Messier 81

Messier 81 is a grand-design spiral galaxy in Ursa Major, about 12 million light-years from Earth. Its bright nucleus and well-defined spiral arms make it one of the sky’s prominent nearby galaxies.

Spiral galaxy: A galaxy whose disk contains stars and gas arranged in spiral arms around a central region. M81’s grand-design arms place it within this broad galaxy type.
M81 Group: A nearby galaxy group centered on Messier 81 and containing several dozen known galaxies. M81 gives this group its name and is its dominant member.
Density wave theory: A model in which spiral arms are large-scale patterns of enhanced density moving through a rotating galactic disk. It explains how M81 can maintain prominent arms without those arms being fixed collections of stars.
Hubble Space Telescope: A space observatory launched in 1990 that observes the universe mainly in ultraviolet, visible, and near-infrared light. Its images resolve stars and structural features across M81’s disk.
Messier 101: A large face-on grand-design spiral galaxy in Ursa Major, also called the Pinwheel Galaxy. Its similarly clear spiral pattern offers a nearby comparison with M81’s structure.
Grand-design spiral galaxy: A spiral galaxy with two or more prominent, coherent arms extending from its central region. M81 is a classic example, with arms that remain distinct across much of its disk.
Messier 82: A nearby starburst galaxy in Ursa Major, also known as the Cigar Galaxy. M82 is M81’s close companion and a prominent member of its galaxy group.
Galactic rotation curve: A graph of orbital speed against distance from the center of a galaxy. M81’s rotation provides evidence about the distribution of its mass.
Cepheid variable: A pulsating star whose period of brightness variation is related to its intrinsic luminosity. Cepheids in M81 help calibrate its distance.
Andromeda Galaxy: The nearest large spiral galaxy to the Milky Way, located about 2.5 million light-years away. It is a closer spiral counterpart whose scale and orientation differ from M81’s.

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Messier 106Related: It is another prominent nearby spiral galaxy used to study galactic structure and evolution.