Spica, whose name means “ear of grain,” is the brightest star in the constellation Virgo and the 15th-brightest star in the night sky. Located approximately 250 light-years from Earth, it appears to be a single blue-white star but is actually a pair of hot, massive stars orbiting each other every four days.
Easy to Find by Following the Arc to Arcturus
Spica is especially prominent from mid-northern latitudes on spring and early-summer evenings. To find it, follow the curve of the Big Dipper’s handle until you reach the bright orange star Arcturus, then continue the same curve southward to blue-white Spica—an observing method commonly remembered as “arc to Arcturus, then spike to Spica.” Because Spica lies close to the ecliptic, it can also occasionally be hidden, or occulted, by the Moon.
The 15th-Brightest Star in the Night Sky
Spica has an apparent magnitude of approximately 0.97, making it the brightest star in Virgo and the 15th-brightest star in the night sky. Although it lies around 250 light-years away, its enormous intrinsic luminosity allows it to appear as a conspicuous first-magnitude star from Earth. Its blue-white color is also noticeably different from the warmer orange appearance of nearby Arcturus.
Spica Is Actually Two Massive Blue Stars
Although Spica appears as a single point of light through an ordinary telescope, astronomers know it is a binary system consisting of two hot B-type stars because their spectral lines shift as they orbit each other. The primary has approximately 11 times the Sun’s mass, about 7.5 times its radius and a surface temperature near 25,000 kelvins. Its companion has roughly 7 solar masses, 3.7 times the Sun’s radius and a temperature of around 21,000 kelvins. Together, the two stars radiate more than 20,000 times as much energy as the Sun.
Its Two Stars Orbit Every Four Days
Spica’s two stars complete one orbit around their common center of mass every 4.0145 days. They are separated by approximately 18 million kilometers (11 million miles), only about one-eighth of the average distance between Earth and the Sun. Despite moving around each other at enormous speeds, the stars are so distant and closely spaced that they still appear as a single point of light from Earth.
Gravity Pulls Its Stars Out of Shape
Spica’s components orbit so close together that their mutual gravity distorts them into elongated shapes rather than perfect spheres. As the stars orbit, the projected surface area presented toward Earth changes slightly, causing the system’s combined brightness to vary. Spica is therefore classified as an ellipsoidal variable, although these small changes cannot be detected with the naked eye.
Its Primary Star Pulsates in Several Ways
In addition to the variation caused by Spica’s distorted shapes, its primary star also pulsates. Astronomers have identified three separate pulsation modes: one causes the star to expand and contract radially, while the other two move different regions of its surface in less symmetrical patterns. One pulsation is closely tied to the orbital motion, suggesting that it is driven or influenced by the gravitational pull of the companion star.
Only Around 12.5 Million Years Old
Spica’s two stars are estimated to be only about 12.5 million years old, compared with approximately 4.6 billion years for the Sun. Massive stars consume their nuclear fuel far more rapidly than lower-mass stars, giving them much shorter lifespans. With roughly 11 times the Sun’s mass, Spica’s primary is expected eventually to undergo core collapse and explode as a supernova, although interaction between the two close stars could affect how the system evolves.
Spica Helped Reveal Earth’s Axial Precession
In the second century BC, the Greek astronomer Hipparchus compared his measurement of Spica’s position with observations recorded more than a century earlier. He found that the star’s position relative to the autumnal equinox had shifted, helping him recognize that the equinox points slowly move against the background stars. This effect, now called axial precession, is caused by the gradual wobble of Earth’s rotational axis over a cycle lasting approximately 26,000 years.