A star (or "sun") is a massive energy-producing sphere of plasma and gas located in space. The region around a star that is held by its gravity, including any planets, moons, comets, and asteroids, is called a star system.
Stellar life cycle

Stars are born out of huge gaseous nebulae. Inside these nebulae, centers of higher density form, slowly accumulating more mass as the center's gravity increases, to form a protostar. Pressure in the interior of the protostar rises, in turn increasing the density and temperature until the gas turns to plasma, where the atomic nuclei and the electrons are dissociated from each other. At a sufficient temperature and pressure, nuclear fusion is initiated at the core, producing light: the star is born.
Risan sunsetThe two suns of Risa
Star evolution

Stars can be made up of various different elements depending on their age.

Young stars mainly consist of hydrogen, which is fused to helium thereby increasing the star's helium ratio over time and producing large quantities of energy. This energy, in turn, creates extreme pressure, preventing the star from collapsing under its own gravity.

Dr. Tolian Soran used a trilithium weapon in 2371 to stop all fusion reactions inside the Amargosa sun, thereby collapsing the star and altering the gravitational conditions in the system. (Star Trek Generations)

As a sun gets older it begins to fuse heavier elements, like helium, as the lighter elements like hydrogen are depleted. This, however, releases more energy, causing the star to swell, which increases its surface area from which the energy is emitted. This phase marks the beginning of the star's end.

In 2367, Dr. Timicin of the planet Kaelon II tried to save the dying star Kaelon by regulating its ever-increasing temperature with the bombardment of photon torpedoes. However, the experiment failed after testing the procedure with a star in an uninhabited solar system. (TNG: "Half a Life")

Because of its larger surface area, the star turns red and is then called a red giant. After the sun runs out of light elements and the number of fusion reactions decreases, its own gravity causes it to collapse and to expel its outer layers of matter, creating beautiful "planetary nebulae". The remnant of the star is called white dwarf.

Every star has to pass these stages of evolution. However, depending on their masses, some suns experience further changes.

Below 1.5 Sol masses: After one to ten billion years any nuclear reactions inside the white dwarf finally cease and the star turns to a "black dwarf", a very small stellar corpse.

In 2370, the USS Prometheus hosted Dr. Gideon Seyetik's successful attempt to re-ignite the stellar corpse Epsilon 119 by using a protomatter-laden shuttlepod remotely sent into the star. In the case of a failure, the star could have exploded in a supernova instead. (DS9: "Second Sight")

Above 1.5 Sol masses: The white dwarf swells again, fusing all elements up to iron. After the last iron is depleted, the star turns into a supernova, where the outer layers of the sun explode, which, in turn, causes a massive shock wave. The remains of this explosion are a vast matter nebula and a tiny neutron star, which is so dense, that all protons and electrons are neutralized to neutrons. A special form of neutron stars are pulsars.

In 2269, the star Beta Niobe turned to a supernova, when crewmembers of the Federation starship USS Enterprise were almost trapped on its planet Sarpeidon. (TOS: "All Our Yesterdays")

The resulting electromagnetic pulse from the supernova of Beta Magellan in 2364 was so feared by the computer-dependent Bynar in the nearby Beta Magellan system that they commandeered the USS Enterprise-D as a temporary dump for their planetary computer. (TNG: "11001001")

In 2373, a series of supernovae witnessed by the USS Voyager in the Delta Quadrant turned out to be the real-time result of "battles" during a civil war in the Q Continuum. They were actually created by spatial disruptions in the Continuum, which created a negative-density false vacuum that sucked nearby matter into the Continuum. (VOY: "The Q and the Grey")

If the remnant of a supernova is more massive than 2.5 Sol masses, it collapses to a black hole.
A star is a massive, luminous sphere of plasma held together by gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on the planet. Some other stars are visible from Earth during the night when they are not obscured by atmospheric phenomena, appearing as a multitude of fixed luminous points because of their immense distance. 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.

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. Once a star's hydrogen is nearly exhausted, almost all naturally occurring elements heavier than helium are created, either via stellar nucleosynthesis during their lifetimes or by supernova nucleosynthesis when very massive stars explode. Near the end of its life, a star can also contain a proportion of degenerate matter. Astronomers can determine the mass, age, metallicity (chemical composition), and many other properties of a star by observing its motion through space, luminosity, and spectrum respectively. The total mass of a star is the principal determinant of 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.

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, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process.[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, a star with at least 0.4 times the mass of the Sun[2] expands 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 its matter into the interstellar environment, where it will form a new generation of stars with a higher proportion of heavy elements.[3] Meanwhile, the core becomes a stellar remnant: a white dwarf, a neutron star, or (if it is sufficiently massive) a black hole.

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 star cluster or a galaxy.

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