Milky Way
The Milky Way is the barred spiral galaxy that contains the Solar System. It includes hundreds of billions of stars, gas, dust, and dark matter.
Galactic center: The central region of a galaxy, where stellar density and gravitational activity are often greatest. The Milky Way’s central region contains its bar, dense stellar populations, and a supermassive black hole.
Galactic rotation curve: A graph of orbital speed versus distance from a galaxy’s center. The Milky Way’s rotation curve reveals the distribution of mass beyond its bright disk.
Galaxy formation: The growth of galaxies from early matter through gravitational collapse, star formation, and mergers. The Milky Way’s structure records the general processes that built galaxies in the early universe.
Solar System: The Sun and the objects gravitationally bound to it, including planets, moons, and smaller bodies. The Solar System occupies a small region in the Milky Way’s disk, far from its center.
Galactic disk: A flattened region of stars, gas, and dust in a disk-shaped galaxy. Most of the Milky Way’s visible stars and star-forming gas lie in its disk.
Differential rotation: Rotation in which different parts of a system orbit at different angular speeds. The Milky Way’s inner regions orbit faster than distant regions, continually reshaping its spiral pattern.
Gaia Sausage: A population of stars on elongated orbits in the Milky Way, interpreted as debris from an ancient merger. Its stars preserve evidence of a major collision in the Milky Way’s early history.
Orion Arm: A minor spiral feature of the Milky Way containing the Solar System. The Sun lies within this local spiral feature, between the larger Sagittarius and Perseus arms.
Galactic bulge: A dense, roughly spheroidal concentration of stars surrounding the central region of a spiral galaxy. The Milky Way’s central bar and bulge shape the dense stellar region around its nucleus.
Spiral arm: A curved region of enhanced stars, gas, and star formation in a spiral galaxy’s disk. The Milky Way’s spiral arms concentrate gas and young stars, though their exact structure remains mapped from within.