Each star is a self-luminous gaseous globe similar to the Sun. Some are larger, some smaller, some hotter, some cooler, some more brilliant, and some inherently dimmer. All are located at distances that are many orders of magnitude greater than the separation between the Earth and the Sun. Our knowledge of stars has been built up primarily from measurements of their brightnesses and changes in brightness, their positions and changes in position, and their colors and spectra, as well as by applying our knowledge of physics and chemistry to interpreting these observations.

BRIGHTNESS AND MAGNITUDES

The brightness of a star as seen in the sky is described by a quantity called apparent magnitude – a measure of the amount of light arriving here on Earth that owes its origins to the work of Hipparchus, the outstanding Greek observer of the second century B.C. Hipparchus divided stars into six classes, or magnitudes, with the brightest stars being designated first magnitude and the faintest visible stars, sixth magnitude.

The magnitude system was put on a firm mathematical footing in 1856 by the English astronomer N. R. Pogson. He defined the magnitude scale in such a way that a difference of five magnitudes (e.g., between magnitude 1 and magnitude 6 or between magnitude 6 and magnitude 11) corresponds to a difference in brightness of a factor of precisely one hundred, and a difference of one magnitude corresponds to a brightness factor equal to the fifth root of one hundred, which is 2.512.