Understanding Spica (Alpha Virginis)
The familiar star Spica (Alpha Virginis) is the fifteenth brightest star in the night sky, in part because it is relatively nearby, only about 250 light-years away. It is easy to find by following the arc of the Big Dipper's handle and using the mnemonic, "Arc to Arcturus (Alpha Bootes) and then spike to Spica." In fact Spica is a "spectroscopic" binary, two stars orbiting each other and too close together to separate visually. They were revealed as being a binary pair in 1890 when spectroscopic observations discovered that the stellar lines were doubled due to each star having a slightly different velocity and corresponding Doppler shift. The stars in Spica are, moreover, an unusual pair: They are very close, separated by about twenty-eight solar-radii, and orbit each other in only 4.01 days. This puts them so close together that their mutual gravity tidally distorts their atmospheres, with the result...