Spica

alpha virginis, spica star

Spica, Alpha Virginis (α Vir), is a blue-white spectroscopic binary star located at a distance of 250 light years from Earth in the constellation Virgo. With an apparent magnitude of 0.97, it is the brightest star in Virgo and the 16th brightest star in the sky. It is only slightly fainter than Altair in the constellation Aquila, Acrux in Crux, Aldebaran in Taurus and Antares in Scorpius, and it just outshines Pollux in Gemini and Fomalhaut in Piscis Austrinus.

alpha virginis, spica star
Spica (Alpha Virginis), image: Wikisky

Star system  

Spica is a spectroscopic binary star consisting of two stars in a close orbit with an orbital period of 4.0145 days. The stars are too close together – they are separated by only 0.12 astronomical units – to be resolved in a telescope and can only be detected as a pair through their spectral lines. Spica is a double-lined spectroscopic binary (SB2), which means that spectral lines of both components are visible and the lines are sometimes double and sometimes single.

Spica is classified as a rotating ellipsoidal variable, a spectroscopic binary star whose two components are so close together that their shape is distorted by their gravitational interaction. Both components are ellipsoidal. The stars do not eclipse each other during their orbit, but show changes in brightness as a result of the changes in what the observer sees of the stars due to their rotation. Their brightness varies because they are ellipsoidal in shape and their apparent diameters change as they orbit, not as a result of any physical changes to the stars themselves.

Rotating ellipsoidal variable stars typically show variations in brightness of up to 0.1 magnitudes and their periods correspond to their orbital motion. Spica is the brightest star of this type in the sky. Its brightness varies by only 0.03 magnitudes and the variations cannot be noticed visually.

The primary star in the system has the stellar classification B1III-IV, indicating a blue giant or subgiant star. It has an estimated mass 11.43 times that of the Sun and a radius 7.47 times solar. With an effective temperature of about 25,300 K, it is 20,512 times more luminous than the Sun, but a lot of its energy output is in the invisible ultraviolet. The star is a fast spinner, with a projected rotational velocity of 165.3 km/s. Its estimated age is 12.5 million years. It is one of the nearest stars to the solar system that is massive enough to be a Type II supernova candidate.

The primary component is classified as a Beta Cephei variable, a star showing small, rapid fluctuations in brightness as a result of pulsations of its outer layers. It varies in brightness with a period of 0.1738 days. Other well-known stars in this class include Hadar (Beta Centauri), Mimosa (Beta Crucis), Imai (Delta Crucis), Mirzam (Beta Canis Majoris), Algenib (Gamma Pegasi), and Shaula (Lambda Scorpii).

Alpha Virginis B is cooler, smaller and less massive, with a temperature of 20,900 K, a radius 3.74 times that of the Sun and a mass of 7.21 solar masses. It is a main sequence star of the spectral type B2V. It has a luminosity 2,254 times that of the Sun. Its projected rotational velocity is 58.8 km/s. The strength of the star’s spectral lines varies during the orbital motion, becoming weaker as the star moves away from the observer, possibly as a result of a strong stellar wind from Spica A scattering its light. This is known as the Struve-Sahade effect.

Both stars rotate more rapidly than their orbital period, and the lack of synchronization, coupled with the highly elliptical orbit, may indicate that the star system is still young. The stars’ gravitational interaction over time may reduce the eccentricity of their orbit and lead to more synchronous rotation.

A study published in 2016 revealed that Spica is a polarimetric variable. Most of the polarimetric signal comes from the reflection of the primary star’s light off the companion and vice versa. The two stars were the first to have their geometric albedo (reflectivity) measured. A study published in 2019 yielded geometric albedos of 3.61 percent and 1.36 percent for Spica A and Spica B. Even though the stars reflect very little of the incident light, the reflected light is highly polarized.

Facts 

Spica is one of the 58 bright stars selected for navigation. Navigational stars play a special role in the field of celestial navigation because they are some of the brightest and most identifiable stars in the sky. Spica is the only navigational star in the constellation Virgo.

