A Supernova (plural: supernovae or supernovas) is a stellar explosion. They are extremely luminous and cause a burst of radiation that may briefly outshine an entire galaxy before fading from view over several weeks or months. During this short interval, a supernova can radiate as much energy as the Sun could emit over its life span. The explosion expels much or all of a star's material at a velocity of up to a tenth the speed of light, driving a shock wave into the surrounding interstellar medium. This shock wave sweeps up an expanding shell of gas and dust called a supernova remnant.
On average, supernovae occur about once every 50 years in a galaxy the size of the Milky Way and play a significant role in enriching the interstellar medium with heavy elements. Furthermore, the expanding shock waves from supernova explosions can trigger the formation of new stars.
Before their extinction at the hands of the Flood, and the Halo Array, the Forerunner fleet had "the capacity to force premature stellar collapse," creating an artificial supernova, used to destroy entire star systems taken by the Flood.
In the Battle of Shield 0459 in 2531, The UNSC moved the UNSC Spirit of Fire's Slipspace Drive into the Apex Site of Shield 0459 and detonated it close to the internal miniature star. The chain reaction sent the star into supernova state, obliterating the Installation and the fleet of Forerunner Ships, preventing the Covenant from using it and saving Humanity from total defeat and extinction.
During the closing hours of the Second Battle of Requiem, Jul 'Mdama activated one of Requiem's failsafes, pulling the Shield World into its star. His fleet then left the system, leaving the UNSC Infinity trapped in orbit above Requiem. The Infinity eventually freed itself and entered Slipspace just as Requiem was pulled into Epoloch. This triggered the star's artificial supernova that destroyed the system and nearly incinerated the Infinity.