The Alwanir Nebula was an interstellar gas cloud.

In late 2369, the lateral sensor arrays of the USS Enterprise-D conducted three spectral analyzes of the Alwanir Nebula, each on a different frequency, while en route to the Gariman sector. (TNG: "Rightful Heir")

"al-WAH-ner" was the pronunciation for this nebula's name from the script pronunciation guide.
According to Star Trek: Star Charts (p. 64) and Stellar Cartography: The Starfleet Reference Library ("Beta Quadrant: Local Space Extract"), the Alwanir Nebula was located in the Beta Quadrant.