To determine a star's brightness, it's important to differentiate between two specific terms: absolute magnitude and apparent magnitude.
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Absolute Magnitude: This measures a star's intrinsic brightness, or the brightness it would have if it were placed at a standard distance of 10 parsecs (about 32.6 light years) from Earth. Absolute magnitude is a good measure for comparing the true brightness of stars regardless of their distance from us.
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Apparent Magnitude: This measures how bright a star appears from Earth, which can be affected by its distance, the presence of interstellar material, and light pollution.
In summary, if you're looking for the best measure of a star’s brightness, absolute magnitude is the more appropriate property, as it represents the true brightness of the star itself, independent of its distance from the observer.
Other properties like age and size do not directly measure brightness. Age is related to the evolutionary stage of the star, while size (or radius) can influence brightness but is not a direct measure of how "bright" a star is in terms of absolute or apparent magnitude.