We have already known that the brightness of the stars is measured in terms of its magnitude. Each drop in magnitude corresponds to a drop of 2.5 in the brightness of the source as seen from earth. Thus, a sixth magnitude star is approximately 100 times dimmer than a first magnitude. It is not at all unusual today for astronomers using state-of-the-art telescopes to detect twenty-fourth magnitude objects in the sky.
The apparent brightness of a star depends on how far away it is and on how much energy it is giving off (its ‘luminosity’). To eliminate the ambiguity associated with the distance to the stars, astronomers have defined the ‘absolute magnitude’ of a star as the brightness it would have if it were seen from a distance of thirty-three light-years. The absolute magnitude does not depend on the distance to a star, but measures something intrinsic to the star itself.
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