Spica (Alpha Virginis) is a blue-white giant star located around 260 light-years away in the constellation of Virgo. This 1st magnitude star is the 15th brightest in the entire night sky, and derives its name from the Latin phrase meaning “ear of grain” or “ear of wheat.” Below are some more quick facts about the star Spica:
• Constellation: Virgo
• Coordinates: RA 13h 25m 11.579s|DEC-11°09’40.75
• Distance to Earth: 260 light years
• Star Type: Blue-white Variable Giant (B1 III-IV/B2 V)
• Mass: Primary 10 solar masses|Secondary 7 solar masses
• Radius: Primary 7.40 solar radii|Secondary 3.64 solar radii
• Apparent Magnitude: +0.97 (0.97 – 1.04)
• Luminosity: Primary 12,100 sol|Secondary 1,500 sol
• Surface Temperature: Primary 22,400K|Secondary 18,500K
• Other Designations: Azimech, Spica Virginis, Alaraph, Dana, a Virginis
Spica is actually a binary system whose primary star is around 10 times the mass of our Sun, with its companion star of about 7 solar masses. The primary component is a blue main sequence star that is about midway through the transition from being a sub-giant to becoming a true giant star with sufficient mass to go supernova when it ends its life. Being a Beta Cephei-type variable, the primary star’s luminosity varies slightly over a period of 0.1738 days.
The two components of the binary system are too close together to be resolved optically, with the average distance between the two stars only about 18 million kilometers (11 million miles). They orbit each other at about 90 km/sec (56 m/sec), and complete one orbit every four days.
Spica is the most conspicuous star in the constellation Virgo, and from the northern hemisphere is best seen from March to July, and from the Southern hemisphere in Autumn and Winter. An easy way to find Spica is to look for the right-angled ‘Spring Triangle‘ formed by the bright stars of Spica, Arcturus, and Regulus. The line between Spica and Regulus forms the base of the triangle.
Spica is located so close to the ecliptic that it can be occulted by the Moon, and less frequently by the planets. Venus last occulted Spica in November 1783, and will do so again in September 2197.
Spica holds pride of place as the star that allowed the Greek astronomer Hipparchus to prove that the stars are moving across the sky, a phenomenon that we know today as “precession of the equinoxes.” According to extant manuscripts, Hipparchus (190BC to 120BC) had used data from earlier times to show that Spica had moved measurably from its position relative to the Temple of Menat that was built in about 3,200 B.C.
In later ages, Nicolaus Copernicus confirmed the original measurements taken by Hipparchus, and further confirmed precession with measurements taken with a home-made instrument called a triquetrum, used for measuring the altitudes of the stars and other astronomical bodies.
In classical Greek and Roman mythology, “Virginis” was almost universally taken to refer to Dike, the daughter of Zeus and Themis, and a goddess of justice. Dike is said to have been among the last of the gods to depart from the Earth at the conclusion of the Golden Age, a period of primordial harmony presided over by the Titan Cronus
In some Egyptian traditions, Spica was associated with the goddess Isis, and honoured as “The Lute Bearer.” One ancient Egyptian legend holds that Pharaoh Akhenaton had a temple built in honour of his wife Nefertiti, whose beauty was said to compare to that of even the bright star Spica.