Magnitude (astronomy)

The magnitude of a star or other celestial object is a number that measures its brightness. Apparent magnitude () of a celestial object is a number that measures its brightness as seen by an observer on Earth, and absolute magnitude is its actual brightness adjusted for distance. The brighter an object appears, the lower its magnitude value. The Sun, at apparent magnitude of −27, is the brightest object in the sky.

Magnitude is a logarithmic measure.  It is measured in a specific wavelength or passband, usually in optical or near-infrared wavelengths.  A full moon is −13 magnitude and the brightest planet, Venus, measures −5.  The brightest man-made objects, Iridium flares, are ranked at −9 and the International Space Station measures −6.ISS - Satellite Information. 

 History 
The Greek astronomer Hipparchus first invented our system of apparent magnitude.Leila Belkora, Minding the Heavens: The Story of our Discovery of the Milky Way (Bristol: Institute of Physics, 2003), pp. 19–20 He gave the brightest stars a magnitude of 1 and increased the numbers for dimmer stars. About 300 years later Ptolemy of Alexandria made an encyclopedia of stars based on Hipparchus' work. The Persian astronomer Al-Sufi took Ptolemy's work and gave Arabic names to the stars some 800 years later. Medieval European astronomers simply translated Al-Sufi's work into Latin which is why many stars today have Arabic names.

In recent centuries astronomers have been able to measure the distance of stars, and calculate their absolute magnitude.

 Magnitude systems 
Visible totypicalhuman eyeApparentmagnitudeBrightnessrelativeto VegaNumber of stars brighter thanapparent magnitudein the night skyYes−1.0250%10.0100%41.040%152.016%483.06.3%1714.02.5%5135.01.0%1 6026.00.40%4 8006.50.25%9 096Bright Star CatalogueNo7.00.16%14 0008.00.063%42 0009.00.025%121 00010.00.010%340 000

 Apparent magnitude m is a measure of the brightness of a celestial body as seen by a person on Earth (without the earth's atmosphere). The brighter the object appears, the lower the numerical value of its magnitude.Robert Bruce Thompson; Barbara Fritchman Thompson, Astronomy Hacks (Sebastopol, CA: O'Reilly, 2005), p. 88 For example, Alpha Centauri has higher apparent magnitude (i.e. lower value) than Betelgeuse, because it is much closer to the Earth.
 Absolute magnitude is the apparent magnitude, m, an object would have if it were at a standard distance from Earth. The standard distance is 32.6 light years from Earth.Rony De Laet, The Casual Sky Observer's Guide: Stargazing with Binoculars and Small Telescopes (New York: Springer, 2012), p. 29 The concept of absolute magnitude allows astronomers to compare the brightness of objects without regard to their distance. When measured using absolute magnitude the sun measures less than Sirius at only 4.8.
 Visual magnitude means the amount of brightness of a star on a numbered scale. It is the magnitude based on the sensitivity of the human eye.Hannu Karttunen; et al., Fundamental Astronomy (Berlin; New York: Springer, 2007), p. 86
 Photographic magnitude is different from visual magnitude because photographic film is more sensitive to blue light and radiation the human eye cannot see.

 Related pages 
 List of brightest stars
 List of most luminous known stars
 List of nearest galaxies
 List of nearest stars

 References 

 Other websites 
 Magnitude System
 Magnitudes: Measuring the Brightness of Stars

Category:Astronomy