Characteristics
Spica is a close binary star whose components orbit about each other every four days. They remain sufficiently close together that they can not be resolved as individual stars through a telescope. The changes in the orbital motion of this pair results in a Doppler shift in the absorption lines of their respective spectra, making them a double-lined spectroscopic binary. Initially, the orbital parameters for this system were inferred using spectroscopic measurements. Between 1966 and 1970, the Narrabri interferometer was used to observe the pair and to directly measure the orbital characteristics and the angular diameter of the primary. The latter was determined to be (0.90 ± 0.04) × 10−3 arcseconds, while the angular size of the semi-major axis of the orbit was found to be only slightly larger at (1.54 ± 0.05) × 10−3 arcseconds.
The primary star has a stellar classification of B1 III-IV. The luminosity class matches the spectrum of a star that is midway between a subgiant and a giant star, and it is no longer a B-type main-sequence star. This is a massive star with more than 10 times the mass of the Sun and seven times the Sun's radius. The total luminosity of this star is about 12,100 times that of the Sun, and eight times the luminosity of its companion. The primary is one of the nearest stars to the Sun that has sufficient mass to end its life in a Type II supernova explosion.
The primary is classified as a Beta Cephei-type variable star that varies in brightness over a 0.1738-day period. The spectrum shows a radial velocity variation with the same period, indicating that the surface of the star is regularly pulsating outward and then contracting. This star is rotating rapidly, with a rotational velocity of 199 km/s along the equator.
The secondary member of this system is one of the few stars to display the Struve–Sahade effect. This is an anomalous change in the strength of the spectral lines over the course of an orbit, where the lines become weaker as the star is moving away from the observer. It may be caused by a strong stellar wind from the primary scattering the light from secondary when it is receding. This star is smaller than the primary, with about 7 times the mass of the Sun and 3.6 times the Sun's radius. Its stellar classification is B2 V, making this a main-sequence star.
Spica is a rotating ellipsoidal variable, which is a non-eclipsing close binary star system where the stars are mutually distorted through their gravitational interaction. This effect causes the apparent magnitude of the star system to vary by 0.03 over a time interval that matches the orbital period. This slight dip in magnitude is barely noticeable visually. The rotation rates of both stars are faster than their mutual orbital period. This lack of synchronization and the high ellipticity of their orbit may indicate that this is a young star system. Over time, the mutual tidal interaction of the pair may lead to rotational synchronization and orbit circularization.
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