The upper main sequence (hot and bright with the most mass) produces energy by one type of nuclear fusion involving only hydrogen and helium. Stars get the energy they produce through nuclear fusion of hydrogen into helium. Any star that has a mass greater than 1.5 times the Sun's mass is grouped into the upper main sequence any star with a mass lower than 1.5 solar masses belongs to the lower main sequence.

The main sequence is sometimes divided into two parts. In general, the hottest and brightest stars (in the upper-left of the Hertzsprung-Russell diagram) also have the most mass. It does not spend so much time in its red giant phase. After this happens, the star is no longer on the main sequence. When a star begins to run out of fuel, it gets much bigger and colder and turns into a red giant. They stay on the main sequence for millions or billions of years. Stars are created from nebula and first appear on the main sequence. You can see an example of the band of main-sequence stars to the right. The main sequence appears as a band going from the upper-left (hot and bright) to the lower-right (cooler and less bright). Hertzsprung-Russell diagrams are graphs that measure each star's brightness against its temperature or color. This is because stars on the main sequence are highly stable, so they remain there a long time. Most of the stars in the Milky Way galaxy and most other galaxies are also main-sequence stars. Any star that is plotted in that area is a main-sequence star. The main sequence is a region on a Hertzsprung-Russell Diagram where most stars appear. Hertzsprung-Russell diagram showing stars in the main sequence.

Main sequence star chart