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Color and stellar luminosity.

We all know that some stars are brighter than others; astronomers measure that brightness as magnitude. A star can appear bright for either of two reasons.

It may radiate a large amount of light (be of high luminosity), or it may be unusually close to us so that it appears bright even though it has low luminosity.

The Danish astronomer Ejnar Hertzsprung (1873-1967) suggested that if a star's distance were known, one could calculate what magnitude it would have if it were some standard distance away. The distance chosen was 10 parsecs, or 32.6 light-years. The brightness of the star at that distance would then be its absolute magnitude. Thus, if our Sun were 10 parsecs away from us, it would seem to have a magnitude of 4.86 (a rather dim star), and that would be its absolute magnitude.

In studying the absolute magnitude of various stars, Hertzsprung could calculate their relative luminosities. In 1905 he noticed that there were two kinds of red stars: red stars with very high luminosity (which we now call red giants) and red stars with very low luminosity (which we now call red dwarfs).

The most interesting aspect of the findings was that there were no red stars of intermediate luminosity. Hertzsprung's report did not attract much attention at first (he published it in a journal of photography), but it represented the first step toward understanding the evolution of stars.

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Author:Asimov, Isaac
Publication:Asimov's Chronology of Science & Discovery, Updated ed.
Article Type:Reference Source
Date:Jan 1, 1994
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