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 2.512 times as bright as a star of the third magnitude. The advantage of this particular magnitude ratio, 2.512, is that it coincides closely with the Ptolemaic system; and because 2.512 is the fifth root of 100, a star of the first magnitude is exactly 100 times as bright as a star of the sixth magnitude, a star of the sixth magnitude is exactly 100 times as bright as a star of the 11th magnitude, and so on. The mean of the magnitudes of several hundred stars found in the Bonn Durchmusterung catalog, which was prepared by the German astronomer Friedrich Wilhelm August Argelander about 1860, was taken as the standard of the scale for calibration purposes.
With accurate instruments, such as bolometers and radiometers, astronomers today can measure differences as small as one-hundredth of a magnitude. Stars with magnitudes between 1.5 and 2.5 are called second-magnitude stars. Stars brighter than magnitude 1.5, of which there are 20, are called first-magnitude stars. Thus, the first-magnitude star Aldebaran has an actual magnitude of 1.1; the slightly brighter first-magnitude star Altair has a magnitude of 0.9. The brightest stars are brighter than magnitude zero. Sirius, the brightest star outside the solar system, has a magnitude of -1.6. The sun has a magnitude of -26.7, inasmuch as it is about 10 billion times as bright as Sirius in the earth’s sky.
Absolute magnitude, as opposed to apparent magnitude, indicates the brightness that a star would have if it were placed at a distance from the earth of ten parsecs, or 32.6 light-years. By rating stars in this way, astronomers are able to compare them with respect to intrinsic brightness. The sun, for example, has an absolute magnitude of +4.7. Sirius has an absolute magnitude of +1.4.
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 2.512 times as bright as a star of the third magnitude. The advantage of this particular magnitude ratio, 2.512, is that it coincides closely with the Ptolemaic system; and because 2.512 is the fifth root of 100, a star of the first magnitude is exactly 100 times as bright as a star of the sixth magnitude, a star of the sixth magnitude is exactly 100 times as bright as a star of the 11th magnitude, and so on. The mean of the magnitudes of several hundred stars found in the Bonn Durchmusterung catalog, which was prepared by the German astronomer Friedrich Wilhelm August Argelander about 1860, was taken as the standard of the scale for calibration purposes.
With accurate instruments, such as bolometers and radiometers, astronomers today can measure differences as small as one-hundredth of a magnitude. Stars with magnitudes between 1.5 and 2.5 are called second-magnitude stars. Stars brighter than magnitude 1.5, of which there are 20, are called first-magnitude stars. Thus, the first-magnitude star Aldebaran has an actual magnitude of 1.1; the slightly brighter first-magnitude star Altair has a magnitude of 0.9. The brightest stars are brighter than magnitude zero. Sirius, the brightest star outside the solar system, has a magnitude of -1.6. The sun has a magnitude of -26.7, inasmuch as it is about 10 billion times as bright as Sirius in the earth’s sky.
Absolute magnitude, as opposed to apparent magnitude, indicates the brightness that a star would have if it were placed at a distance from the earth of ten parsecs, or 32.6 light-years. By rating stars in this way, astronomers are able to compare them with respect to intrinsic brightness. The sun, for example, has an absolute magnitude of +4.7. Sirius has an absolute magnitude of +1.4.
