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)
When stars exploded, they emitted an electrical magnetic pulse. This pulse could cause electro-magnetic devices such as computers to fail. Its Shock waves could pose a danger to Starfleet too close to them, and even collapse their warp fields. (TNG: "11001001"; VOY: "The Q and the Grey")
Supernovae, however, generally occurred only approximately once per century in the Milky Way Galaxy. Up until 2373, only two Starfleet crews had ever witnessed supernovae firsthand. That year, a third observation was made by USS Voyager, setting the record for the closest observation at less than 10 billion kilometers. (VOY: "The Q and the Grey")
The Fabrina solar system went nova some 10,000 years before the 2268. (TOS: "For the World is Hollow and I Have Touched the Sky")
In 2269, 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, the Q Civil War caused hazardous repercussions throughout the Milky Way Galaxy, including "galactic crossfire" in the form of supernovas. At least three supernovae went of within three days in a single sector of the Delta Quadrant.
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. Spock still used the red matter to stop the supernova after Romulus was gone but this led to the creation of the alternate reality. In 2258 of the alternate reality, Spock used a Vulcan mind meld to show the alternate James T. Kirk the supernova event. (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.