Galaxies
The observable universe contains an estimated 2 trillion galaxies, each home to hundreds of billions of stars.
🌌 The Milky Way
Our galaxy is a barred spiral about 100,000 light-years across containing 100-400 billion stars. The Sun orbits the galactic center at about 230 km/s and completes one orbit every 225 million years (one galactic year). Sagittarius A* (4 million solar masses) sits at its center.
🔭 Galaxy Types
Elliptical galaxies are smooth, featureless, and contain mostly old stars. Spiral galaxies (like the Milky Way and Andromeda) have arms of star formation. Irregular galaxies have no defined shape, often due to gravitational interactions. Lenticular galaxies are between spiral and elliptical.
💥 Galaxy Collisions
The Andromeda Galaxy is approaching the Milky Way at 110 km/s and will collide in about 4.5 billion years. Despite the collision, individual stars are so far apart that almost none will directly collide. The result will likely be a giant elliptical galaxy, informally called Milkomeda.
🕸️ Galaxy Clusters
Galaxies cluster under gravity. The Local Group contains the Milky Way, Andromeda, and about 80 smaller galaxies. It is part of the Virgo Supercluster, which is itself part of the Laniakea Supercluster spanning 520 million light-years containing 100,000 galaxies.
🌐 Large-Scale Structure
On the largest scales, galaxies form a cosmic web of filaments, sheets, and voids. Galaxy clusters line the filaments; vast empty voids sit between them. The Sloan Great Wall, a superstructure of galaxies, spans 1.37 billion light-years.
🔥 Active Galactic Nuclei
Some galaxies have extremely luminous centers powered by matter falling into a supermassive black hole. Quasars are the most powerful, visible across billions of light-years. The most luminous quasar known outshines our entire galaxy by a factor of 600 trillion.