Saturday, December 6, 2008

A star is a massive, self-luminous celestial body that is held together by its own intense gravity in which the energy generated by its nuclear reactions is balanced by the outflow of energy to the surface. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth. Other stars are visible in the night sky, when they are not outshone by the Sun. For most of its life, a star shines due to thermonuclear fusion in its core releasing energy that traverses the star's interior and then radiates into outer space. Almost all elements heavier than hydrogen and helium were created by fusion processes in stars.

By observing its spectrum, luminosity and motion through space Astronomers, can determine the mass, age, chemical composition and many other properties of a star. The total mass of a star is the principal determinant in its evolution and eventual fate. Other characteristics of a star are determined by its evolutionary history, including the diameter, rotation, movement and temperature.

A star originates as a collapsing cloud of material composed mainly of hydrogen, along with helium and trace amounts of heavier elements. Once the stellar core is sufficiently dense, some of the hydrogen is steadily converted into helium through the process of nuclear fusion. The remainder of the star's interior carries energy away from the core through a combination of radiative and convective processes. The star's internal pressure prevents it from collapsing further under its own gravity.