What is a Star?

A star is a massive luminous ball of plasma. The nearest star to the Earth is the Sun. The Earth is caught in the Sun's gravitational pull and uses the Sun as an energy source. Many other stars are visible in the night sky. A star shines the majority of its life because of the thermonuclear fusion within its core which releases energy throughout the star's interior. The majority of the celestial objects in the Solar System were created by the fusion process in the stars.

Astronomers study stars based on their age, mass, and chemical composition which is defined by their spectrum, luminosity, and motion through space. The total mass of the star is what determines its fate. Diameter, rotation, movement and temperature also take a role in a star's history.

A star begins its life as a cloud of material composed mainly of hydrogen, helium, and minute amounts of other, heavier, elements. Once the core is dense, the hydrogen is slowly converted to helium through nuclear fusion. The star's pressure prevents it from collapsing upon itself under its own gravity. Once the star's hydrogen fuel is exhausted the stars having 0.4 times the mass of the sun become a red giant, which is a luminous star of low or intermediate mass. The star then evolves into a recycling form where it will form a new generation of stars with its heavier elements.

There are multi-star systems which consist of two or more stars that are gravitationally bound and orbit around each other. These two stars orbit relatively closely together which has a large impact on its evolution.

Historically speaking, stars have been important and carefully studied through many civilizations. Records showing the study of astronomy and stars have been found before the earliest reliable source of recorded history.