Introduction to Galaxy Formation and Evolution
Linas JuozėnasShare
Knowledge Ark · Universe · Chapter 03
Galaxy formation and evolution
Why galaxies take different shapes, how they change, and what their histories reveal about our own place in the cosmos.
Dark matter, stars, and gas together create the gravitational environment in which a galaxy develops.
Cooling, fresh supply, and feedback influence where star formation continues and where it slows.
Internal patterns and encounters redistribute material, leaving clues to a galaxy’s past.
A galaxy is a history still unfolding.
A spiral’s bright arms, an elliptical’s smooth glow, and an irregular galaxy’s scattered star-forming regions look very different. Each offers clues to how matter has gathered and changed over billions of years.
In the previous chapter, we followed the first stars and the emergence of cosmic structure. Here, we ask what happened as galaxies continued to grow: how their gas became stars, how their surroundings influenced them, and why their shapes diverged.
This chapter connects what we can see to what we must infer. A photograph reveals structure; measurements of light and motion help reveal mass, chemistry, and activity. Bringing these clues together lets astronomers test possible histories instead of judging a galaxy by appearance alone.
Ten ways to understand a galaxy
Start with the hidden foundations, explore the visible patterns, then follow the possible futures.
Dark matter halos: galactic foundations
What holds a galaxy together beyond the stars we see?
A galaxy’s starlight reveals only part of its mass. In the standard cosmological model, galaxies develop within dark matter halos: extended concentrations of matter detected through their gravitational effects. Orbital speeds and the bending of background light help astronomers investigate this otherwise invisible component.
A halo provides a gravitational setting for galaxy formation, but gathering gas is only the beginning. Gas must lose heat to become dense enough to form stars, while its rotation helps shape the resulting system. This article connects the unseen mass to the luminous galaxy inside it.[1]
Explore article 01→Hubble’s galaxy classification
What can a galaxy’s shape tell us—and what can it hide?
Spirals have disks with winding arms; ellipticals have smooth, rounded or elongated light profiles. Lenticulars, also called S0 galaxies, have disks but little prominent spiral structure. Irregulars do not fit neatly into these regular forms.
Hubble’s famous “tuning fork” organizes ellipticals, lenticulars, and ordinary and barred spirals; irregular galaxies sit outside its main branches. It classifies appearance, without prescribing a life story. The traditional labels “early type” and “late type” describe categories, not a galaxy’s age. We will use this vocabulary while exploring why similar shapes can conceal different histories.[2]
Explore article 02→Collisions and mergers: drivers of galactic growth
What actually collides when two galaxies encounter each other?
Individual stars are so widely separated that direct stellar collisions are rare. Instead, gravity changes their orbits, pulling out streams and tails. Gas can be compressed or driven inward, sometimes producing a burst of star formation. The outcome depends on the galaxies’ gas supplies, masses, and encounter geometry.[3]
Not every close encounter ends in a merger. The fast motions common in rich clusters can make capture difficult, while slower encounters in groups can be more favorable. This article follows how interactions redistribute existing stars and, under suitable conditions, help create new ones.[4]
Explore article 03→Galaxy clusters and superclusters
How does living in a crowd change a galaxy?
A galaxy cluster is a gravitationally bound system containing galaxies, dark matter, and a vast reservoir of hot gas. A galaxy moving through that gas can lose some of its own fuel through ram-pressure stripping—roughly, the pressure of an oncoming wind. Its future star formation may change as a result.[5]
Zoom out and clusters belong to still larger patterns of groups and filaments. These superclusters generally are not bound as complete systems, although dense parts can be. We will explore the distinction between sharing a place in the cosmic web and sharing a gravitationally bound future.[6]
Explore article 04→Spiral arms and barred galaxies
How can a rotating disk keep making such striking patterns?
Spiral arms trace patterns in stars and gas, often highlighted by young, bright stars. They are not rigid spokes rotating as one piece. Some models describe long-lived density waves; others produce arms that form, wind up, and recur. Encounters with nearby galaxies can also help generate spiral structure.[7]
A bar is an elongated stellar structure crossing a galaxy’s center. Bars exchange angular momentum with their surroundings and can help gas move inward, building central structures or supporting star formation. That transport does not guarantee an active black hole: getting gas from a galactic disk to the black hole requires further steps.[8]
Explore article 05→Elliptical galaxies: formation and features
Does a smooth-looking galaxy have a simple past?
Elliptical galaxies often contain predominantly older stars and form relatively few new ones. Their smooth appearance hides considerable variety: they span a wide range of masses, and some show substantial organized rotation. A lack of much cold, star-forming gas also does not mean a complete absence of gas.[9], [15]
Mergers can rearrange stellar orbits and build rounded systems, while later encounters can add stars to their outskirts. How much gas was available—and whether it cooled, formed stars, or was heated—also matters. We will look for clues in stellar motions, chemical composition, and faint outer structures, rather than assuming every elliptical followed the same route.[1]
Explore article 06→Irregular galaxies: chaos and starbursts
Does an uneven appearance always mean something violent happened?
An irregular galaxy may show scattered star-forming patches, an off-center concentration of stars, or an asymmetric outline. Some have been disturbed by neighbors. In many small galaxies, uneven star formation and the influence of stellar explosions also help produce a patchy appearance.
