A star is a massive, compact body of plasma in outer space that is held together by its own gravity and is sufficiently massive to sustain nuclear fusion in a very dense, hot core region. This fusion of atomic nuclei generates the energy that is continuously radiated from the outer layers of the star during much of its life span. Astronomers can determine many of the properties of a star by observing its spectrum, luminosity and motion through space. Individual stars differ in their total mass, chemical composition, and age. The total mass of a star is the principal determinant in its evolution and eventual fate. Other characteristics of a star that are determined by its evolutionary history include the diameter, rotation, movement and temperature. A plot of the star's temperature against luminosity, known as a Hertzsprung-Russell diagram, allows the current age and evolutionary state of the star to be determined. Binary and multi-star systems consist of two or more stars that are gravitationally bound, and generally move around each other in stable orbits. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution.
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_______________________________ Today's selected anniversaries:
1493: Christopher Columbus became the first European to land on Puerto Rico, an island he named San Juan Bautista. (http://en.wikipedia.org/wiki/Puerto_Rico)
1816: The Royal University of Warsaw, later known as Warsaw University, was established. (http://en.wikipedia.org/wiki/Warsaw_University)
1863: U.S. President Abraham Lincoln delivered the Gettysburg Address. (http://en.wikipedia.org/wiki/Gettysburg_Address)
1942: World War II: Soviet forces under General Georgy Zhukov launched the Operation Uranus envelopment at the Battle of Stalingrad, turning the tide of the battle in the Soviet Union's favor. (http://en.wikipedia.org/wiki/Operation_Uranus)
1999: Shenzhou 1, China's first unmanned test flight of the Shenzhou spacecraft, was launched. (http://en.wikipedia.org/wiki/Shenzhou_1)
_____________________ Wikiquote of the day:
Knowledge comes, but wisdom lingers, and I linger on the shore, And the individual withers, and the world is more and more. -- Alfred, Lord Tennyson (http://en.wikiquote.org/wiki/Alfred_Tennyson)