A galaxy is a gravitationally bound system containing stars, gas, dust, and dark matter, often spanning tens of thousands of light-years. We live in the Milky Way, a barred spiral galaxy containing an estimated 100 to 400 billion stars. Our Solar System sits in the Orion Arm, about 26,000 light-years from the galactic center, where radio observations reveal a supermassive black hole called Sagittarius A*. Nearly every large galaxy harbors such a supermassive black hole at its core.
The Milky Way's nearest large galactic neighbor is the Andromeda Galaxy, also known as M31, located about 2.5 million light-years away and currently approaching the Milky Way for a future collision. Together with roughly 80 smaller galaxies, Andromeda and the Milky Way form the Local Group. On still larger scales, gravity gathers hundreds to thousands of galaxies into galaxy clusters, such as the Virgo Cluster about 54 million light-years away, and clusters themselves group into vast superclusters like Laniakea. Separating these structures are enormous cosmic voids, while filaments of galaxies stretch between clusters to form the cosmic web, the large-scale architecture of the observable universe, which spans about 93 billion light-years in diameter.
Some galaxies have unusually brilliant centers, called active galactic nuclei, or AGNs. The most luminous of these are quasars, powered by matter accreting onto supermassive black holes, and a special class called blazars are AGNs whose relativistic jets happen to point nearly toward Earth. Deep exposures such as the Hubble Deep Field and the JWST Deep Field have revealed thousands of distant galaxies, offering a glimpse of the universe as it appeared in its infancy and confirming that structure grew over cosmic time through the gradual amplification of primordial density fluctuations, a process called structure formation.