Tellun was the primary of the Tellun system, in the Alpha Quadrant.
In 2256, this star's location was labeled on the star chart "Alpha/Beta Quadrant Overview" in the ready room aboard the USS Discovery. (DIS: "Magic to Make the Sanest Man Go Mad")
In 2257, during the first Federation-Klingon War, this star's system was occupied by the Klingons. (DIS: "The War Without, The War Within")
This star was only mentioned in writing.
According to Star Trek: Star Charts (p. 62) and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), Tellun was a multi-star with a G-class star and an A-class star.