A star’s absolute magnitude is the apparent magnitude it would have if it were 10 parsecs away (an arbitrarily chosen distance) and there were no intervening gas or dust. Symbol, M (in contrast to the lowercase “m” for apparent magnitude).
A big, bright star that is far away can easily be fainter than a dimmer star that is nearer us–such as the sun. For stars’ real brightnesses to be apparent visually, they would all have to be at the same distance. The measure that indicates real brightness is absolute magnitude.
Absolute magnitude is calculated from the star's apparent magnitude and its distance.
M = m − 5logD + 5 − A
where D is the distance to the star in parsecs and A is a correction for the light absorbed by the gas and dust between Earth and the star.
The absolute magnitude of the sun is about 4.8. Most stars have absolute magnitudes between 0 and 15; the extreme range is -10 to +19.
The absolute bolometric magnitude of a star is the bolometric magnitude it would have if it were at a distance of 10 parsecs.
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Last revised: 16 April 2004.