The Hobus star was located in the Beta Quadrant's Devron sector, 500 light-years from Romulus, in the outermost reaches of the Romulan Star Empire. It was one of the oldest stars in the galaxy. (ST comic: "Countdown")
In the year 2387, the Hobus star exploded in a supernova which threatened the galaxy. The first indications of the disaster were observed on stardate 64333.4 by the crew of the Romulan Mining Guild starship Narada, who were mining the first planet in the star system, Hobus I, for the rare mineral decalithium. Wild fluctuations in Hobus's radiation output forced the Narada to abandon a mining operation and evacuate the area at warp speed. The star erupted, destroying the planet. Ambassador Spock warned the Romulan Senate that the imminent supernova would be "unlike any the galaxy has ever seen" and that the star had converted the planet's mass into energy, increasing its own power. (ST comic: "Countdown")
27 hours after stardate 64444.5, the Hobus star exploded, the effects propagating through subspace at multiwarp speeds, destroying the planets Romulus and Remus. On stardate 64471.6, Ambassador Spock, using the prototype ship Jellyfish equipped with red matter, created a black hole, absorbing the exploding star. (STO website: The Path to 2409; TOS movie, novelization & comic adaptation: Star Trek; ST comic: "Countdown")
The supernova left behind the parsecs-deep, amber-hued Hobus Nebula. At the center of the nebula, the Hobus stellar remnant, known as the Hobus Cinder, is still active and can be seen pulsing through the nebula gasses (STO novel: The Needs of the Many)
In 2388, Admiral Kathryn Janeway flew the first survey mission of the Hobus Nebula aboard a variant of the Delta Flyer with a crew of volunteers. It was discovered that the supernova's neutrino output was somehow being shunted into subspace, and that the Hobus Cinder continued to output intense streams of X-rays, gamma rays, delta rays, epsilon radiation, Berthold rays and tetryon particles. (STO novel: The Needs of the Many; STO missions: "Revelation", "Ground Zero")
As mandated by the Federation Science Council, Starfleet conducts annual surveys of the Hobus Nebula. In the early 2400s, Admiral Janeway commanded one such mission aboard the Excalibur-class USS Tucker, where she was interviewed by Jake Sisko. In the interview, Janeway classified Hobus as a type 1a supernova with unexplained light speed properties. In other words, the wave was propagating outward at multi-warp speeds, something no natural supernova should be able to do. (STO novel: The Needs of the Many)
In 2409, a Romulan Republic warbird captained by the Virinat survivor and Tovan Khev investigated the Hobus system based on intelligence found in Tal Shiar files acquired during an undercover operation to infiltrate the Tal Shiar. During the investigation, the Warbird discovered a hidden base on an asteroid near Hobus II. As they explored the base, they found working Iconian technology, some of which had been hybridized by Romulan technology. Upon reaching the base's gateroom, the away team found Praetor Taris who had been missing since Sela's take over of Romulan Star Empire.
Taris revealed she had given the order to detonate the device (which was Iconian in origin) in the Hobus star, causing the supernova. However, she had been duped by her right hand man, Colonel Hakeev of the Tal Shiar. Hakeev had told her the device would allow her to contact the Iconians, something Taris now suspected had been a fabrication to advance Hakeev's own cause. Taris pleaded with the Virinat survivor to be allowed to continue her work, in an attempt to contact the Iconians and diplomatically solving the issues the race had seeded throughout the galaxy. Unfortunately, the Tal Shiar tracked Taris and the Virinat survivor (his cover exposed) to this base. Taris fled through the Iconian gateway to her base on Iconia, the Virinat's survivor's crew returned to the ship, and the survivor fled through the gateway as the station was destroyed. (STO mission: "Revelation")
A few weeks later, a Starfleet ship performed a more thorough scan than Admiral Janeway's of the system and found a Reman base on Hobus I. Inside the base they found evidence that the destruction of the Hobus star was caused by protomatter explosives injected into the core of the star. It seems that the shock wave fed on energy from the surrounding planets, allowing it to travel as fast as it did. This explained the strange behavior of the supernova, but it still remained a mystery to Starfleet who had done this. (STO mission: "Ground Zero")
Spoiler warning: Plot and/or ending details follow.
Various aspects of the Hobus supernova were documented in the comic series Star Trek: Countdown, the Star Trek Online game and the STO novel: The Needs of the Many, as well as in canon in TOS movie: Star Trek. In particular, the Hobus explosion reacting with the decalithium was said to have triggered a rare subspace explosion that could move at warp speeds, which is why the Romulus system was affected by the far-off destruction. Later canon sources, specifically Star Trek: Picard mention the supernova having been the explosion of the Romulus system's star(s). While this might represent a changed premise, the capability of a subspace detonation from the earlier sources could account for the Hobus explosion moving at high speeds and triggering the explosion of additional suns, which means the later canon reference might not necessarily invalidate this aspect of the earlier stories.