How is the magnitude of a star defined?
The measure of the star's brightness is called magnitude.
Astronomers refer to two different magnitude for stars. Apparent magnitude is the brightness of the star as it appears to the earth. the magnitude scale is based on apparent brightness. With this, astronomer can determine the actual absolute magnitude of the star. Absolute magnitude of the star is the actual amount of light the star is giving off.
Stars that have the same absolute magnitude often have different apparent magnitudes. Different apparent magnitudes because they have different distances from the Earth.