The Delos sun was the primary of the Delos star system.
In 2364, this star was "undergoing large-scale magnetic field changes", which was generating "violent, gigantic flares." The Federation sent the USS Enterprise-D on a mission to study this star in detail.
As the starship approached the star, Data reported there was an unusual number of sunspots and eruptive prominences. As well, he noted that the magnetic field was extremely irregular.
Soon after, Wesley Crusher informed the bridge that the ship's deflector shields were being impacted by huge bursts of X-rays. As systems across the ship were being affected, Worf adjusted the deflectors to compensate.
Before the ship could investigate the star further, it picked up a distress signal from the Ornaran freighter Sanction. (TNG: "Symbiosis")
The name of this star is from the script.