A?star?is a luminous sphere of?plasma?held together by its own?gravity. The nearest star to?Earth?is the?Sun. Many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped into?constellations?and?asterisms, the brightest of which gained proper names. Astronomers have assembled?star catalogues?that identify the known stars and provide standardized?stellar designations. However, most of the stars in the?Universe, including all stars outside our?galaxy, the?Milky Way, are invisible to the naked eye from Earth. Indeed, most are invisible from Earth even through the most powerful?telescopes.
For at least a portion of its life, a star shines due to?thermonuclear fusion?of?hydrogen?into?helium?in its core, releasing energy that traverses the star's interior and then?radiates?into?outer space. Almost all naturally occurring elements heavier than helium are created by?stellar nucleosynthesis?during the star's lifetime, and for some stars by?supernova nucleosynthesis?when it explodes. Near the end of its life, a star can also contain?degenerate matter.?Astronomers?can determine the?mass, age,?metallicity?(chemical composition), and many other properties of a star by observing its motion through space, its?luminosity, and?spectrum?respectively. The total mass of a star is the main factor that determines its?evolution?and eventual fate. Other characteristics of a star, including diameter and temperature, change over its life, while the star's environment affects its rotation and movement. A plot of the temperature of many stars against their luminosities produces a plot known as a?Hertzsprung?Russell diagram?(H?R diagram). Plotting a particular star on that diagram allows the age and evolutionary state of that star to be determined.
A star's life?begins?with the?gravitational collapse?of a gaseous?nebula?of mat
Uploaded: Dec 25, 2017