09 Galaxies and Their Evolution
Learn how galaxies form and change, how active galactic nuclei influence them, and how galaxies are organized across the cosmic web.
shapes and what they reveal
Galaxies are vast systems of stars, gas, dust, and bound together by gravity. They are not isolated: they grow and change as gas moves into and out of them, stars form, and galaxies interact with one another and their surroundings.
Astronomers classify galaxies mainly by visible shape:
Spiral galaxies have a flattened disk, often with spiral arms, around a central bulge. Gas and dust in the disk can form new stars. The Milky Way is a barred spiral, with an elongated central bar.
Elliptical galaxies range from nearly round to elongated. Their stars follow less orderly orbits, and these galaxies generally contain less cold gas and dust than spirals. Many have little ongoing star formation.
Lenticular galaxies have a disk and central bulge but lack prominent spiral arms. They tend to have older stars and relatively little current star formation.
Irregular galaxies have no clear spiral or elliptical structure. Some are small and gas-rich; others have been distorted by gravitational encounters.
A ’s shape describes its appearance, not a complete history or a fixed stage in a universal sequence. Interactions can distort a disk, and gas loss or heating can reduce the supply for forming stars. The takeaway: morphology offers useful clues, but it does not by itself explain how a formed or evolved.
How galaxies form and evolve
Gravity drew matter into denser regions in the early universe. formed extended halos whose gravity helped attract ordinary matter, especially gas. As gas accumulated in these halos, it could cool, settle, and form stars. Galaxies grew through both the addition of gas and the merging of smaller systems.
evolution continues after formation. Gas supplies the material for new stars, while stellar winds and supernovae return energy and elements to their surroundings. A can acquire gas from its environment or lose gas through energetic outflows. These processes affect its star-formation activity and change its contents over time.
During close encounters, gravity can draw long streams of stars and gas from galaxies, compress gas into star-forming regions, or alter their shapes. In a merger, stars usually pass without directly colliding because the distances between them are enormous, though gas clouds can interact. A major merger can strongly disrupt a disk and contribute to the formation of an elliptical . However, some disks survive encounters, and others can form again from later-acquired gas. There is no single progression that every follows.
Environment also matters. In dense groups and clusters, encounters and interactions with surrounding hot gas can change a ’s gas supply and star formation. Galaxies in less crowded regions may retain cold gas for longer. These are broad tendencies, not rules for every .
Takeaway: Gas, gravity, stellar activity, mergers, and environment all contribute to evolution; no single factor or shape determines every ’s path.
Active galactic nuclei and
An is a compact, exceptionally energetic region at a ’s center. Its power comes from matter falling toward a supermassive black hole. Before that material crosses the event horizon, some forms a hot that radiates across the electromagnetic spectrum. Magnetic fields can also help launch fast jets and broader winds from the region around the black hole.
AGN are observed in several forms, including Seyfert galaxies, quasars, and blazars. Some differences reflect how luminous the source is and how it is viewed. A blazar has a powerful jet aimed nearly toward Earth, making it appear especially bright and variable. A quasar is an extremely luminous AGN whose central light can outshine its host .
AGN can affect their host galaxies through . Jets and winds may heat or expel gas, reducing the supply of cool material available for star formation. The strength and long-term effects vary: gas may later cool or be replenished. AGN therefore connect the growth of central black holes with the evolution of their host galaxies.
Takeaway: An AGN is powered by matter falling toward a supermassive black hole, and its outflows can alter—but do not permanently determine—the gas available for star formation.
Groups, clusters, and the
Galaxies occupy a hierarchy of larger structures. Groups contain relatively small numbers of gravitationally bound galaxies; the Milky Way belongs to the Local Group. Clusters contain hundreds or thousands of galaxies and also hold large amounts of hot gas and . Groups and clusters occur within still larger patterns, but superclusters are generally not bound together as single systems by gravity.
Across the universe, galaxies and groups trace filaments, sheets, and dense intersections, with vast voids containing relatively few galaxies between them. This pattern is the . Gravity acting on matter, including , helps build it over time. Dense intersections can grow into clusters, while filaments connect them and provide routes along which galaxies and gas accumulate.
A ’s position in this web affects the conditions it experiences. A entering a crowded cluster may encounter very different conditions from an isolated or one in a smaller group. Comparing galaxies in different environments helps reveal how individual systems and large-scale structure develop together.
Takeaway: evolution occurs within a changing cosmic environment, from a ’s local group or cluster to the filaments, sheets, and voids of the wider universe.