Earthsky Tonight
Spica: Speed on
June 29, 2009 - Brightest Stars
How to see it
The best evening views of Spica come from spring to late summer when this star arcs across the southern sky. Spica rises in the east-southeast as the sunset glow fades in mid-April. At that time, it is visible most of the night. Two months later it beams at its highest point to the south at early evening. By the end of August, Spica can be viewed only briefly in the west-southwestern sky as darkness falls.
Here’s how you can ’speed on’ to find Spica. First look for the Big Dipper in the northern sky. The Dipper is highest in the sky in spring and summer. Notice that the Big Dipper has a bowl and a long, curved handle. Find the Dipper, and then follow the curve of the its handle outward, away from the Dipper. The first bright star you come to is orange Arcturus, but if you continue past it in the curving path, the next star you come to is Spica. Old-timers remember this with the saying, ‘Arc to Arcturus, then speed on to Spica.’
Spica is the brightest light in the constellation Virgo the Maiden, and it is the 15th brightest star visible from anywhere on Earth. It’s virtually the same brightness as Antares in the constellation Scorpius, though, so sometimes Antares is listed as the 15th and Spica as the 16th brightest. No matter. Identify this beautiful blue-white star, and – with the Big Dipper’s help to spot it in the sky – it’ll be your friend for life.
History and Myth
Spica is from the Latin word for ‘ear,’ and the general connotation is that it refers to an ‘ear of wheat.’ Indeed, the star and the constellation Virgo itself were sometimes associated with the Greek goddess of the harvest, Ceres.
There are many names and stories for Spica’s constellation – Virgo – in mythology, and by association with Spica as well. Fewer stories refer to Spica independently. Many classical references refer to Virgo’s stars as a goddess or in some associations with wheat or the harvest. In Greece and Rome she typically was Astraea, the very personification of Justice; or Persephone, daughter of Ceres. In Egypt, Virgo was identified with Isis, and Spica was considered her lute bearer. In ancient China, Spica was a special star of spring known as The Horn.
On Arabic name was Azimech, derived from words meaning Defenseless One or Solitary One. This title may be in references to Spica’s solitary status with no other bright stars nearby. But Spica is not the _most_ solitary star. That honor goes to Fomalhaut, sometimes called the Autumn Star.
Science
Spica is about 262 light-years away, based on data from the Hipparcos Space Astrometry Mission of the 1990’s. Spica is a binary star, consisting of two main components which are telescopically indistinguishable from a single point of light. The dual nature of this star was revealed only by analysis of its light with a spectroscope. Both stars are larger and hotter than the sun, with the larger of the two, in fact , a blue giant or subgiant. The surface temperatures are estimated at 22,400 K and 18,500 K for the larger and smaller components, respectively. This compares to about 5,800 K for the sun. (“K” stands for “kelvins”, a degree on the absolute scale. At high values, each kelvin is about 5/9ths the corresponding Fahrenehit degree, so these two stars are about 40,000 F and 33,000 F, compared to 10,000 F for the sun.)
Separated by only less than 18 million km (about 11 million miles), they orbit a common center of gravity in only 4 days. The resulting forces on these stars are so great they they likely have been distorted, being more egg-shaped than spherical. Although this cannot be visually seen, it is supported by slight magnitude variations. The magnitude changes could be the result of the stars showing more or less surface area as they move through different orientations in their orbits. Taken together, the light from these two stars averages more than 2200 times brighter than the sun, and their diameters are estimated to be 7.8 and 4 times the sun’s diameter.
Spica is one of several bright stars that the moon can occult (eclipse). Based on observations of how the star’s light is extinguised when the moon passes in front, some astronomers think that it many not just be a spectroscopic binary star. Instead, they feel that there may be as many as 3 other stars in the system. This would make Spica not a single or even double star, but in fact a quintuple star!
Spica’s position is RA: 13h 25m 12s, dec: -11° 09′ 41″