Magnitude
Magnitude (astronomy), term used in astronomy to designate the brightness of a star . Magnitude, also called apparent magnitude, describes the brightness of a star as viewed from the earth . The term absolute magnitude refers to the brightness of a star as viewed from a standard distance of 10 parsecs, or about 32 light-years. The ancient Alexandrian astronomer Ptolemy originally divided all visible stars into six magnitudes: the brightest were called first magnitude, those barely visible to the naked eye were called sixth magnitude, and the other visible stars were assigned intermediate positions. After the introduction of the telescope in the 17th century, this system of magnitudes was used and extended to the fainter stars in different ways by different astronomers. In the 19th century a standard system was finally adopted under which a star of any given magnitude is 2.512 times as bright as a star of the next higher magnitude; thus, for example, a star of the second magnitude is 2