From Halopedia, the Halo wiki
When the UNSC Pillar of Autumn's reactors detonated on September 22, 2552, Installation 04 attempted to activate a safety measure and make a slipspace jump. However, this self-preservation protocol was only partly successful, and only a small portion of the ring later designated Alpha Shard managed to enter slipspace before the Autumn's reactors overloaded and destroyed the ring. The surviving fragment of the Halo reentered normal space in the vicinity of Thotael, a red giant star some distance away from the Orrichon system, and was caught in the star's orbit.
While most of Installation 04's debris either fell into Threshold's or Basis' gravity wells or remained in the orbital debris field, at least 30% of the mass and curvature remained unaccounted for by the UNSC. In late 2555 the Office of Naval Intelligence took note of information recorded by an automated scout vehicle, Recon-14, in an area designated Location 6201. Recon-14 had been deployed from the Pillar of Autumn before the ship's crash-landing on the Halo installation. The signal data from the drone was analyzed by multiple ONI specialists, among them ONI attaché for astrophysical phenomena Dr. Madeline Tress. She correctly concluded that the signal originated from the missing fragment of Installation 04, indicating that a part of the ring either generated or was enveloped in a significant slipspace event, one far exceeding the capabilities of either human or Covenant technology.
Uniquely, the extreme heat of the Autumn's explosion, comparable to temperatures produced by a supernova, caused the formation of an entirely new element known as Element 120.V in a small area of the fragment — one not found elsewhere in the known universe. This element was later illegally mined from the fragment and smuggled to human colonial space, specifically the planet Sedra. It was discovered that the element caused catastrophic breakdown of DNA in certain humans but did not affect other species. The element was refined into a bioweapon which a Sangheili terrorist unleashed in Sedra City, killing or hospitalizing many of the planet's citizens. A joint ONI-Sedran Colonial Guard team led by Colonel Randall Aiken and Lt. Commander Locke embarked on a mission to the Halo fragment in order to capture the smugglers who mined the element and destroy the deposits with a HAVOK nuclear warhead. However, the team became stranded on the shard, hunted by resident Lekgolo swarms that had commandeered and crashed their D81-LRT Condor dropship. Most of the group were killed during the mission and only two members—Lieutenant Commander Locke and Private Macer—made it off the shard aboard the smugglers' Bactrian-class freighter while Colonel Aiken sacrificed himself to manually detonate the HAVOK nuclear device, destroying the deposits.
Despite the extreme temperatures and radiation produced by its vicinity to the star the Halo shard has managed to sustain a habitable environment, although only when the terraformed side faces away from the star; the fragment's spin about itself produces a day-night cycle in which the terraformed side was in shadow for approximately sixteen hours, during which the temperatures remained high but survivable. However, when the habitable side rotated to face the star, not even the ring's systems could sustain livable temperatures or protect the occupants from the stellar radiation. Despite this, the UNSC took note of the ring's impressive redundant systems in functionalities including but not limited to those involved in maintaining gravitational field integrity, atmospheric composition, and even biodiversity levels. While most of the shard's terrain was rocky and barren due to its extreme daytime temperatures, one particular area had managed to sustain a river and even simple moss-like vegetation. The vegetation was speculated to possibly not be a carbon-based lifeform which would explain its survival. While the river existed, it was shown to hold boiling water, even at nighttime when the shard was semi-habitable.
List of appearances