main sequence

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A major grouping of stars that forms a relatively narrow band from the upper left to the lower right when plotted according to luminosity and surface temperature on the Hertzsprung-Russell diagram.

from the GNU version of the Collaborative International Dictionary of English.

• noun (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• proper noun astronomy : A major grouping of stars that creates a narrow band from the upper left to the lower right when plotted by luminosity and surface temperature on a Hertzsprung-Russell diagram.
• proper noun The area on the Hertzsprung-Russell diagram in which the main sequence is located