|Cross section of a solar type star|
|Stellar life cycle|
A star is a massive, luminous ball of plasma. 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 because thermonuclear fusion in its core releases 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.
Astronomers can determine the mass, age, chemical composition and many other properties of a star by observing its spectrum, luminosity and motion through space. 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 begins as a collapsing cloud of material composed primarily 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. This is called the main sequence stage in a stars life cycle. Our sun is in this main sequence stage. In around 5 billion years, the sun will become a red gaint engulfing the orbit of the venus.
Then the following deviations occur in a star's life cycle depending on the mass of the star. Once the hydrogen fuel at the core is exhausted, then--