What is the name of the largest known ring of stars in the Milky Way's outer disk?

The largest known ring of stars in the Milky Way’s outer disk is the Monoceros Ring.

This long, complex structure was first reported in 2002 after surveys found an unusual stellar overdensity in the direction of the constellation Monoceros, the Unicorn. It is estimated to contain roughly 100 million solar masses and may stretch about 200,000 light-years while wrapping around the Milky Way several times.

The Monoceros Ring is often described as a stellar stream or ring because its stars form a broad, elongated structure around the Galaxy rather than a compact cluster. One influential explanation proposes that it is debris pulled from the Canis Major Dwarf Galaxy by the Milky Way’s gravity.

Astronomers still debate that interpretation. Other studies argue that the structure may instead be part of the Milky Way’s warped, flared outer disk, or a combination of disk features and tidal debris. That debate concerns its origin and physical nature; Monoceros Ring remains the canonical name used for the observed structure.