The main sequence is a continuous and distinctive band of stars that appear on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung-Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell. After a star has formed, it generates energy at the hot, dense core region through the nuclear fusion of hydrogen atoms into helium. During this stage of the star's lifetime, it is located along the main sequence at a position determined primarily by its mass, but also based upon its chemical composition and other factors. All main sequence stars are in hydrostatic equilibrium, where outward thermal pressure from the hot core is balanced by the inward gravitational pressure from the overlying layers. The main sequence is sometimes divided into upper and lower parts, based on the dominant process that a star uses to generate energy. Stars below about 1.5 times the mass of the Sun primarily fuse hydrogen atoms together in a series of stages to form helium, a sequence called the proton-proton chain. Above this mass, in the upper main sequence, the nuclear fusion process mainly uses atoms of carbon, nitrogen and oxygen as intermediaries in the production of helium from hydrogen atoms. Main sequence stars below 0.4 solar masses undergo convection throughout their mass. In general, the more massive the star the shorter its lifespan on the main sequence.

Read the rest of this article: http://en.wikipedia.org/wiki/Main_sequence

_______________________________ Today's selected anniversaries:

1778:

English explorer James Cook landed on Vancouver Island and claimed it for Great Britain. http://en.wikipedia.org/wiki/Vancouver_Island

1854:

Commodore Matthew C. Perry of the U.S. Navy signed the Treaty of Kanagawa, forcing the opening of Japanese ports to American trade. http://en.wikipedia.org/wiki/Matthew_C._Perry

1903:

New Zealand inventor Richard Pearse reportedly flew in one of the first powered flying machines for a distance of several hundred metres, about nine months before the Wright brothers flew their Wright Flyer. http://en.wikipedia.org/wiki/Richard_Pearse

1917:

The Danish West Indies became the U.S. Virgin Islands after the United States paid Denmark US$25 million for the Caribbean islands. http://en.wikipedia.org/wiki/United_States_Virgin_Islands

1995:

American singer Selena, known as "The Queen of Tejano music", was shot and killed in Corpus Christi, Texas, by the president of her fan club, Yolanda Saldívar. http://en.wikipedia.org/wiki/Selena

2004:

Iraq War: Iraqi insurgents in Fallujah ambushed a convoy containing four American contractors from the private security company Blackwater USA. http://en.wikipedia.org/wiki/31_March_2004_Fallujah_ambush

_____________________________ Wiktionary's word of the day:

crescendo (n): 1. (music) An instruction to play gradually more loudly, denoted by a long, narrow angle with its apex on the left ( < ).

2. (figuratively) A gradual increase of anything, especially to a dramatic climax http://en.wiktionary.org/wiki/crescendo

___________________________ Wikiquote quote of the day:

Let us roll all our strength and all

Our sweetness up into one ball, And tear our pleasures with rough strife

Thorough the iron gates of life: Thus, though we cannot make our sun

Stand still, yet we will make him run. --Andrew Marvell http://en.wikiquote.org/wiki/Andrew_Marvell