That is to say there exists a mathematical relationship connecting these three quantities a star s apparent magnitude, its intrinsic magnitude and its distance. It should be noted that measuring distances is one of the most important tasks in astronomy. Not only is it required to know the intrinsic brightness of stars, it is also important in order to know the spatial distribution of stars and galaxies. For the nearby stars, their distances can be measured using the method of trigonometry parallax. That is, using the fact that a star will appear to project onto a slightly different part of the sky when it is viewed half a year apart.
Astronomers use a distance unit called parsec .
Then the method of similar objects has to be applied. This method assumes that all stars of the same type have the same intrinsic brightness. Remember that if the distance of a star is known, and the apparent magnitude can always be measured, then the absolute magnitude of the star can be determined. Therefore, with the nearby stars which parallactic distances can be obtained, a table of spectral types vs. absolute magnitudes can be obtained. The diagram on the left shows the relationship between luminosity and temperature of stars.