The apparent magnitude of a celestial object, such as a star or galaxy, is the brightness measured by an observer at a specific distance from the object. The smaller the distance between the observer and object, the greater the apparent brightness.
Two objects that have the same apparent magnitude, as seen from the Earth, may either be:
To convert the apparent magnitude, m, of a star into a real magnitude for the star (absolute magnitude, M), we need to know the distance, d to the star. Alternatively, if we know the distance and the absolute magnitude of a star, we can calculate its apparent magnitude. Both calculations are made using:
The apparent magnitudes, absolute magnitudes and distances for selected stars are listed below:
Although Rigel and Deneb have the same real brightness (the same absolute magnitude), Rigel appears brighter than Deneb on the sky (it has a smaller apparent magnitude) because it is much closer to the Earth.