Spica
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Spica is the brightest object in the constellation of Virgo and one of the 20 brightest stars in the night sky. It has the Bayer designation α Virginis, which is Latinised to Alpha Virginis and abbreviated Alpha Vir or α Vir. Analysis of its parallax shows that it is located 250Template:± light-years from the Sun.<ref name="van Leeuwen2007"/> It is a spectroscopic binary star and rotating ellipsoidal variable; a system whose two stars are so close together they are egg-shaped rather than spherical, and can only be separated by their spectra. The primary is a blue giant and a variable star of the Beta Cephei type.
Spica, along with Arcturus and Denebola—or Regulus, depending on the source—forms the Spring Triangle asterism, and, by extension, is also part of the Great Diamond together with the star Cor Caroli.
NomenclatureEdit
In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)<ref name=WGSN/> to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016<ref name=WGSN1/> included a table of the first two batches of names approved by the WGSN; which included Spica for this star. It is now so entered in the IAU Catalog of Star Names.<ref name="IAU-CSN"/> The name is derived from the Latin spīca virginis "the virgin's ear of [wheat] grain". It was also anglicized as Virgin's Spike.
α Virginis (Latinised to Alpha Virginis) is the system's Bayer designation. Johann Bayer cited the name Arista.
Other traditional names are Azimech Template:IPAc-en, from Arabic السماك الأعزل al-simāk al-ʼaʽzal 'the unarmed simāk (of unknown meaning, cf. Eta Boötis); Alarph, Arabic for 'the grape-gatherer' or 'gleaner', and Sumbalet (Sombalet, Sembalet and variants), from Arabic سنبلة sunbulah "ear of grain".<ref name=allen/>
In Chinese, {{#invoke:Lang|lang}} ({{#invoke:Lang|lang}}), meaning Horn (asterism), refers to an asterism consisting of Spica and ζ Virginis.<ref name=zh/> Consequently, the Chinese name for Spica is {{#invoke:Lang|lang}} ({{#invoke:Lang|lang}}, Template:Langx).<ref name=aeea/>
In Hindu astronomy, Spica corresponds to the Nakshatra Chitrā.
Observational historyEdit
As one of the nearest massive binary star systems to the Sun, Spica has been the subject of many observational studies.<ref name=bass33/>
Spica is believed to be the star that gave Hipparchus the data that led him to discover the precession of the equinoxes.<ref name=evans98/> 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.<ref name=allen03/> Nicolaus Copernicus made many observations of Spica with his home-made triquetrum for his researches on precession.<ref name=jrasc37_4/><ref name=moesgaard73/>
ObservationEdit
Spica is 2.06 degrees from the eclipticTemplate:Citation needed and 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 (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.<ref name=tonight/> 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.<ref name=breit10/>
A method of finding Spica is to follow the arc of the handle of the Big Dipper (or Plough) 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."<ref name=rao/><ref name=news/>
Stars that can set (not in a circumpolar constellation for the viewer) culminate at midnight—noticeable where viewed away from any polar region experiencing midnight sun—when at opposition, meaning they can be viewed from dusk until dawn. This applies to α Virginis on 12 April, in the current astronomical epoch.<ref name=InTheSky/>
Physical propertiesEdit
Spica is a close binary star whose components orbit 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.<ref name=apj704_1/> 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 Template:Nowrap, and the angular size of the semi-major axis of the orbit was found to be only slightly larger at Template:Nowrap.<ref name=mnras151/>
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.<ref name="apj295"/> 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.<ref name="aass125_1"/>
Spica is a polarimetric variable, first discovered to be such in 2016.<ref name="cotton16"/> The majority of the polarimetric signal is the result of the reflection of the light from one star off the other (and vice versa). The two stars in Spica were the first ever to have their reflectivity (or geometric albedo) measured. The geometric albedos of Spica A and B are, respectively, 3.61 percent and 1.36 percent,<ref name=Bailey2019/> values that are low compared to planets.
The MK spectral classification of Spica is typically considered to be an early B-type main-sequence star.<ref name="johnson"/> Individual spectral types for the two components are difficult to assign accurately, especially for the secondary due to the Struve–Sahade effect. The Bright Star Catalogue listed a spectral class of B1III-IV for the primary and B2V for the secondary,<ref name="bsc"/> but later studies have given various different values.<ref name="popper"/><ref name="odell"/>
The primary star has a stellar classification of B1III-IV.<ref name=aaa483_3/> 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 main-sequence star. The evolutionary stage has been calculated to be near or slightly past the end of the main-sequence phase.<ref name=odell/> This is a massive star with more than 10 times the mass of the Sun and seven times its radius. The bolometric luminosity of the primary is about 20,500 times that of the Sun, and nine times the luminosity of its companion.<ref name=Tkachenko2016/> 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.<ref name=kaler/><ref name=Firestone2014/> Since Spica has only recently left the main sequence, this event is not likely to occur for several more million years.
The primary is classified as a Beta Cephei 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.<ref name=apj704_1/>
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.<ref name=bass33/> It may be caused by a strong stellar wind from the primary scattering the light from secondary when it is receding.<ref name=apj479/> This star is smaller than the primary, with about 4 times the mass of the Sun and 3.6 times the Sun's radius.<ref name=apj704_1/> Its stellar classification is B4-7 V, making this a main-sequence star.<ref name=aaa483_3/>
In cultureEdit
Both a rocket and crew capsule designed and under development by Copenhagen Suborbitals, a crowd-funded space program, is named Spica. Spica aims to make Denmark the first country to launch its own astronaut to space after Russia, the US and China.<ref name=copenhagensuborbitals/>
Spica is one of the Behenian fixed stars. In his Three Books of Occult Philosophy, Cornelius Agrippa attributes Spica's kabbalistic symbol File:Agrippa1531 Spica.png to Hermes Trismegistus.Template:Cn.
See alsoEdit
ReferencesEdit
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