Spica
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 Location of Spica (circled) |
|
| Observation data Epoch J2000 Equinox J2000 |
|
|---|---|
| Constellation | Virgo |
| Pronunciation | /ˈspaɪkə/ |
| Right ascension | 13h 25m 11.579s[1] |
| Declination | −11° 09′ 40.75″[1] |
| Apparent magnitude (V) | +0.97[2] (0.97 - 1.04[3]) |
| Characteristics | |
| Spectral type | B1 III-IV/B2 V[4] |
| U−B color index | −0.94[2] |
| B−V color index | −0.23[2] |
| Variable type | β Cep + Rotating ellipsoid[3] |
| Astrometry | |
| Radial velocity (Rv) | +1.0[5] km/s |
| Proper motion (μ) | RA: −42.35 ± 0.62[1] mas/yr Dec.: −30.67 ± 0.37[1] mas/yr |
| Parallax (π) | 13.06 ± 0.70[1] mas |
| Distance | 250 ± 10 ly (77 ± 4 pc) |
| Absolute magnitude (MV) | −3.55 (−3.5/−1.5)[6] |
| Orbit[7] | |
| Period (P) | 4.0145898 d |
| Eccentricity (e) | 0.067 ± 0.014 |
| Inclination (i) | 54 ± 6° |
| Periastron epoch (T) | 2440678.09 |
| Argument of periastron (ω) (secondary) |
140 ± 10° |
| Details | |
| Primary | |
| Mass | 10.25 ± 0.68[7] M☉ |
| Radius | 7.40 ± 0.57[7] R☉ |
| Luminosity | 12,100[8] L☉ |
| Surface gravity (log g) | 3.7 ± 0.1[6] cgs |
| Temperature | 22,400[6] K |
| Rotational velocity (v sin i) | 199 ± 5[7] km/s |
| Secondary | |
| Mass | 6.97 ± 0.4[7] M☉ |
| Radius | 3.64 ± 0.28[7] R☉ |
| Luminosity | 1,500[8] L☉ |
| Temperature | 18,500[8] K |
| Rotational velocity (v sin i) | 87 ± 6[7] km/s |
| Other designations | |
Spica (/ˈspaɪkə/; α Vir, α Virginis, Alpha Virginis) is the brightest star in the constellation Virgo, and the 15th brightest star in the night sky. Located 250 ± 10 light years from Earth, it is actually a spectroscopic binary and rotating ellipsoidal variable—a system whose two main stars are so close together they are egg-shaped rather than spherical, and can only be separated by their spectrum. The primary is a blue giant and a variable star of the Beta Cephei type.
Contents
Observation history
Spica is believed to be the star that gave Hipparchus the data that led him to discover the precession of the equinoxes.[10] A temple to Menat (an early Hathor) at Thebes was oriented with reference to Spica when it was built in 3200 BC, and, over time, precession slowly but noticeably changed Spica's location relative to the temple.[11] Nicolaus Copernicus made many observations of Spica with his home-made triquetrum for his researches on precession.[12][13]
As one of the nearest massive binary star systems to the Sun, Spica has been the subject of many observational studies.[14]
Analysis of its parallax shows that it is 250 ± 10 light years from Earth.[1]
Visibility
Spica is 2.05 degrees from the ecliptic and can be occulted by the Moon and sometimes by the planets. The last planetary occultation of Spica occurred when Venus passed in front of the star (as seen from Earth) on November 10, 1783. The next occultation will occur on September 2, 2197, when Venus again passes in front of Spica.[15] The Sun passes a little more than 2° north of Spica around October 16 every year, and the star's heliacal rising occurs about two weeks later. Every 8 years, Venus passes Spica around the time of the star's heliacal rising, as in 2009 when it passed 3.5° north of the star on November 3.[16]
A method of finding Spica is to follow the arc of the handle of the Big Dipper to Arcturus, and then continue on the same angular distance to Spica. This can be recalled by the mnemonic phrase, "arc to Arcturus and spike to Spica."
