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Absolute magnitude is the measure of a celestial object's intrinsic brightness. It is the hypothetical apparent magnitude of an object at a standard luminosity distance of exactly 10.0 parsecs or about 32.6 light years from the observer, assuming no astronomical extinction of starlight. This allows the true energy output of astronomical objects to be compared without regard to their variable distances. As with all astronomical magnitudes, the absolute magnitude can be specified for different wavelength intervals; for stars the most commonly quoted absolute magnitude is the absolute visual magnitude, which is the absolute magnitude in the visual (V) band of the UBV system. Also commonly used is the absolute bolometric magnitude, which is the total luminosity expressed in magnitude units; it takes into account energy radiated at all wavelengths, whether observed or not.
The absolute magnitude uses the same conventions as the visual magnitude: brighter objects have smaller magnitudes, and 5 magnitudes corresponds exactly to a factor of 100, so 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 with an absolute magnitude of −25.5 is 100 times brighter than our galaxy. 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. Similarly, Canopus has an absolute visual magnitude of about -5.5, while Ross 248 has an absolute visual magnitude of +14.8, for a difference of slightly more than 20 magnitudes, so if the two stars were at the same distance, Canopus would be seen as about 20 magnitudes brighter; stated another way, Canopus gives off slightly more than 100 million (108) times more visual power than Ross 248.
In stellar and galactic astronomy, the standard distance is 10 parsecs (about 32.616 light years, 308.57 petameters or 308.57 trillion kilometres). A star at 10 parsecs has a parallax of 0.1" (100 milli arc seconds). Galaxies (and other extended objects) are much larger than 10 parsecs, their light is radiated over an extended patch of sky, and their overall brightness cannot be directly observed from relatively short distances, but the same convention is used. A galaxy's magnitude is defined by measuring all the light radiated over the entire object, treating that integrated brightness as the brightness of a single point-like or star-like source, and computing the magnitude of that point-like source as it would appear if observed at the standard 10 parsecs distance. Consequently, the absolute magnitude of any object equals the apparent magnitude it would have if it was 10 parsecs away.
In using an 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 usually is computed from the visual magnitude plus a bolometric correction, . This correction is needed because very hot stars radiate mostly ultraviolet radiation, while very cool stars radiate mostly infrared radiation (see Planck's law).
Many stars visible to the naked eye have such a low absolute magnitude that they would appear bright enough to cast shadows if they were only 10 parsecs from the Earth: Rigel (−7.0), Deneb (−7.2), Naos (−6.0), and Betelgeuse (−5.6). For comparison, Sirius has an absolute magnitude of 1.4 which is brighter than the Sun, whose absolute visual magnitude is 4.83 (it actually serves as a reference point). The Sun's absolute bolometric magnitude is set arbitrarily, usually at 4.75.  Absolute magnitudes of stars generally range from −10 to +17. The absolute magnitudes of galaxies can be much lower (brighter). For example, the giant elliptical galaxy M87 has an absolute magnitude of −22 (i.e. as bright as about 60,000 stars of magnitude −10).
For a negligible extinction, one can compute the absolute magnitude of an object given its apparent magnitude and luminosity distance :
where is the star's luminosity distance in parsecs, where 1 parsec is 206,265 astronomical units, approximately 3.2616 light-years. For very large distances, the cosmological redshift complicates the relation between absolute and apparent magnitude, because the radiation observed at one wavelength was radiated at a significantly different one. For comparing the magnitudes of very distant objects with those of local objects, a k correction might have to be applied to the magnitudes of the distant objects.
For nearby astronomical objects (such as stars in our galaxy) luminosity distance DL is almost identical to the real distance to the object, because spacetime within our galaxy is almost Euclidean. For much more distant objects the Euclidean approximation is not valid, and General Relativity must be taken into account when calculating the luminosity distance of an object.
In the Euclidean approximation for nearby objects, the absolute magnitude of a star can be calculated from its apparent magnitude and parallax:
where p is the star's parallax in arcseconds.
You can also compute the absolute magnitude of an object given its apparent magnitude and distance modulus :
Rigel has a visual magnitude of and distance about 860 light-years
Vega has a parallax of 0.129", and an apparent magnitude of +0.03
Alpha Centauri A has a parallax of 0.742" and an apparent magnitude of −0.01
The Black Eye Galaxy has a visual magnitude of mV=+9.36 and a distance modulus of 31.06.
Given the absolute magnitude , for objects within our galaxy you can also calculate the apparent magnitude from any distance (in parsecs):
For objects at very great distances (outside our galaxy) the luminosity distance DL must be used instead of d (in parsecs).
Given the absolute magnitude , you can also compute apparent magnitude from its parallax :
Also calculating absolute magnitude from distance modulus :
Bolometric magnitude corresponds to luminosity, expressed in magnitude units; that is, after taking into account all electromagnetic wavelengths, including those unobserved due to instrumental pass-band, the Earth's atmospheric absorption, or extinction by interstellar dust. For stars, in the absence of extensive observations at many wavelengths, it usually must be computed assuming an effective temperature.
The difference in bolometric magnitude is related to the luminosity ratio according to:
which makes by inversion:
In this case, the absolute magnitude (H) is defined as the apparent magnitude that the object would have if it were one astronomical unit (AU) from both the Sun and the observer. Because the object is illuminated by the Sun, absolute magnitude is a function of phase angle and this relationship is referred to as the phase curve.
To convert a stellar or galactic absolute magnitude into a planetary one, subtract 31.57. A comet's nuclear magnitude (M2) is a different scale and can not be used for a size comparison with an asteroid's (H) magnitude.
The absolute magnitude can be used to help calculate the apparent magnitude of a body under different conditions.
is the phase integral (integration of reflected light; a number in the 0 to 1 range).
A full-phase diffuse sphere reflects ⅔ as much light as a diffuse disc of the same diameter.
Note: because Solar System bodies are never perfect diffuse reflectors, astronomers use empirically derived relationships to predict apparent magnitudes when accuracy is required.
How bright is the Moon from Earth?