3 Galaxies and Their Evolution
Learn how galaxies are classified, how their structures and environments shape their evolution, and how they fit into the cosmic web.
types and visible shapes
A is a gravitationally bound system of stars, gas, dust, stellar remnants, and dark matter. Galaxies range from faint dwarfs to enormous systems containing trillions of stars. They are not isolated: they form, change, and gather into larger structures over cosmic time.
Astronomers commonly classify galaxies by their visible shapes. These categories describe patterns, not a fixed sequence that every must follow. A 's appearance can offer clues about its stars, gas, and past interactions, but shape alone does not reveal its entire history.
Spiral galaxies have a rotating, flattened disk, often with spiral arms, and usually a central bulge. The arms contain gas and dust where many young stars form; older stars are also found in the disk, bulge, and surrounding halo. A barred spiral has a bar-shaped concentration of stars across its center. The Milky Way is a barred spiral.
Elliptical galaxies range from nearly round to elongated. Their stars move in many different directions rather than following an orderly disk. They generally contain less cold gas and dust and form fewer new stars than gas-rich spirals.
Lenticular galaxies have a disk and central bulge but lack prominent spiral arms. They often have relatively little ongoing star formation.
Irregular galaxies lack a clear spiral or elliptical shape. Some are small and gas-rich; others have been distorted by gravitational encounters.
Structure and the movement of gas
A 's visible structure may include a thin disk, a thicker disk, a central bulge or bar, and a diffuse containing old stars and star clusters. Beyond the visible stars lies an extended . Dark matter does not emit or reflect detectable light, but its gravity affects how stars and gas move and helps shape the distribution of matter around galaxies.
Most large galaxies also contain a central supermassive black hole. When gas falls toward one, the energetic central region can become an active galactic nucleus.
Gas is essential to continued galactic growth because cold gas can collapse under gravity to form stars. Stars return some material to space through stellar winds and explosions, enriching the surrounding gas with elements made inside stars. Gas can also enter from intergalactic space or be stripped away through interactions with neighboring galaxies. These exchanges influence how quickly a forms stars and what it becomes.
Formation and ongoing evolution
Galaxies grew from early concentrations of matter in the young universe. Gravity gathered dark matter into halos; ordinary gas collected within them, cooled, and formed stars. Small systems could then grow by drawing in gas and merging with other galaxies. The precise early steps, and the relationship between galactic growth and the growth of central black holes, remain active areas of research.
evolution is not simply a smooth change in shape. A continuing supply of cold gas can sustain star formation. When available star-forming gas is used up, heated, or removed, star formation can slow or stop.
Changes within a also matter. Bars and other internal structures can redistribute gas and stars. Energy from young stars and active black holes can heat or push gas around, affecting later star formation.
Interactions and mergers
A close gravitational encounter can distort a , pull out long streams of stars and gas, or compress gas clouds and trigger a burst of star formation. In a , the galaxies eventually combine. Their stars do not usually collide directly because the distances between stars are enormous. Mergers can rearrange a 's structure and send gas toward its center, sometimes feeding its central black hole.
A large can also gradually absorb smaller companions, adding their stars and gas to its own. This process is sometimes called .
A gas-rich spiral experiencing a major may lose its orderly disk and end up with a more elliptical appearance. However, there is no universal spiral-to-elliptical pathway: galaxies have diverse histories, and their surroundings, gas supply, and interactions all matter.
Galaxies in the
Gravity groups galaxies into groups and clusters. The Milky Way belongs to the Local Group, which also includes Andromeda and many smaller galaxies. Clusters contain hundreds or thousands of galaxies, along with hot gas and substantial dark matter.
Groups and clusters are arranged in larger patterns of filaments, walls, and sheets, with vast, relatively empty voids between them. Together these structures form the . Gravity draws matter into dense regions and filaments, where galaxies and clusters gather. Superclusters describe broad associations of groups and clusters; they are generally not all bound together as one system.
A 's environment can influence its evolution. Close neighbors make encounters and mergers more likely, while hot gas in a dense cluster can affect a 's gas supply. The history of individual galaxies is therefore connected to the growth of structure across the universe.
Observing galactic history
Astronomers reconstruct galactic histories by comparing galaxies at different distances. Because light takes time to travel, distant galaxies are seen as they were in the past, providing snapshots of earlier cosmic eras. This allows the study of evolution across cosmic time without implying that every follows the same path.