In 2387, a star in close proximity to Romulus went supernova, threatening nearby systems. To stop the threat, AmbassadorSpock used red matter to create a black hole that consumed the star's energy. Before he could successfully carry out his mission, however, the supernova reached Romulus, destroying the planet and all of its inhabitants. 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. While Nero confronted Spock, 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)
Having a supernova threaten the entire galaxy is a physical impossibility under normal circumstances: the law of special relativity limits the shockwave'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. This means it should be impossible for any such event to cause severe damage to a warp-capable, multi-planetary society such as the Romulan Star Empire, let alone the entire galaxy. This discrepancy was explained several different ways in non-canon works.
In the comic book series Star Trek: Countdown, this star and its system were named "Hobus". The comic explained that 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 Star Treknovelization instead stated that the star had merely threatened systems in its own vicinity rather than the entire galaxy.
A third explanation appears in the backstory of Star Trek Online, which 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-AdmiralTaris 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.
In the more recent reference bookStellar 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.