Ring galaxy

A galaxy whose stars and gas form a prominent ring around a central region. Such rings often arise from collisions or resonances.

Head-on galaxy collision: A near-central collision between galaxies that can launch an outward-moving density wave through a disk. The wave from a small galaxy crossing a disk can gather gas and stars into a ring.
Cartwheel Galaxy: A ring galaxy in the constellation Sculptor, with a bright outer ring and spokes extending from its center. Its structure is a widely studied example of a ring launched by a nearly central collision.
Disk galaxy: A galaxy whose stars, gas, and dust are arranged predominantly in a rotating disk. Most ring galaxies are understood as altered disks, rather than systems built around a ring from the start.
Spiral galaxy: A disk galaxy with spiral arms extending from its central region. Spiral arms wind through a disk, whereas a ring galaxy has a prominent closed or nearly closed structure.
Density wave: A propagating pattern of enhanced density that can organize material in a rotating medium. An outward-moving density wave explains how collision debris forms an expanding ring.
Hoag's Object: A galaxy consisting of a bright, nearly circular ring of stars around a separate central body. Its detached ring makes it a striking case whose formation remains less settled than the Cartwheel's.
Galactic bar: An elongated structure of stars crossing the center of some disk galaxies. A bar can drive gas toward resonant radii where a ring forms.
Lenticular galaxy: A disk galaxy with a central bulge and smooth disk but little visible spiral structure. A smooth disk can resemble a ring system at a glance, but lacks its defining prominent ring.
Lindblad resonance: A resonance in a rotating disk where orbital and pattern frequencies satisfy a specific relation. Resonances with a bar or companion can trap disk material into rings without a direct central collision.
Lindsay-Shapley Ring: A ring galaxy in the Large Magellanic Cloud, distinguished by its unusually large ring structure. It provides a nearby example of ring morphology beyond the classic Cartwheel system.

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Tidal strippingRelated: Some ring systems arise as encounters redistribute and strip galactic material.