Spica: The Ear of Wheat

Spica is the brightest star in Virgo and the 15th brightest star in the night sky. This brilliant blue-white beacon marks the ear of wheat held by the maiden figure of Virgo and serves as the endpoint of one of the most famous star-hopping routes in astronomy. Spica played a pivotal role in the history of science, leading to one of the most important astronomical discoveries of the ancient world. Find it with StarGlobe.

How to Find Spica

The classic route to Spica begins with the Big Dipper in Ursa Major. Follow the arc of the handle to reach Arcturus in Bootes, then continue the same gentle curve further south to Spica. The mnemonic "arc to Arcturus, speed on to Spica" has guided generations of stargazers. The distance from Arcturus to Spica is roughly the same as from the Dipper's handle to Arcturus.

Spica is easy to confirm because it sits in a relatively dim area of sky. Its blue-white color contrasts noticeably with the warm orange of Arcturus above it. Spica lies very close to the ecliptic, so the Moon and planets frequently pass near it, occasionally creating beautiful conjunctions.

A Close Binary System

Spica appears as a single star, but it is actually a close binary system consisting of two hot blue stars orbiting each other every 4.01 days. The primary, Spica A, is a blue giant with about 11 times the Sun's mass, 7.4 times its radius, and about 12,100 times its luminosity. The secondary, Spica B, has about 7 times the Sun's mass and roughly 1,500 times its luminosity.

The two stars orbit so closely that they are tidally distorted into egg shapes, with each star's gravity pulling the near side of its companion into a bulge. This distortion causes the system's brightness to vary slightly as the stars orbit, presenting different cross-sections to our view. The variation is about 0.03 magnitudes, too subtle for the naked eye but detectable with instruments.

The system is also a spectroscopic binary, meaning the two components can only be distinguished by the regular shifting of their spectral lines as they orbit. The stars are too close to separate visually in any telescope.

The Discovery of Precession

Spica's greatest contribution to science came around 129 BCE, when the Greek astronomer Hipparchus compared his observations of the star's position to measurements made about 150 years earlier by Timocharis. He found that Spica had shifted its position relative to the equinoxes by about 2 degrees. This led Hipparchus to discover the precession of the equinoxes, the slow wobble of Earth's rotation axis that causes the celestial poles and equinoxes to drift through the sky over a cycle of roughly 25,772 years.

This discovery was one of the most significant achievements in ancient astronomy and demonstrated the extraordinary precision of naked-eye observing that Hipparchus achieved.

Physical Properties

The Spica system lies about 250 light-years from Earth. The primary star has a surface temperature of approximately 22,400 Kelvin, making it one of the hottest first-magnitude stars. Both stars are evolving rapidly and are expected to end their lives as supernovae, though not for millions of years.

Spica is classified as a Beta Cephei variable, with the primary star pulsating slightly over a period of about 4.17 hours. This pulsation is distinct from the orbital variations and adds another layer of complexity to the system.

Cultural Significance

The name Spica comes from the Latin "spica virginis," meaning "the ear of wheat of the virgin." The star has been associated with agricultural goddesses across many cultures, representing the grain held by Virgo. In ancient Egypt, a temple at Thebes was aligned with the setting of Spica. In Chinese astronomy, Spica was the chief star of the Horn mansion, one of the 28 lunar mansions.

The association between Spica and agriculture reflects the practical importance of spring stars in ancient farming communities. The visibility of Spica in evening skies coincided with planting seasons across the Northern Hemisphere.

Best Time to Observe

Spica is best observed during April and May, when it culminates during convenient evening hours. The star is visible from February through July. Because it lies near the celestial equator (declination about -11 degrees), Spica is well-placed for observers in both hemispheres. From southern latitudes, it appears in the northern sky during autumn months.

Neighboring Stars and Constellations

Arcturus shines above Spica to the north. Leo and Regulus lie to the northwest. Corvus, the Crow, sits to the southwest. Libra extends to the east. The Virgo Cluster of galaxies spreads across the northern part of Virgo above Spica. Map the spring sky from Spica with StarGlobe.

Quick Facts

Spica has an apparent magnitude of 1.04, a distance of about 250 light-years, and a combined luminosity approximately 13,600 times the Sun. The orbital period is 4.01 days. Its right ascension is 13h 25m, and its declination is -11 degrees 10 minutes.