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The Hertzsprung-Russell diagram is a plot of stars classified according to their surface temperature and their intrinsic luminosity. The horizontal axis is labelled with temperature ranging from 2000 degrees on the right, to 30 000 degrees on the left. The brightness scale runs from solar luminosities at the bottom left to solar luminosities at the top left. The Main Sequence runs diagonally from top left to bottom right, depicting stars in the main part of their lifetime. The Sun is identified approximately two thirds of the way down this main sequence, with a surface temperature of approximately 6000 degrees and a luminosity of 1. A red cloud at top right identifies red giant stars, and white dwarfs are labelled in blue at the lower left of the diagram. The letters O B A F G K M along the top of the diagram identify the spectral classes of the stars – from the hottest and brightest blue-white 'O' class stars at the top left to the cool and faint red 'M' class stars at the bottom right.