Spica, the brightest star in the constellation Virgo, is a prominent blue-white point of light in the night sky. The star’s striking color immediately suggests an extreme temperature, setting it apart from the cooler, redder stars. As one of the sky’s 20 brightest stars, Spica is a massive object of intense study, giving astronomers insight into the lives of hot, giant stars.
Surface Temperature and Spectral Classification
The primary star in the system, Spica A, has an effective surface temperature estimated to be around 25,300 Kelvin (K). This temperature is approximately 45,000 degrees Fahrenheit or 25,000 degrees Celsius, which is over four times hotter than the surface of our own Sun.
This extreme thermal energy dictates Spica’s brilliant blue-white color, a direct consequence of Wien’s law, where hotter objects emit light at shorter, bluer wavelengths. The secondary star, Spica B, is also very hot, with an estimated surface temperature of about 20,900 K. Both stars belong to the spectral classification of B-type stars.
The classification for Spica A is specifically B1III-IV, indicating it is an evolved star between a subgiant and a giant star. Stars of this class generate enormous amounts of energy from hydrogen fusion in their core, achieving high surface temperatures due to the rapid consumption of nuclear fuel.
Spica’s Immense Energy Output
Spica’s extreme temperature and immense physical size result in high luminosity. The primary star, Spica A, has a radius roughly 7.5 times that of the Sun. This massive surface area, coupled with its heat, causes the primary star to shine with a luminosity over 20,500 times greater than the Sun.
The secondary star, Spica B, is smaller, with a radius about 3.7 times that of the Sun and a luminosity of over 2,200 times solar. The system is highly luminous, producing a significant amount of light in the invisible ultraviolet spectrum due to its high temperature.
Because of their high mass—Spica A is over 11 times the mass of the Sun—these stars have a much shorter lifespan than less massive stars. Spica’s existence on the main sequence is measured in tens of millions of years. The primary star is massive enough that it is expected to eventually end its life in a core-collapse supernova.
The Close Binary System
Spica is a close binary system composed of two massive B-type stars, Spica A and Spica B, orbiting a common center of mass. They are separated by an average distance of only about 18 million kilometers, which is significantly closer than Mercury is to the Sun. Due to this proximity, they cannot be resolved as two separate stars even through a powerful telescope.
Their mutual gravitational pull is strong enough that both stars are tidally distorted into an egg-like shape rather than being perfectly spherical. They complete a full orbit around each other in just over four days. This rapid, close orbit classifies Spica as a double-lined spectroscopic binary, where their orbital motion is detected by analyzing the shift in their light’s spectral lines.
Spica A is the larger, more luminous component and is classified as an evolved star nearing the end of its main-sequence life, while Spica B is still a main-sequence star. The intense stellar wind from the primary star affects the spectrum observed from the secondary star, a phenomenon known as the Struve-Sahade effect.
Locating Spica in the Night Sky
Spica is the most conspicuous star in the constellation Virgo, located about 250 light-years from Earth. Despite this vast distance, its inherent luminosity allows it to appear as one of the brightest stars we can see.
A well-known method for finding Spica involves using the handle of the Big Dipper asterism. Trace the curve to “arc to Arcturus” in the constellation Boötes, and then continue that line to “speed to Spica.” Its position near the ecliptic—the path the Sun, Moon, and planets appear to follow—also means it is occasionally occulted, or hidden, by the Moon.