JON LOMBERG / SCIENCE PHOTO LIBRARY JON LOMBERG / SCIENCE PHOTO LIBRARY
Stellar classification using a Hertzsprung-Russell diagram, a graph that plots the absolute magnitude (luminosity relative to the Sun) of a star against its surface temperature. This shows most stars on the main sequence curve (from lower right to upper left). A new star's position on the sequence depends upon its mass. Low-mass stars are cool and red. Medium-mass stars (like the Sun, circled) are yellow. High-mass stars are hot & blue. A medium- mass star leaves this curve at the end of its life as it expands and cools, moving right and above the curve as a red giant (like Betelguese, upper right). It then contracts to a white dwarf (like Sirius B, lower centre), ending below the curve.
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