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::Magnitude (astronomy)

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Night sky with a very bright satellite flare
Hubble Ultra Deep Field part.jpgCometBorrelly1002.jpg
  • Top: Light sources of different magnitudes. A very bright satellite flare can be seen in the night sky.
  • Bottom: The Hubble Ultra-Deep Field detected objects as faint as 30th magnitude (left). Comet Borrelly, the colors show its brightness over the range of three orders of magnitude.

In astronomy, magnitude is the logarithmic measure of the brightness of an object, measured in a specific wavelength or passband, usually in the visible or near-infrared spectrum. An imprecise but systemic determination of the magnitude of objects was introduced in ancient times by Hipparchus.

Astronomers use two different definitions of magnitude: apparent magnitude and absolute magnitude. The apparent magnitude (m, or vmag for the visible spectrum) is the brightness of an object as it appears in the night sky from Earth, while the absolute magnitude (Mv, V and H) describes the intrinsic brightness of an object as it would appear if it were placed at a certain distance from Earth. This distance is 10 parsecs for stars and 1 astronomical unit for asteroids and planets. The size of an asteroid is typically estimated based on its absolute magnitude.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

The brighter an object appears, the lower the value of its magnitude, with the brightest objects reaching negative values. The Sun has an apparent magnitude of −27, the full moon −13, the brightest planet Venus measures −5, and Sirius, the brightest visible star in the night sky is at −1.5. An apparent magnitude can also be assigned to man-made objects in Earth orbit. The brightest satellite flares are ranked at −9, and the International Space Station appears at a magnitude of −6. Since the scale is logarithmic, each step of one magnitude changes the brightness by a factor of about 2.512. A magnitude 4 star is exactly a hundred times brighter than a magnitude 9 star, as the difference of five magnitude steps corresponds to (2.512)5 or 100.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>


Magnitude (astronomy) sections
Intro   History    Scale    Apparent and absolute magnitude    Problems    See also    Notes    References    External links   

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