Hertzsprung-Russell diagram

The Hertzsprung-Russell diagram (usually referred to by the abbreviation H-R diagram or HRD, also known as a Colour-Magnitude diagram, or CMD) shows the relationship between absolute magnitude, luminosity, classification, and effective temperature of                                                        stars. The diagram was created circa 1910 by Ejnar Hertzsprung and Henry Norris Russell, and represented a huge leap forward in understanding stellar evolution, or the 'lives of stars'.
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A graph which has the absolute magnitudes of stars plotted against their spectral classification. Luminosity can be substituted for absolute magnitudes and OBAFGKM pattern and thus go from blue to yellow and to red. The positions of the stars on the diagram are not random and specific regions are seen to be present. The main sequence is a curving line from the top left of the diagram to the bottom right. It is here that all the stable hydrogen -burning stars are clustered. The Sun is amongst these stars and the values on the axes are usually chosen so that it is plotted in the centre of the diagram. At the top right of the diagram are the giants and supergiants. At the bottom left are the white dwarfs. As stars evolve, they move their position on the Hertzsprung-Russell diagram. For instance, they drop onto the zero age main sequence along Hayashi tracks . As the hydrogen in the core of a star is depleted, the star will evolve across the main sequence. As the helium is fused in it the star becomes a red giant . When fusion stops, a low mass star will collapse and become a white dwarf but a high mass star will explode as a supernova . The Hertzsprung-Russell diagram continues to be an important instrument for understanding stellar evolution.