As an example, in the image above, we see the stars Castor and Pollux in the constellation Gemini. From earth, we see Pollux slightly brighter than Castor. (Remember that in the magnitude scale, the bigger the number, the fainter it is).
(STELLAR) ABSOLUTE MAGNITUDE
The stars as seen from earth appear different in magnitude (brightness) due to two main reasons. One is that stars are at different distances and hence the further the star is, the fainter it is. Secondly, some stars are actually brighter or fainter than each other due to their sizes and the type of star that they are.
In order to help visualize how bright stars really are, we use a standard distance between an observer and the star that will show the stars brightness as they would appear at this standard distance.
The distance used is 10 parsecs (pc), (1 parsec is 206 265 Astronomical units) which equates to approx 32.6 light years (lt.yr).
As
an example, in the image above, we see the stars Castor and Pollux in
the constellation Gemini. From earth, we see Pollux slightly brighter
than Castor. (Remember that in the magnitude scale, the bigger the
number, the fainter it is).
In the graphic to the right, we show the stars at their real distances from the earth; Castor at 52 light years and Pollux at 34 light years. In the bottom half of the same graphic, we place both stars at the standard 10 parsec distance and it now shows Castor is the brighter of the two stars. The magnitudes at this 10 parsec distance are the stars absolute magnitudes.