The Hertzsprung–Russell Diagram

Apparent magnitude, m, is the relative brightness of a star (or other stellar object) as viewed from the Earth; the lower the magnitude, the brighter the star. Sirius, the brightest star, has an apparent magnitude of -1.46, while the faintest stars visible to the naked eye have magnitudes of about 6. A difference of five magnitudes between two objects is a factor of 100 in brightness. The apparent brightness of a star depends on two factors: its actual brightness, and its distance from the Earth. Absolute magnitude, M, measures the actual brightness of a star, so the distance of the star from the Earth is needed. The absolute magnitude reflects the true amount of light, and therefore energy, emitted by the star. Often this value is expressed as the luminosity of the star.