Spike (star)

Spiga or Spica (Alpha Virginis / α Vir / 67 Vir) is the brightest star in the constellation Virgo and the fifteenth brightest in the night sky. Of apparent magnitude +1.04, it is 260 light years from the solar system. It is thought that Hipparco de Nicaea discovered the precession of the equinoxes from the data obtained from Espiga.

Name

Espiga or Spica are names that come from the Latin spica virginis, the ear of grain (usually wheat) of The Virgin. For the Romans this star symbolized Ceres, goddess of agriculture. Spicum, Spigha, Stachys —from the Greek stakhus, “ear of wheat”— and Arista—Latin for “ear of grain”—were other Roman names for the star. Other names such as Sunbala or Sumbela come from the Arabic word sunbula, a synonym for spike.

An alternative name for the star is Azimech, from the Arabic as-simak al-a'zal, "the defenseless one", alluding to its separation from from nearby stars. A third name, Alaraph, has been used to designate this star as well as the neighboring stars Vindemiatrix (ε Virginis) and Zavijava (β Virginis).

In Babylonia, representing the entire constellation, she personified the «wife of Bēl» and, as Sa-Sha-Shirū, «the girdle of the Virgin», she marked the twentieth ecliptic asterism of the same name. She was also Emuku Tin-tir-Ki, a common title for Babylon itself. In ancient Egypt she was known as "the bearer of the lute", her importance being remarkable, since another Egyptian name she received was Repā , "the Lord". Some authors maintain that one of the temples of Thebes, built in honor of Menat around the year 3200 B.C. C., is oriented towards this star.

In Chinese astronomy it was called Kió, "the horn" or "the spike", and in ancient times it was Keok or Guik, the special star of spring. Together with Heze (ζ Virginis) it forms the Chinese constellation of Jiăo (角). For the Hindus, Espiga corresponds to the nakshatra or astrological mansion of Citrā.

Physical characteristics

Spiga forms a binary system whose components, of spectral type B, are very close to each other. The separation between the two is just 0.12 astronomical units (au) and their orbital period is 4.0145 days. It is an intense source of X-rays that appear to be generated by the collision of the strong stellar winds emitted by both stars.

The main star, Spike A, has a spectral type B1 —classified as a giant or subgiant— and a surface temperature of 22,400 K. With an intrinsic luminosity 13,400 times that of the Sun, its radius is 7.4 times larger than the solar radius—nearly 30% of the separation between the two stars—and it rotates very fast with a rotation speed at its equator of 199 km/s. It has a metallicity somewhat lower than solar ([M/H] = -0.12 ± 0.08). It has a mass 11 times greater than the solar mass, being one of the closest stars to us with enough mass to end its life as a type II supernova.

Due to its proximity to the main star, the parameters of Spike B are poorly known; it appears to be a main sequence star of type B4V with a surface temperature of 18,500 K. Spike B is about 1,700 times more luminous than the Sun, its radius is almost 4 times that of the Sun, and its mass is somewhat less at 7 solar masses. It is one of the few stars where the "Struve-Sahade effect" is observed, an anomalous variation in the intensity of the spectral lines over the course of an orbit, the lines becoming fainter as the star moves away from the observer. It may be caused by the strong stellar wind from the primary scattering light from the secondary as it recedes.

Spiga A+B is a variable star whose brightness variation between magnitude +0.92 and 1.04 is visible to the naked eye. A variation of 0.03 magnitudes is due to the fact that Spike is a rotating ellipsoidal variable, the brightest of its kind. In these variables the two stars are so close that, due to tidal forces, their shapes are not spherical but ellipsoidal. This causes its brightness to fluctuate depending on the area visible by the observer at a given instant. In turn, Spike A is a Beta Cephei type variable, like Murzim (β Canis Majoris), whose brightness variations are caused by non-radial pulsations on the surface over a period of 0.17 days.

Being close to the ecliptic, Spike can be occulted by the Moon and sometimes by planets. The next occultation by a planet will take place on September 2, 2197, when Venus passes in front of Spike.