The Latin word spicum refers to the ear of wheat Virgo holds in her left hand. In Greek and Roman mythology, the constellation and the Spica star were associated with Demeter (Ceres), who was the goddess of the harvest.
The Spica Star – Location and Distance
Spica is located in the constellation of Virgo and is around 262 light years away from Earth. It is bright not only because of its size, but also because of its proximity to Earth. Its right ascension is 13h 25m 11.579s and its declination is −11° 09′ 40.75″.
Spica star can be found by following the arc of the Big Dipper‘s handle, formed by the bright Alkaid, Mizar and Alioth. The first bright star along the imaginary line is Arcturus, the Bear Watcher, and following the same curving path, the second bright star that appears is Spica.
It is located close to the ecliptic, roughly 10 degrees south of the celestial equator, and can be occulted by the Moon, sometimes even by planets. The last time it was occulted by a planet was on November 10, 1783, when Venus passed in front of it.
Spica is best observed from spring to late summer. The Sun passes about 2 degrees north of the star every year around October 16.
Spica Star System, Size and Composition
The star Spica is a blue subgiant star that is really a close binary star system. It is one of the nearest massive binary stars to the Solar System.
It is classified as a double-lined spectroscopic binary star, showing a Doppler shift in the absorption lines of the stars’ spectra. This is a result of the changes in the star system’s orbital motion.
The two components in the system, Spica A and Spica B, have a radius 7.40 and 3.64 times that of the Sun, which is one of the reasons why it is so bright. Both of these stars are much brighter than the Sun, with total luminosities, including UV radiation, of 12,100 and 1,500 times solar. They are also much hotter than the Sun, with estimated surface temperatures of 22,400 K and 18,500 K.
Spica A and Spica B both belong to the spectral class B, which makes them bluish-white in colour. The star is classified as a Beta Cephei variable, and exhibits variations in brightness over a 0.1738-day period. The star is a very fast spinner, with a projected rotational velocity of 199 km/s at the equator.
Spica A is halfway between a subgiant and giant on the evolutionary scale. It belongs to the spectral class B1 III-IV. Because it has a mass more than 10 times that of the Sun, it is massive enough to end its life in a Type II supernova explosion, and is one of the nearest stars to Earth to be able to do so. 80 percent of the light in the system comes from Spica A.
Spica B belongs to the spectral class B2 V, which means that it is still on the main sequence. It is smaller and less massive than the primary star, with only 7 solar masses.
The two components are very close together, only 0.12 AU apart, and therefore complete an orbit around each other every four days. For this reason also, they can’t be resolved in a telescope.
Spica exhibits variations in magnitude of 0.03 over a period matching the stars’ orbital period. The variations in the magnitude are likely the result of the two stars showing more or less surface area as they orbit each other.
Spica A and B orbit a common centre of mass, also known as a barycentre, at about 56 miles per second at a distance of only 18 million kilometres. It is also a rotating ellipsoidal variable. This means that the two stars in the system are distorted by their gravitational interaction. The system is a non-eclipsing binary, with neither star passing in front of the other from our point of view.
The system is a massive source of X-rays. At least some of the X-rays are believed to be produced by violent collisions of the stars’ stellar winds.
Spica moves through the galaxy at a speed of 18.9 km/s relative to the Sun.
Spica is believed to be one of the bright stars that made it possible for Hipparchus (160 – 120 BC) to discover the precession of the equinoxes, after comparing his data to that of the Alexandrian Timochares, who had observed Spica and Regulus around 300 BC. It was later observed by Nicolaus Copernicus, who also used it to study precession.
Spica has had many different names over the years. The 17th century German astronomer Johann Bayer called the star Arista. In medieval times, the names for the star included Azimech and Alarph. Both were derived from Arabic; Azimech from al-simāk al-a‘zal, meaning “the undefended” or “the solitary one.” Alarph, from the Arabic phrase meaning “the grape gatherer.”
Babylonian observers called the star Sa-Sha-Shirū, meaning “the virgin’s girdle”. The Ancient Chinese considered Spica to be a special star of spring and called it Kio, the Horn.
- Spica Star Image – https://earthsky.org/upl/2017/04/spica-artist.jpg
- Location Of Spica – https://earthsky.org/upl/2013/04/13april05_430.jpg
- Spica size comp – https://img.bhs4.com/3f/b/3fb3982655ed315e3d963b731be142cb7c77ea2f_large.jpg