A ball of mostly hydrogen and helium gas that shines extremely brightly. Our Sun is a star. A star is so massive that its core is extremely dense and hot. At the high stellar core temperatures, atoms move so fast that they sometimes stick to other atoms when they collide with them, forming more massive atoms and releasing a great amount of energy. This process is known as nuclear fusion.

Stars are often classified based on their visual character:

ClassTemperature [K]
B25000 to 11000
A11000 to 7500
F7500 to 6000
G6000 to 5000
K5000 to 3500

A spectral class is subdivided into subclasses, designated by a number (0-9).

See also: Asterism, Atmosphere, Binary Star, Black Hole, Blue Supergiant, Brown Dwarf, Carbon-Nitrogen-Oxygen Cycle, Chemical Enrichment, Galaxy, Hertzsprung-Russell Diagram, Neutron Star, Nova, Proton-Proton Chain, Protoplanetary Disc, Pulsar, Red Giant, Red Supergiant, Stellar, Stellar Wind, Sun, Supergiant, Universe, White Dwarf, Yellow Dwarf, Yellow Supergiant, Young Stellar Object.

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Subjects: Astronomy Physics