What is Main sequence?
[[Image:Hertzsprung-russel diagram.png|thumb|right|280px|Hertzsprung-Russell diagram showing stars in the main sequence.]] The '''main sequence''' is a region on a [[Hertzsprung-Russell Diagram]] where most [[star]]s appear. Any star that is plotted in that area is a '''main-sequence star'''. The [[Sun]] is a main-sequence star. Most of the stars in the [[Milky Way]] [[galaxy]] and most other galaxies are also main-sequence stars. This is because stars on the main sequence are highly stable, so they remain there a long time. Hertzsprung-Russell diagrams are [[graph]]s that measure each star's brightness against its temperature or color. The main sequence appears as a band going from the upper-left (hot and bright) to the lower-right (cooler and less bright). You can see an example of the band of main-sequence stars to the right. Stars are created from [[nebula]]s and first appear on the main sequence. They stay on the main sequence for millions or billions of years. When a star begins to run out of fuel, it gets much bigger and colder and turns into a [[red giant]]. After this happens, the star is no longer on the main sequence. It does not spend so much time in its red giant phase. In general, the hottest and brightest stars (in the upper-left of the Hertzsprung-Russell diagram) also have the most [[mass]]. The main sequence is sometimes divided into two parts. Any star that has a mass lower than 1.5 times the Sun's mass is grouped into the ''lower main sequence''; any star with a mass greater than 1.5 solar masses belongs to the ''upper main sequence''. Stars get the energy they produce through [[nuclear fusion]] of [[hydrogen]] into [[helium]]. The lower main sequence (cooler and less bright with less mass) produces energy by one type of nuclear fusion involving only hydrogen and helium. The upper main sequence (hotter and brighter with more mass) produces energy by another type of nuclear fusion. This type uses other [[Chemical element|elements]] (for example, [[carbon]], [[nitrogen]] and [[oxygen]]) with