Glossary of Astronomical Terms/main sequence

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Main sequence is a term that describes those stars that are classified to reside on the predominate, slightly curved, diagonal line running from the upper left corner to the lower right corner of the Hertzsprung-Russell diagram. This diagram is as important to astronomy as the periodic table of the elements is to chemistry.

Stars spend 90% of their lives as a main sequence star. During their stay here the star converts the hydrogen in its core into helium. Main sequence stars obey the mass-luminosity law. As stars spend so much time here, then we can say that 90% of the stars we see, are main sequence, even those in other galaxies.

Stars that have yet to settle down as a main sequence star are still in their formation stages and are known as protostars such as T-Tauri stars. Stars that have moved away (or have "turned off") from the main sequence line are in the process of converting their helium into higher elements and in the late stages of their lives. The star's position on the H-R diagram then leaves the main sequence line, which way they go depends on their mass. Star that generally have at least 0.4 solar mass, then they'll move to the upper right, where it may become a giant_stars or a supergiant stars. Less massive stars, which are most of the stars that exist are only small, cool stars called red dwarfs. Red dwarf stars consume their fuel very slowly and can survive for a hundred billion years. As the universe is 13.7 billion years old, no red dwarf stars have ever died of old age.