A Star (or "Sun") is a sphere of matter emitting energy. The region around a star is called solar system or star system.
Evolution of Stars
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, which causes the sun not to collapse.
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 compounds 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.
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 ran 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 1-10 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 (NCC-71201) hosted Dr. Gideon Seyetik's succesful attempt to re-ignite the stellar corpse Epsilon 199 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 is a vast matter nebula and a tiny neutron star, which is so dense, that all protons and electrons are neutralized to neutrons.
In 2269 the star Beta Niobe turned to a supernova, when crewmembers of the Federation starship USS Enterprise (NCC-1701) were almost trapped on its planet Sarpeidon. (TOS: "All Our Yesterdays")
The resulting electro-magnetic pulse of a supernova in 2364 was so feared by the computer-dependent Bynars in the nearby Beta Magellan system that they commandeered the USS Enterprise (NCC-1701-D) as a temporary dump for their planetary computer. (TNG: "11001001")
In 2373, a series of supernovae witnessed by the USS Voyager in the Gamma 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.
Classification of Stars
Stars are assigned to different spectral classes.
Furthermore, a plot of stellar luminosities versus stellar spectral types on two axes, is called Hertzsprung-Russell diagram.