be the star that provided Hipparchus with the data that led him to discover the precession of the equinoxes. A temple to Menat (an early form of Hathor) at Thebes was aligned with Spica when built around 3200 BC; over time, precession shifted the star’s position relative to the temple. Nicolaus Copernicus also made many observations of Spica with his homemade triquetrum for his own precession research.
Spica lies about 2.06 degrees from the ecliptic, so the Moon and sometimes planets can occult it.
- Bayer designation
- α Virginis (Alpha Virginis)
- Distance from sun
- 250 light-years
- Apparent magnitude variation
- 0.03 over orbital period
- Orbital period
- 4 days
- Primary spectral type
- B1III-IV
- Primary mass
- more than 10 times the Sun's mass
- Primary radius
- 7 times the Sun's radius
Lore & Background
The star's name derives from Latin spīca virginis 'the virgin's ear of grain', also anglicized as Virgin's Spike. In 2016, the International Astronomical Union's Working Group on Star Names approved Spica as the proper name. In the Sumerian Starlist from Uruk (3200–1500 BC), Spica is called absinnu or šer'u, 'The Seed-Furrow'. Other traditional names include Azimech (from Arabic 'the unarmed simāk'), Alaraph ('the grape-gatherer'), and Sumbalet (from Arabic 'ear of grain'). In Chinese astronomy, Spica is part of the Horn asterism; in Hindu astronomy, it corresponds to the Nakshatra Chitrā.
Spica is believed to be the star that gave Hipparchus the data leading to his discovery of the precession of the equinoxes. A temple to Menat at Thebes was oriented with reference to Spica when built in 3200 BC, and precession slowly changed its location relative to the temple. Nicolaus Copernicus made many observations of Spica with his home-made triquetrum for his researches on precession. Spica is 2.06 degrees from the ecliptic and can be occulted by the Moon and sometimes by planets; the last planetary occultation occurred when Venus passed in front of the star on November 10, 1783, and the next will occur on September 2, 2197.
Reader's Guide
Spica is notable as one of the nearest massive binary star systems to the Sun, making it a subject of many observational studies. It is a rotating ellipsoidal variable, a non-eclipsing close binary where the stars are mutually distorted by gravity, causing a slight 0.03-magnitude variation matching the orbital period. Both stars rotate faster than their mutual orbital period, and the lack of synchronization and high orbital ellipticity may indicate a young system. Spica is also a polarimetric variable, first discovered as such in 2016, with the polarimetric signal mostly from reflection of one star's light off the other; these two stars were the first to have their geometric albedos measured (3.61% for Spica A, 1.36% for B). The primary is classified as a Beta Cephei variable, pulsating with a 0.1738-day period, and is one of the nearest stars to the Sun massive enough to end its life in a Type II supernova, though this is not likely for several million years. The secondary exhibits the Struve–Sahade effect, an anomalous change in spectral line strength over the orbit. Spica forms the Spring Triangle asterism with Arcturus and Denebola (or Regulus, depending on the source), and by extension is part of the Great Diamond with Cor Caroli. A common method to find Spica is to follow the arc of the Big Dipper's handle to Arcturus and continue the same angular distance to Spica, recalled by the mnemonic 'arc to Arcturus and spike to Spica.'
Did You Know?
- Spica is a rotating ellipsoidal variable whose two stars are egg-shaped due to their gravitational interaction.
- The last planetary occultation of Spica occurred when Venus passed in front of it on November 10, 1783.
- The two stars in Spica were the first ever to have their geometric albedos measured.
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