Two arms? Four arms? Both.
Old stars trace two major arms, while gas and young stars map four spiral arms. The mismatch remains unexplained.Milky Way: The Spiral‑Arm Mystery
Messy spirals
Arms branch, merge, and twist. The Sun sits on the inner edge of the Orion Arm—a local spur, not a grand arm.Milky Way: The Spiral‑Arm Mystery
