A considerable number of the brighter and more important stars are known by individual names, such as Sirius, Canopus, Achernar, Arcturus, Vega, Rigel, Betelgeuse, Procyon, Spica, Aldebaran, Regulus, Altair, and Fomalhaut. Astronomers usually designate the principal stars of each constellation by the letters of the Greek alphabet, �, �, �, etc., the brightest star in the constellation bearing the name of the first letter, the next brightest that of the second letter, and so on.