Main sequence

In Astronomy the term main sequence refers to a phase in a star's lifetime when it shines by fusing Hydrogen into Helium.

The term comes from a kind of chart, called a Hertzprung-Russell diagram, that plots a stars color, along the x-axis, against its luminosity along the y-axis. The stars color reflects the temperature of its surface.

Phases of a star's life

Stellar nebulae

Inside massiver gas clouds regions of greater density can coalesce. If that region of greater density is massive enough it may form a spinning disk of gas and dust, while gravity its gravity will form a knot at its center that will become a proto-star. The proto-star is considered to have graduated to an actual star, when internal pressure and tempertatur at its center reach levels where hydrogen fusion occurs.

Giant star phase

Star that exhaust the hydrogen that powers the fusion reactions that keep their gravity from collapsing them will either collapse to a greater density to where Helium will fuse to yet higher elements. Stars without enough mass to fuse Helium will go directly to the white dwarf phase. Most stars are dim, low mass red dwarf stars that will not have enough mass to get dense enough to fuse Helium. But they are also very long-lived stars, and, at approximately 15 billion years old, none of these stars has got to the point where it has exhausted its Hydrogen.

• red dwarf stars, which will spend hundreds of millions of years, or even a billion years, on the main sequence. They are dim low mass stars that fuse their Hydrogen into Helium very slowly.

• orange dwarf stars

most of their