The Romulan supernova
In the 2380s it was discovered that the Romulan sun would soon go supernova, threatening to destroy the Romulan system and its inhabited planets and threatening the lives of billions of people in the Beta Quadrant.
The Romulan Star Empire asked the United Federation of Planets for help with evacuation efforts, and Starfleet agreed to provide aid at the behest of Jean-Luc Picard. Picard left the USS Enterprise-E to command a rescue armada of ten thousand Wallenberg-class transports, with the intent to relocate 900 million Romulan citizens to worlds outside the blast of the supernova. He would later compare this rescue effort to the Dunkirk evacuation of World War II. (PIC: "Remembrance")
The plan was not popular in the Federation. Many felt that there were better uses for the Federation's resources than aiding its oldest enemy, and fourteen member species even threatened to secede if aid was given to the Romulan Empire. The final blow came on April 5th, 2385, when the evacuation fleet was destroyed before it could depart from Mars by a group of compromised synths during a surprise attack on the planet, secretly orchestrated by a secret Romulan cabal known as the Zhat Vash. Starfleet subsequently called off the rescue and abandoned the Romulans, a move that drove Picard to resign in protest. (PIC: "Remembrance", "Maps and Legends", "The End is the Beginning", "Broken Pieces")
In 2387, shortly before the supernova was projected to occur, Ambassador Spock of Vulcan concocted a plan to use red matter to create a black hole that would absorb the exploding star's energy, and promised to do so in time to save Romulus. However, the supernova began before he could arrive, and Romulus and its remaining population were destroyed. Among those killed were the wife and unborn child of Nero, who swore vengeance against Spock, Vulcan, and the Federation for letting his world die.
Shortly after Spock released the red matter from the Jellyfish, successfully halting the supernova's expansion, Nero confronted Spock from the Narada. As Spock attempted to escape, both of their ships were pulled into the black hole that had consumed the star, sending them back in time and resulting in the creation of the alternate reality. (Star Trek)
In the aftermath of the supernova, Romulan refugees lived dispersed. While some migrated to Earth, many made a living on the former relocation hub of Vashti. Backed by the Tal Shiar, a new political entity succeeding the Star Empire was the Romulan Free State. (PIC: "Maps and Legends", "Absolute Candor", "The Impossible Box")
Prior to the release of Star Trek Nemesis, which establishes that the Romulan sun was a single star, Star Trek Maps (p. 29) and The Worlds of the Federation (p. 148) both placed the Romulan star system around a binary star system of two white dwarfs.
The Star Trek: Picard press kit  states that Admiral Picard was ordered to leave the Enterprise in 2381 to assist the Romulan people "during their time of great crisis". This might indicate that the impending supernova had been known at least since then.
Having a supernova threaten the entire galaxy, as was claimed to be the case by Spock in Star Trek, is a physical impossibility under normal circumstances: the law of special relativity limits the shock wave's expansion rate to below the speed of light, meaning it would be a minimum of several years before it affected even nearby stars. The expansion of the blast wave would also result in it weakening and dissipating over time.
Even if Spock consumed the star in time to prevent it from destroying Romulus, doing so would still leave the planet without a source of heat and light, resulting in it becoming uninhabitable anyway, albeit physically intact.
In the anti-time future depicted in "All Good Things...", Romulus remained intact and inhabited as of 2395, suggesting that the supernova either did not occur or was successfully mitigated by Spock's actions.
Prior to the release of PIC: "Remembrance", which established that the supernova of 2387 was the Romulan sun itself, various non-canon works stated that its source was a neighboring star called "Hobus". In the comic book series Star Trek: Countdown, the star's galactic threat came from its unusual ability to transform anything it contacted into energy, which it then consumed, increasing its power and causing it to expand farther and faster throughout the galaxy.
The backstory for Star Trek Online, which follows the Countdown continuity, further states that the shockwave from the Hobus explosion propagated through subspace at faster-than-light speeds and reached the Romulan system in just over a day, destroying both Romulus and Remus mere hours into an evacuation expected to take six weeks. A scandal occurred several years later involving rumors that the Vulcan Science Academy knew about the instability of the star but did nothing until Spock attempted to save Romulus with the Jellyfish. The scandal resulted in several resignations and sowed seeds of distrust towards the Vulcans.
It is revealed during the course of Star Trek Online itself that the supernova was not a natural occurrence: the supernova and the unusual behavior of its shockwave were caused by an Iconian-designed doomsday weapon deployed by rogue members of the Tal Shiar. Then-Admiral Taris led the operation but claims to the player character that she was deceived about the nature of the Iconian device by her then-science officer, Hakeev (β).
In the more recent reference book Stellar Cartography: The Starfleet Reference Library ("Stellar Cartography", p. 35), the explosion of Hobus was called a "subspace supernova". This event resulted in the destruction of the entire Romulan star system, with billions killed. The cause of the explosion was unclear. The star was located in the Beta Quadrant. There was some contention on where precisely the star was located. On p. 35, the star was located in the Romulan Neutral Zone. Yet, in the "Federation Historical Highlights, 2161-2385" and "Position of the Romulan Star Empire", the star was located "southeast" of Romulus and quite some distance from the Neutral Zone. On the "Position of the Romulan Star Empire", which was a map of the Romulan Empire – prepared by the Stellar Archive Artist Lsel Essik, from the Romulan Master Data Catalog, in 2366 – Hobus was identified as a Romulan system.
The Star Trek novelization instead stated that the star had merely threatened systems in its own vicinity rather than the entire galaxy.
This information was superseded by canon, when the Romulan sun was identified as the star which supernovaed in the episode "Remembrance".