Unfortunately, most stars evolve quite slowly, and astronomers must examine many stars to gain an adequate understanding of how stars work. It is also important to realize that we don't have access to a star in a laboratory, so astronomers must make use of measurements of distant stars recorded by instruments and then analyze those measurements and formulate theories about stellar evolution and makeup. The first instruments used to gather information about the stars were our eyes. Astronomers now have electronic instruments for measuring light intensity and measurements of magnitude have evolved to today's current logarithmic method. Astronomers need to know the absolute magnitude of the stars to carry out meaningful research.
Absolute magnitude is basically the apparent magnitude adjusted for distance.
Astronomers have chosen a standard distance of 10 parsecs which is used to compute the absolute magnitude of stars, and allows the accurate comparison of stellar light output from star to star. If you examine several stars in the night sky with the naked eye, binoculars or a telescope, you will notice that stars do have color. Vega and Sirius are distinctly blue, while Antares and Betelgeuse are distinctly red. While we can say that a star is red or yellow or blue, astronomers must accurately measure a star's color in a way that allows comparison to other stars. That measurement is known as a star's Color Index.