Characteristics
Spica is a close binary star whose components orbit about each other every four days. They stay close enough together that they cannot be resolved as two 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.[7] Initially, the orbital parameters for this system were inferred using spectroscopic measurements. Between 1966 and 1970, the Narrabri Stellar Intensity Interferometer was used to observe the pair and to directly measure the orbital characteristics and the angular diameter of the primary, which was found to be (0.90 ± 0.04) × 10−3 arcseconds, and the angular size of the semi-major axis of the orbit was found to be only slightly larger at (1.54 ± 0.05) × 10−3 arcseconds.[6]
The primary star has a stellar classification of B1 III–IV. The luminosity class matches the spectrum of a star that is midway between a subgiant and a giant star, and it is no longer a B-type main-sequence star.[4] This is a massive star with more than 10 times the mass of the Sun and seven times the Sun's radius. The total luminosity of this star is about 12,100 times that of the Sun, and eight times the luminosity of its companion. The primary is one of the nearest stars to the Sun that has enough mass to end its life in a Type II supernova explosion.[8]
The primary is classified as a Beta Cephei-type variable star that varies in brightness over a 0.1738-day period. The spectrum shows a radial velocity variation with the same period, indicating that the surface of the star is regularly pulsating outward and then contracting. This star is rotating rapidly, with a rotational velocity of 199 km/s along the equator.[7]
The secondary member of this system is one of the few stars whose spectrum is affected by the Struve–Sahade effect. This is an anomalous change in the strength of the spectral lines over the course of an orbit, where the lines become weaker as the star is moving away from the observer.[14] It may be caused by a strong stellar wind from the primary scattering the light from secondary when it is receding.[17] This star is smaller than the primary, with about 7 times the mass of the Sun and 3.6 times the Sun's radius.[7] Its stellar classification is B2 V, making this a main-sequence star.[4]
Spica is a rotating ellipsoidal variable, which is a non-eclipsing close binary star system where the stars are mutually distorted through their gravitational interaction. This effect causes the apparent magnitude of the star system to vary by 0.03 over an interval that matches the orbital period. This slight dip in magnitude is barely noticeable visually.[18] Both stars rotate faster than their mutual orbital period. This lack of synchronization and the high ellipticity of their orbit may indicate that this is a young star system. Over time, the mutual tidal interaction of the pair may lead to rotational synchronization and orbit circularization.[19]
Spica is an polarimetric variable, which suggests that protostellar material is entrained between the two stars.[20]
Names
The name Spica derives from Latin spīca virginis "the virgin's ear of [wheat] grain". It was also anglicized as Virgin's Spike. Johann Bayer cited the name Arista. Another alternative name is Azimech, from Arabic السماك الأعزل al-simāk al-a‘zal 'the Undefended', and Alarph, Arabic for 'the Grape Gatherer'. Sumbalet (Sombalet, Sembalet and variants) is from an Arabic sunbulah "corn ear". [21]
In his Three Books of Occult Philosophy, Cornelius Agrippa attributes its kabbalistic symbol 
to Hermes Trismegistus.
In Chinese astronomy, the star is known as Jiao Xiu 1 (角宿一), i.e. the first star of the Jiao Xiu asterism.
In Hindu astronomy, Spica corresponds to the Nakshatra Chitra.
A blue star represents Spica on the flag of the Brazilian state of Pará. Spica is also the star representing Pará on the Brazilian flag.
Namesakes
Both USS Spica (AK-16) and USNS Spica (T-AFS-9) were named after this star whilst USS Azimech (AK-124), a Crater-class cargo ship, was given one of the star's medieval names. A South Korean Girl Group was named after the star.
Spring Triangle and Great Diamond Asterisms
Spica, along with Denebola or Regulus depending on the source and Arcturus, is part of the Spring Triangle asterism, and by extension, also of the Great Diamond together with Cor Caroli.
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Lua error in package.lua at line 80: module 'strict' not found. Vizier catalog entry
- ↑ 2.0 2.1 2.2 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ 3.0 3.1 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ 4.0 4.1 4.2 Lua error in package.lua at line 80: module 'strict' not found.
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- ↑ 6.0 6.1 6.2 6.3 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ 7.0 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ 8.0 8.1 8.2 8.3 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
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- ↑ Lua error in package.lua at line 80: module 'strict' not found.
- ↑ 14.0 14.1 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Earth-Sky Tonight, March 26, 2010
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
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- ↑ Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Star Names - Their Lore and Meaning, by Richard Hinckley Allen
