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n. (Astronomy) That region on a two-dimensional graph of luminosity versus temperature for stars (the Herzsprung-Russel diagram), which runs from high temperature and high luminosity to low temperature and low luminosity, in which most of the stars (plotted as points on the diagram) are found. A normal star such as the earth's sun will spend most of its time over billions of years within this region of temperature and luminosity, as it progressively converts more of its original hydrogen into heavier elements. After the hydrogen is consumed, a star may become a red giant or evolve into other types of star not within the main sequence region. [ PJC ]