From Halopedia, the Halo wiki
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.
In early Forerunner history, the Forerunners attempted to manipulate several stars in their region of space. They subsequently failed and caused several stars to supernova, leaving their homeworld heavily irradiated and uninhabitable.
Before their defeat at the hands of the Flood, and the firing of 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.
During the Battle of the Etran Harborage in 2531, Professor Ellen Anders came up with a plan to destroy the shield world and its fleet of Forerunner Sojourner-class dreadnoughts by taking the FTL reactor of the UNSC Spirit of Fire and detonating it inside the installation's internal star. This would cause the star to go supernova and obliterate the shield world. Due to damage sustained during an attack by Covenant forces, Sergeant John Forge was forced to take the reactor into the star and detonate it manually. As the Spirit of Fire escaped the shield world, the internal star went supernova, destroying the planet and the Forerunner fleet, thwarting the Covenant plan. A debris field persisted until 2557 when a massive UNSC fleet destroyed the debris with a massive bombardment of nuclear weapons.
In 2552, Spartan John-117 destroyed Halo Installation 04 by detonating the fusion reactors of the crashed UNSC Pillar of Autumn. The explosion created supernova-like temperatures and created a new element on the Alpha Shard, a fragment of the installation that was propelled through slipspace to orbit a red giant. This new element was fatal to humans and was turned into a bioweapon by terrorists. After the source of the new element was traced to Alpha Shard following the terrorist attack on Sedra City, Private Talitha Macer correctly surmised that the supernova effects of the destruction of Installation 04 created it, confirmed by the lack of the element anywhere else in the known universe. A joint mission by an ONI-Sedran Colonial Guard team destroyed the deposits of this new element with a HAVOK tactical nuclear weapon.
At the close of the Requiem Campaign in 2558, the leader of a Covenant remnant, Jul 'Mdama, put the shield world Requiem on a collision course with the system's sun, Epoloch. When the shield world was absorbed by the sun, the star went into a supernova state and exploded, nearly destroying the UNSC Infinity which narrowly escaped the shockwave into slipspace.
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