A supernova or nova was a massive explosion that typically occurred upon the death of an old star. When a massive star ran out of nuclear fuel, the star had no counteractive force against its own gravity and collapses. The resulting shock wave blew the outer layers of the star into space, possibly leaving a core that becomes either a neutron star or a black hole, depending on the remaining mass. A different kind of supernova occurred (more rarely in our Milky Way Galaxy) when a white dwarf star in a tight binary system swallowed material from her companion, eventually becoming too massive and collapsing. (TOS: "All Our Yesterdays"; TAS: "The Counter-Clock Incident"; TNG: "The Naked Now", "The Last Outpost", "Tin Man"; VOY: "The Q and the Grey"; Star Trek)
Supernovae, however, generally occur only approximately twice per century in the Milky Way Galaxy. As of the 24th century, only three Starfleet vessels ever witnessed supernovae firsthand. The record for the closest observation was less than 10 billion kilometers and was set by the USS Voyager in 2373. (VOY: "The Q and the Grey")
The Fabrina solar system went nova some ten thousand years before the 23rd century. (TOS: "For the World is Hollow and I Have Touched the Sky")
In 2268, the crew of the USS Enterprise observed the supernova Beta Niobe and the destruction of planet Sarpeidon. A few weeks earlier, she had already witnessed the supernova Minara. (TOS: "The Empath", "All Our Yesterdays")
In 2366, the star Beta Stromgren exploded as a supernova. The last stages of the star's life were observed by the USS Enterprise-D while taking first contact with the creature Gomtuu. (TNG: "Tin Man")
In 2373, a number of supernovae occurred within a small region of the Delta Quadrant, as a result of the Q Civil War. These supernovae were actually the result of spatial disruptions within the Q Continuum.
The crew of Voyager, acting on information provided by the female Q, used the explosion of one of these supernovae to enter the Continuum themselves, in order to rescue Q and Captain Janeway. (VOY: "The Q and the Grey")
Several weeks later, in the Alpha Quadrant, the Changeling posing as Doctor Julian Bashir attempted to trigger a supernova in the Bajoran sun using trilithium, tekasite and protomatter. The Founder was stopped by Kira Nerys, Jadzia Dax and the crew of the USS Defiant. (DS9: "By Inferno's Light")
In 2387, a supernova threatened the galaxy. Ambassador Spock stopped the supernova by using red matter to create a black hole, which consumed the supernova. Before he could, however, the supernova destroyed Romulus. (Star Trek)
Background information Edit
In the shooting script of Star Trek: The Motion Picture, the film would have begun in deep space. The camera would focus in on a white star in a star system "...suddenly changing, brightening, flaring unbelieavable intensity: supernova." Pausing briefly as witness to this celestial event, the camera would move on searching for V'ger.
It was never clarified if the spatial disruptions in the Q Continuum were created deliberately or merely side-effects of various Q weaponry, but only that the Q civil war, and Q's actions, were having galactic consequences.
In Star Trek: Countdown, the official comic book prequel to Star Trek, the star which went supernova and destroyed Romulus was called the Hobus star. It was explained that the Hobus supernova was unlike any previously seen: as the supernova grew, it converted mass into energy, which increased its power and allowed it to expand. As a result, its threat reached beyond the Hobus system and potentially the entire galaxy.