Spica

Spica or Alpha Virginis (Alp Vir) is the brightest naked eye star in the constellation Virgo. With an apparent magnitude of 0.98v, Spica is the 15th brightest star in the entire sky (see: 50 Brightest Stars ). Its absolute magnitude is -3.55 and its distance is 262 light years. The Equinox J2000 equatorial coordinates are RA = 13h 25m 11.6s, Dec = -11° 09' 41".

Spica has a spectral type of B1V, a surface temperature of 22,400° Kelvin and a luminosity 12,100 times the Sun. It has a mass of 10.3 solar masses and a diameter 7.4 times the Sun.

The image above shows the uncropped view of Spica (North is up) through the Takahashi E-180 Astrograph.

A method of finding Spica is to follow the arc of the handle of the Big Dipper to Arcturus, and then continue on the same angular distance to Spica. This can be recalled by the mnemonic phrase, "follow the arc to Arcturus and speed on to Spica."

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. The orbital parameters for this system were first inferred using spectroscopic measurements.

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 star 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.

The description above is based on the Spica entry in Wikipedia. For more information about Spica, see Stars (Jim Kaler).

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