Spica lies only 2.06 degrees from the ecliptic, which means that it can be occulted by the Moon and, more rarely, by planets. The last occultation by a planet occurred on November 10, 1783, when the planet Venus passed in front of the star. The next planetary occultation (also by Venus) will take place on September 2, 2197.  The Sun and the planets pass near the star more often. The Sun comes within a little over 2 degrees north of Spica in mid-October every year, two weeks before the star’s heliacal rising (rising above the horizon at dawn after a period of invisibility). Venus passes near the star in late October or early November every 8 years.

Spica is believed to have played a crucial role in the discovery of the Earth’s axial precession (precession of the equinoxes). The discovery is usually attributed to the Greek astronomer Hipparchus (190-120 BCE), who measured the longitude of Spica and a number of other bright stars. After comparing his measurements with those given by Timocharis and Aristillus (3rd century BCE), Hipparchus found that Spica had moved two degrees relative to the autumnal equinox (the instant when the plane of the Earth’s equator aligns with the path of the Sun and the Sun can be seen directly above the equator). He concluded that the equinoxes were moving through the constellations of the zodiac and that the precession rate was at least a degree every century.

The autumnal equinox lies halfway between Spica and the bright Regulus in the constellation Leo.

Spica is one of the 27 stars represented on the flag of Brazil. Each star symbolizes a Brazilian Federative Unit and Spica represents the state of Pará. It is the only star above the white band, symbolizing a part of the territory north of the equator.

Spica was of some importance in ancient Egypt and Greece. In Egypt, it was known as the Lute-Bearer and Repa, “the lord.” A temple to Menat (one of the names of the goddess Hathor), built around 3200 BCE, and the temple of the Sun at Tell el-Amarna, built around 2000 BCE, were oriented to Spica’s setting. Due to axial precession, the star’s location relative to the temple has changed over time. Several temples in Greece were also found that were constructed relative to Spica’s motion across the sky, including two temples at Rhamnus from 1092 and 747 BCE, temples to Hera at Olympia, Argos and Girgenti, and the temple of Nike Apteros at Athens (1130 BCE).

In medieval astrology, Spica was one of the 15 Behenian fixed stars, believed to be a source of special astrological power. The star was linked with the planets Mercury and Venus and associated with emerald and sage, which were used in rituals to bring out the star’s influence.

Name 

The name Spica (pronunciation: /ˈspaɪkə/) comes from the Latin phrase spīca virginis, meaning “the virgin’s ear of grain.” The name was officially approved by the International Astronomical Union’s (IAU) Working Group on Star Names (WGSN) on June 30, 2016. It formally applies only to the component Alpha Virginis Aa.

The star has been known by many other names throughout history. The Latin Spica Virginis was translated into English as Virgin’s Spike. The German uranographer Johann Bayer used the name Arista. The Arabic derivations included Alarph, “the grape gatherer”, Sumbalet, “ear of grain,” and, Azimech, derived from Al Simak al A’zal, meaning “the unarmed” or “the defenceless,” unguarded by any other star.

The 13th century Persian astronomer Zakariya al-Qazwini called the star Sak al Asad, meaning “the shin-bone of the Lion.” Spica was a part of an enormous asterism, the Lion, which included several other stars in Virgo that outlined one of the lion’s legs, with Spica and Arcturus marking the shin-bones, the stars of Corvus forming the hindquarters, Pollux and Castor in Gemini one of the front paws, Canis Minor stars the other paw, and Regulus in Leo the forehead. The asterism occupied about a third of the sky.

The Chinese name for the star is 角宿一 (Jiǎo Xiù yī), meaning the First Star of Horn. In Chinese astronomy, Spica forms an asterism known as Horn with Heze, Zeta Virginis.

In Hindu astronomy, Spica was known as Chitra (“the bright one”) and associated with the 12th lunar mansion (nakshatra) of the same name.

In Babylonian astronomy, the star represented the wife of Bel and was known as Sa-Sha-Shiru, meaning “the virgin’s girdle.”

Location

Spica is easy to find because it lies on the imaginary line arching away from the three stars of the Big Dipper’s handle, Alioth, Mizar and Alkaid. Following the arc of the handle leads first to the bright Arcturus in the constellation Boötes and then to Spica.

