Sirius. Image credit: Hubble
The official definition of absolute magnitude is the intrinsic brightness of a celestial body computed as if viewed from a distance of 10 parsecs, or 32.6 light-years( standard luminosity distance). It is also known as absolute visual magnitude when measured in the standard V phometric band. If you want to derive the absolute magnitude from the observed apparent magnitude of a celestial object its value is corrected for distance to the observer. Absolute magnitude then equals the apparent magnitude an object would have if it were at a standard luminosity distance away from the observer, in the absence of astronomical extinction. It allows the true brightnesses of objects to be compared without regard to distance.
The absolute magnitude uses the same convention as the visual magnitude, with a factor of 1000.2a, difference in brightness between steps in magnitude. As an example: the Milky Way 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. 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.
For planets, comets, and asteroids a different definition of absolute magnitude is used which is more meaningful for non-stellar objects. For these bodies the absolute magnitude is defined as the apparent magnitude that the object would have if it were 1 AU from both the Sun and the observer and at a phase angle of zero degrees. This is physically impossible, as it requires the observer to be located at the center of the Sun, but it is convenient for purposes of calculation. For a meteor, the standard distance for measurement of magnitudes is at an altitude of 100 km at the observer’s zenith.
There is a nice article about absolute magnitude here. It even gives the equations needed to determine it. Here on Universe Today we have a great article about the brightest stars and their absolute magnitudes. Astronomy Cast offers a cool episode about space dust. I included that one to get you to think about if it effects the absolute magnitude of a star by our calculations.
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