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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. Different types of apparent magnitude (visual, photographic, etc.) lead to different absolute magnitudes.
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.
The stellar magnitude a meteor would have if placed in the observer's zenith at a height of 100 kilometers.
International Meteor Organization Glossary. See www.imo.net/glossary (Retrieved 23 July 2011)
The stellar magnitude a comet would have if it were 1 AU from both the Earth and the Sun. The magnitude is not a fixed quantity; it often differs from one appearance of the comet to another, and can also vary as the comet approaches and departs.
| Comet | Year | Absolute Magnitude |
|---|---|---|
| Sarabat | 1729 | −3.0 |
| Comet of 1577 | 1577 | −1.8 |
| Hale-Bopp | 1997 | −1.0 |
| De Cheseaux | 1747 | −0.5 |
| Comet | Year | Absolute Magnitude |
|---|---|---|
| Halley | 1986 | 5.5 |
| Humason | 1962 | 1.5 |
| Wild 2 | 2003 | 6.5 |
| C/2006 P1 (McNaught) | 2007 |
JPL Comet Glossary
The absolute magnitude of an asteroid is the apparent visual magnitude it would have if it were 1 AU from Earth, 1 AU from the Sun, and lay on a line passing through the Sun and the observer on Earth (i.e., at zero phase angle). A table converting absolute magnitudes to an estimate of the asteroid's diameter can be found at http://neo.jpl.nasa.gov/glossary/h.html
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Last revised: 22 July 2011.