A star is type of astronomical object consisting of a luminous spheroid of plasma held together by its own gravity.
The nearest star to Earth is the Sun. Many other stars are visible to the naked eye from Earth during the night,
appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically,
the most prominent stars were grouped into constellations and asterisms, the brightest of which gained proper
names. Astronomers have assembled star catalogues that identify the known stars and provide standardized
stellar designations. However, most of the stars in the Universe, including all stars outside our galaxy, the Milky
Way, are invisible to the naked eye from Earth. Indeed, most are invisible from Earth even through the most
For at least a portion of its life, a star shines due to thermonuclear fusion of hydrogen into helium 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 are created by stellar nucleosynthesis during the star's lifetime, and for some stars
by supernova nucleosynthesis when it explodes. Near the end of its life, a star can also contain degenerate matter.
Astronomers can determine the mass, age, metallicity (chemical composition), and many other properties of a star
by observing its motion through space, its luminosity, and spectrum respectively. The total mass of a star is the
main factor that determines its evolution and eventual fate. Other characteristics of a star, including diameter and
temperature, change over its life, while the star's environment affects its rotation and movement. A plot of the
temperature of many stars against their luminosities produces a plot known as a Hertzsprung–Russell diagram (H
–R diagram). Plotting a particular star on that diagram allows the age and evolutionary state of that star to be
A star's life begins with the gravitational collapse of a gaseous nebula of material composed primarily of hydrogen,
along with helium and trace amounts of heavier elements. When the stellar core is sufficiently dense, hydrogen
becomes steadily converted into helium through nuclear fusion, releasing energy in the process. The remainder
of the star's interior carries energy away from the core through a combination of radiative and convective heat
transfer processes. The star's internal pressure prevents it from collapsing further under its own gravity. A star with
mass greater than 0.4 times the Sun's will expand to become a red giant when the hydrogen fuel in its core is exhausted.
In some cases, it will fuse heavier elements at the core or in shells around the core. As the star expands it throws a
part of its mass, enriched with those heavier elements, into the interstellar environment, to be recycled later as new
stars.Meanwhile, the core becomes a stellar remnant: a white dwarf, a neutron star, or if it is sufficiently massive a black hole.