A starburst is an unusually intense episode of star formation, and it is not a defining feature of every irregular galaxy. Surveys of nearby dwarf galaxies find strong bursts in only a minority at a given time. This article explores both spectacular bursts and the quieter systems that show how galaxies develop with relatively little mass.[10]
Explore article 07→Evolutionary paths: secular vs. merger-driven
Can a galaxy transform without a major collision?
Secular evolution means gradual rearrangement over many orbits. Bars and other disk structures can move angular momentum around, redistribute stars and gas, and build central concentrations. A galaxy can therefore change substantially through its own internal dynamics.[8]
Mergers provide another route, adding material and altering stellar motions. These processes can overlap within one galaxy’s history; fresh gas supply and feedback also influence what happens next. We will compare the evidence for different contributions and ask why changing shape and stopping star formation are related questions, but not identical events.[12]
Explore article 08→Active galactic nuclei and quasars
How can a galactic center outshine billions of stars?
When gas reaches a supermassive black hole, it can release enormous energy before crossing the event horizon. This bright central region is an active galactic nucleus, or AGN. Quasars are especially luminous examples. The radiation comes from matter around the black hole; it does not escape from inside the horizon.[11]
Radiation, winds, and—in some systems—jets can heat or displace surrounding gas. This feedback can affect the supply available for new stars, but finding an active nucleus or an outflow does not by itself prove that star formation has stopped. We will examine how black hole growth and galaxy evolution influence one another.[12]
Explore article 09→Galactic futures: Milkomeda and beyond
What might our galaxy become billions of years from now?
“Milkomeda” is a nickname for a possible future merger remnant of the Milky Way and Andromeda. Their eventual merger remains uncertain. A 2025 study and a 2026 follow-up obtained different probabilities, showing how strongly the forecast depends on measurements of the galaxies’ motions and the gravitational influence of their neighbors.[13], [14]
This final article extends the story into the future: how encounters could reshape galaxies, how changing gas supplies affect star formation, and how cosmic expansion influences the largest structures. Forecasting those outcomes means identifying which conclusions are robust and which depend on measurements or assumptions that may still change.
Explore article 10→What would reveal this galaxy’s history?
A shape is a starting clue. Young stars reveal recent activity; stellar motions trace gravity; gas shows where future stars might form. Faint streams and shells can preserve evidence of earlier encounters.
As you explore, look for connections between these clues. A rounded appearance, a quiet central black hole, or a bright patch of new stars each tells only part of the story.
Why the story remains open
Astronomers still investigate how different processes share the work of building galaxies. How much growth came from fresh gas or mergers? When did internal dynamics reshape a disk? How effectively did black hole activity change the gas around it?[8], [12]
We cannot watch a whole galactic lifetime unfold. Instead, observations of many galaxies and simulations of their evolution help us reconstruct what happened—and test what could happen next.
Sources and further reading
Space-agency explainers introduce the main ideas. The research papers and reviews explore the evidence and its limits, including recent work on the Milky Way’s possible future.
- Somerville & Davé (2015) — Physical Models of Galaxy Formation in a Cosmological FrameworkHalos, gas accretion, star formation, and the processes that shape galaxies.
- ESA / Hubble — The Hubble tuning forkThe morphological classification scheme, including lenticular and barred galaxies.
- Di Matteo et al. (2008) — On the frequency, intensity, and duration of starburst episodes triggered by galaxy interactions and mergersWhy interacting galaxies show a range of star-formation responses.
- Pearson et al. (2024) — The effects of galaxy environment on the merger fractionThe roles of environment and relative velocities in galaxy merging.
- Kravtsov & Borgani (2012) — Formation of Galaxy ClustersCluster assembly, dark matter, and the hot intracluster medium.
- Chon, Böhringer & Nowak (2015) — On the definition of superclustersThe difference between an observed association and a system that will remain bound.
- Dobbs & Baba (2014) — Dawes Review 4: Spiral Structures in Disc GalaxiesSpiral-arm mechanisms, simulations, and observational evidence.
- Kormendy & Kennicutt (2004) — Secular Evolution and the Formation of Pseudobulges in Disk GalaxiesBars, angular momentum transport, and gradual changes within disks.
- Emsellem et al. (2011) — The ATLAS3D project, III: A census of the stellar angular momentum within the effective radius of early-type galaxiesThe range of stellar rotation in nearby elliptical and lenticular galaxies.
- Lee et al. (2009) — Dwarf Galaxy Starburst Statistics in the Local VolumeHow often nearby dwarf galaxies experience strong starbursts.
- NASA / Webb — What Are Active Galactic Nuclei?An accessible explanation of accretion-powered galactic centers.
- Kormendy & Ho (2013) — Coevolution (Or Not) of Supermassive Black Holes and Host GalaxiesConnections between black hole growth, galaxy structure, and feedback.
- Sawala et al. (2025) — No certainty of a Milky Way–Andromeda collisionA forecast that includes orbital uncertainties and neighboring galaxies.
- Wu et al. (2026) — The Fate of the Milky Way–Andromeda System: To Merge or Not?A follow-up favoring a higher merger probability but emphasizing sensitivity to measured motions; accepted by The Astrophysical Journal Letters.
- NASA — Galaxy TypesAn introduction to galaxy forms, stellar populations, and star-forming gas.