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This article is about the astronomical object. For other uses, see
Star (disambiguation).
A star forming region in the
Large Magellanic Cloud.
NASA/
ESA image.
A star is a massive, luminous ball of
plasma that is held together by
gravity. 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. Historically, the most prominent stars on the
celestial sphere were grouped together into
constellations, and the brightest stars gained proper names. Extensive
catalogues of stars have been assembled by astronomers, which provide standardized
star designations.
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.
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 plot of the temperature of many stars against their luminosities, known as a
Hertzsprung-Russell diagram (H–R diagram), allows the age and evolutionary state of a star to be determined.
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.
[1] 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. Once the hydrogen
fuel at the core is exhausted, those stars having at least 0.4 times the mass of the Sun
[2] expand to become a
red giant, in some cases fusing heavier
elements at the core or in shells around the core. The star then evolves into a degenerate form, recycling a portion of the matter into the interstellar environment, where it will form a new generation of stars with a higher proportion of heavy elements.
[3]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.
[4] Stars can form part of a much larger gravitationally bound structure, such as a
cluster or a
galaxy.
