Absolute magnitude

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In astronomy, absolute magnitude (also known as absolute visual magnitude when measured in the standard V photometric band) measures a celestial object's intrinsic brightness. To derive absolute magnitude from the observed apparent magnitude of a celestial object its value is corrected from distance to its observer. The absolute magnitude then equals the apparent magnitude an object would have if it were at a standard luminosity distance (10 parsecs, or 1 AU, depending on object type) away from the observer, in the absence of astronomical extinction. It allows the true brightnesses of objects to be compared without regard to distance. Bolometric magnitude is luminosity expressed in magnitude units; it takes into account energy radiated at all wavelengths, whether observed or not.

The absolute magnitude uses the same convention as the visual magnitude: a factor of 100.4 (≈2.512) ratio of brightness corresponds to a difference of 1.0 in magnitude. The Milky Way, for example, has an absolute magnitude of about −20.5. So a quasar at an absolute magnitude of −25.5 is 100 times brighter than our galaxy (because (100.4)(-20.5-(-25.5)) = (100.4)5 = 100). If this particular quasar and our galaxy could be seen side by side at the same distance, the quasar would be 5 magnitudes (or 100 times) brighter than our galaxy.


Stars and galaxies (M)

In stellar and galactic astronomy, the standard distance is 10 parsecs (about 32.616 light years, or 3 × 1014 kilometres). A star at 10 parsecs has a parallax of 0.1" (100 milli arc seconds). For galaxies (which are of course themselves much larger than 10 parsecs, and whose overall brightness cannot be directly observed from relatively short distances) the absolute magnitude is defined by reference to the apparent brightness of a point-like or star-like source of the same total luminosity as the galaxy, as it would appear if observed at the standard 10 parsecs distance.

In defining absolute magnitude one must specify the type of electromagnetic radiation being measured. When referring to total energy output, the proper term is bolometric magnitude. The bolometric magnitude can be computed from the visual magnitude plus a bolometric correction, Mbol = MV + BC. This correction is needed because very hot stars radiate mostly ultraviolet radiation, while very cool stars radiate mostly infrared radiation (see Planck's law). The dimmer an object (at a distance of 10 parsecs) would appear, the higher its absolute magnitude. The lower an object's absolute magnitude, the higher its luminosity. A mathematical equation relates apparent magnitude to absolute magnitude, via parallax.

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