# Magnitude (astronomy)

measure of brightness for celestial objects, as seen from Earth
(Redirected from Absolute magnitude)

The magnitude of a star or other celestial object is a number that measures its brightness. Apparent magnitude (m) 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.[1]

## History

The Greek astronomer Hipparchus first invented our system of apparent magnitude.[2] He gave the brightest stars a magnitude of 1 and increased the numbers for dimmer stars.[2] About 300 years later Ptolemy of Alexandria made an encyclopedia of stars based on Hipparchus' work.[2] The Persian astronomer Al-Sufi took Ptolemy's work and gave Arabic names to the stars some 800 years later.[2] Medieval European astronomers simply translated Al-Sufi's work into Latin which is why many stars today have Arabic names.[2]

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

## Magnitude systems

Visible to
typical
human eye
Apparent
magnitude
Brightness
relative
to Vega
Number of stars
brighter than
apparent magnitude[3]
in the night sky
Yes −1.0 250% 1
0.0 100% 4
1.0 40% 15
2.0 16% 48
3.0 6.3% 171
4.0 2.5% 513
5.0 1.0% 1 602
6.0 0.40% 4 800
6.5 0.25% 9 096[4]
No 7.0 0.16% 14 000
8.0 0.063% 42 000
9.0 0.025% 121 000
10.0 0.010% 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.[5] 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.[6] 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.[6]
• Visual magnitude means the amount of brightness of a star on a numbered scale.[7] It is the magnitude based on the sensitivity of the human eye.[8]
• Photographic magnitude is different from visual magnitude because photographic film is more sensitive to blue light and radiation the human eye cannot see.[8]

## References

1. ISS - Satellite Information. [1]
2. Leila Belkora, Minding the Heavens: The Story of our Discovery of the Milky Way (Bristol: Institute of Physics, 2003), pp. 19–20
3. "Magnitude". National Solar Observatory—Sacramento Peak. Archived from the original on 2008-02-06. Retrieved 2006-08-23.
4. Bright Star Catalogue
5. Robert Bruce Thompson; Barbara Fritchman Thompson, Astronomy Hacks (Sebastopol, CA: O'Reilly, 2005), p. 88
6. Rony De Laet, The Casual Sky Observer's Guide: Stargazing with Binoculars and Small Telescopes (New York: Springer, 2012), p. 29
7. Nick Greene. "Star Magnitudes". About.com. Archived from the original on 12 July 2014. Retrieved 20 September 2014.
8. Hannu Karttunen; et al., Fundamental Astronomy (Berlin; New York: Springer, 2007), p. 86