More commonly, however, they exist in pairs, multiple systems with several members, or clusters consisting of numerous components. Furthermore, stars vary greatly in brightness, colour, temperature, mass, size, chemical composition, and age. Sirius, with an apparent magnitude of -1.5. Stars visible to the unaided eye have values of six or lower. A star's apparent magnitude is not a direct indicator of its luminosity.
Stars differ widely in colour, ranging from blue-white to red.
Stellar colours can be measured by several methods. The most widely employed technique requires the use of a photoelectric photometer, which can measure stellar colours with the aid of filters. The atmospheric, or surface, temperature of a star can be estimated by studying its spectrum, because temperature largely determines the types of absorption lines present. In nearly all stars, hydrogen is the most abundant element, but it is completely ionized in the outer layers of stars having a temperature in excess of 25,000 K and thus produces virtually no absorption lines. The spectra of stars with still lower surface temperatures manifest different absorption line patterns.