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.
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Today's selected anniversaries:
English explorer James Cook landed on Vancouver Island and claimed it
for Great Britain.
Commodore Matthew C. Perry of the U.S. Navy signed the Treaty of
Kanagawa, forcing the opening of Japanese ports to American trade.
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.
The Danish West Indies became the U.S. Virgin Islands after the United
States paid Denmark US$25 million for the Caribbean islands.
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,
Iraq War: Iraqi insurgents in Fallujah ambushed a convoy containing
four American contractors from the private security company Blackwater
Wiktionary's word of the day:
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
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
Thorough the iron gates of life:
Thus, though we cannot make our sun
Stand still, yet we will make him run.
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