Luminosity and absolute magnitude scales compared. Image used with permission.
Absolute magnitude, like luminosity, is a measure of the true brightness, or radiative power output, of an object in space. It is based on a logarithmic scale. Absolute magnitude M is the apparent magnitude of a star, or other luminous object, when seen from a standard distance of 10 parsecs (32.6 light-years). It can be found from the object's apparent magnitude m and its parallax π in arcseconds using the formula
M = m + 5 + 5 logπ
The Sun, for example, has an absolute magnitude of 4.8, while Aldebaran, the brightest star in the constellation Taurus, has an absolute magnitude of -0.63, which is equivalent to saying that Aldebaran is about 350 times more luminous than the Sun.
In the case of an asteroid or comet, the absolute magnitude is the apparent magnitude the object would have at zero phase angle and at a distance of 1 AU (astronomical unit) from both the Sun and the Earth.
See also distance modulus.