Report Text Star
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For at least a
portion of its life, a star shines due to thermonuclear fusion of
hydrogen 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 were created by stars, either via stellar
nucleosynthesis during their lifetimes or by supernova nucleosynthesis
when stars explode. 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 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.
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Star
A star is a massive, luminous ball of plasma held together by gravity.
At the end of its lifetime, a star can also contain a proportion of
degenerate matter. The nearest star to Earth is the Sun, which is the
source of most of the energy on Earth. Other stars are visible from
Earth during the night when they are not outshone by the Sun or blocked
by atmospheric phenomena. Historically, the most prominent stars on the
celestial sphere were grouped together into constellations and
asterisms, and the brightest stars gained proper names. Extensive
catalogues of stars have been assembled by astronomers, which provide
standardized star designations.
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. Once the hydrogen fuel at the
core is exhausted, those stars having at least 0.4 times the mass of the
Sun 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.
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. Stars can form part of a much
larger gravitationally bound structure, such as a cluster or a galaxy.
Taken from: http://en.wikipedia.org/wiki/Star
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