The Big Dipper, Arcturus and Spica, image: Wikisky

Spica is part of two prominent spring asterisms, the Spring Triangle and the Great Diamond, also known as the Diamond of Virgo. The Spring Triangle is formed by Spica with Arcturus and Regulus, the brightest stars in the constellations Boötes and Leo, while the Great Diamond consists of Spica, Arcturus, Denebola in Leo and Cor Caroli, the luminary of Canes Venatici.

The Spring Triangle and the Great Diamond, image: Wikisky

Constellation

Spica is located in the constellation Virgo. It marks the ear of grain held in the celestial Virgin’s left hand. Like other zodiac constellations, Virgo is one of the Greek constellations, first listed by Ptolemy in the 2nd century CE. It is the second largest constellation in the sky, after Hydra, occupying an area of 1,294 square degrees.

Virgo constellation, image: Roberto Mura

Virgo contains as many as 11 Messier objects (bright deep sky objects catalogued by the 18th century French astronomer and comet hunter Charles Messier), most of them part of the Virgo Cluster, the nearest large galaxy cluster to our galaxy. Its brightest members are visible in small telescopes. The centre of the cluster is found within the Spring Triangle, about halfway between Denebola in Leo and Vindemiatrix in Virgo.  Member galaxies include Virgo A (Messier 87), a supergiant elliptical galaxy whose supermassive black hole was the first to be directly imaged, the bright elliptical galaxy Messier 49, the barred spiral galaxies Messier 58, Messier 61 and NGC 4639, and the interacting pairs NGC 4435 and NGC 4438, also known as the Eyes Galaxies, and NGC 4567 and NGC 4568, nicknamed the Siamese Twins.

Virgo is also home to the famous Sombrero Galaxy (Messier 104), located 11.5 degrees west of Spica, and the supergiant elliptical galaxy IC 1101, one of the largest galaxies ever discovered.

The best time of year to observe the stars and deep sky objects of Virgo is during the month of May.

The 10 brightest stars in Virgo are Spica (Alpha Vir, mag. 0.97), Porrima (Gamma Vir, mag. 2.74), Vindemiatrix (Epsilon Vir, mag. 2.826), Heze (Zeta Vir, mag. 3.376), Minelauva (Delta Vir, mag. 3.39), Zavijava (Beta Vir, mag. 3.604), 109 Virginis (mag. 3.72), Mu Virginis (mag. 3.88), Zaniah (Eta Vir, mag. 3.89), and Nu Virginis (mag. 4.04).

Spica – Alpha Virginis

Spectral classB1V (B1III-IV + B2V)
Variable typeRotating ellipsoidal variable
U-B colour index0.94
B-V colour index-0.23
Apparent magnitude0.97 (0.97 – 1.04)
Absolute magnitude-3.55
Distance250 ± 10 light years (77 ± 4 parsecs)
Parallax13.06 ± 0.70 mas
Radial velocity+1.0 km/s
Proper motionRA: −42.35 ± 0.62 mas/yr
Dec.: −30.67 ± 0.37 mas/yr
ConstellationVirgo
Right ascension13h 25m 11.579s
Declination−11° 09′ 40.75″
DesignationsSpica, Alpha Virginis, α Vir, 67 Virginis, HD 116658, HR 5056, HIP 65474, SAO 157923, FK5 498, GCTP 18144, GC 18144, GCRV 7963, BD-10°3672, CCDM 13252-1109, EUVE J1325-11.1, IRAS 13225-1054, 2MASS J13251158-1109404, PPM 227262, TYC 5547-1518-1

Alpha Virginis A

Spectral classB1III-IV
Variable typeBeta Cephei
Absolute magnitude-3.5
Mass11.43 ± 1.15 M
Luminosity20,512 L(16,482 – 25,527 L)
Radius 7.47 ± 0.54 R
Temperature25,300 ± 500 K
Age12.5 million years
Rotational velocity165.3 ± 4.5 km/s
Surface gravity3.71 ± 0.10 cgs

Alpha Virginis B

Spectral classB2V
Absolute magnitude-1.5
Mass7.21 ± 0.75 M
Luminosity2,254 L(1,486 – 3,420 L)
Radius3.74 ± 0.53 R
Temperature20,900 ± 800 K
Rotational velocity58.8 ± 1.5 km/s
Surface gravity4.15 ± 0.15 cgs