Pinwheel Galaxy

The Pinwheel Galaxy, also called Messier 101 or NGC 5457, is a large spiral galaxy in Ursa Major seen nearly face-on. Its asymmetric spiral arms contain numerous star-forming regions.

H II region: A cloud of ionized hydrogen surrounding young, massive stars, often marking active star formation. Bright H II regions trace star-forming sites across the galaxy’s spiral arms.
Spiral galaxy: A galaxy with a flattened disk and spiral patterns of stars, gas, and dust. The Pinwheel is a large, loosely wound example of this galaxy type.
Spiral arm: A curved region of enhanced stars, gas, and dust extending through a spiral galaxy’s disk. The Pinwheel’s broad arms contain much of its visible star-forming activity.
Charles Messier: An eighteenth-century French astronomer who compiled a catalog of nebulae and star clusters. He recorded the galaxy as the 101st object in his catalog.
Ursa Major: A northern constellation containing the Big Dipper asterism. The Pinwheel lies within this constellation’s boundaries.
NGC 5474: A dwarf galaxy near Messier 101 with an unusually off-center nucleus and distorted structure. Its proximity and disturbed appearance make it a notable companion of the Pinwheel Galaxy.
Density wave theory: A model in which spiral patterns move through a galactic disk as waves of enhanced density. It explains how spiral structure can persist while stars and gas orbit at different speeds.
Galactic disk: A flattened component of a galaxy containing stars, gas, and dust in orbital motion. Its nearly face-on orientation exposes the Pinwheel’s disk and spiral pattern.
Pierre Méchain: A French astronomer known for discovering many objects later included in Charles Messier’s catalog. He discovered the object that became Messier 101.
M101 Group: A nearby group of galaxies centered on the Pinwheel Galaxy. The galaxy is the dominant member of this loose association.

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