--- title: "Galaxy Collisions | Intro to Astronomy" description: "Galaxy collisions are gravitational encounters where galaxies distort, merge, and trigger starbursts, AGN, and new shapes in Intro to Astronomy." canonical: "https://fiveable.me/intro-astronomy/key-terms/galaxy-collisions" type: "key-term" subject: "Intro to Astronomy" unit: "Unit 28" --- # Galaxy Collisions | Intro to Astronomy ## Definition Galaxy collisions are gravitational encounters in which two or more galaxies pass through, distort, or merge with each other. In Intro to Astronomy, they are a major driver of galaxy evolution, starbursts, and active galactic nuclei. ## What It Is Galaxy collisions are what happen when two galaxies come close enough that gravity starts reshaping both systems. They do not usually look like crash-style impacts between stars, because galaxies are mostly empty space. Instead, the main effect is gravitational interaction, which pulls, stretches, and compresses gas, dust, and star orbits. As the galaxies pass through one another or spiral into a merger, tidal forces can tear out long tails, warp disks, and funnel material toward the center. That rearranged gas is the part that matters most for visible change. When gas clouds get compressed, they can collapse faster and form many new stars at once, which is why galaxy collisions often produce starbursts. The outcome depends on the masses, gas content, and geometry of the encounter. Two spiral galaxies can merge into a single system that gradually loses its ordered disk structure and ends up looking more like an elliptical galaxy. If the galaxies both contain large central black holes, those black holes can eventually spiral together too, after the galactic centers settle. In Intro to Astronomy, galaxy collisions are part of the bigger story of how galaxies grow over time. The Milky Way and Andromeda are a famous example: they are expected to merge in about 4 billion years. That future event is a useful model for how a collision can unfold slowly over hundreds of millions to billions of years, not as a quick explosion. One common misconception is that colliding galaxies destroy everything they touch. In reality, most stars never physically hit each other. The galaxies mostly reorganize themselves through gravity, gas compression, and orbital changes, which is why collisions can create new structures even while they disrupt old ones. ## Why It Matters Galaxy collisions show how galaxies evolve, not just how they look in one snapshot. In Intro to Astronomy, this term connects directly to galaxy formation, star formation, and the growth of central black holes. If you know what collisions do to gas and orbits, a lot of later topics make more sense. This concept also helps you interpret real astronomical images. A warped spiral, a bright central knot, or long tidal tails are clues that gravity has recently disturbed a galaxy. Those features are not random decoration, they are evidence for past or ongoing interactions. Galaxy collisions are also a bridge to active galactic nuclei. When gas gets driven inward during a merger, it can feed the supermassive black hole at the center and produce AGN activity. So one process can lead to several observable outcomes: starbursts, structural changes, and nuclear activity. For course work, this term often shows up in questions asking you to connect cause and effect. You may need to explain why a merger triggers star formation, compare spiral and elliptical structures, or describe what happens when the Milky Way meets Andromeda. The big idea is that galaxies are not fixed objects, they change through repeated encounters over cosmic time. ## Connections ### [Gravitational Interaction](/intro-astronomy/key-terms/gravitational-interaction) This is the force behind every galaxy collision. The galaxies are not smashing together like solid objects, they are responding to each other's gravity, which changes orbits, pulls out tails, and compresses gas. If you understand the interaction, you can predict the visible aftermath in a merger image or diagram. ### Starburst Galaxy Many galaxy collisions trigger starbursts because gas clouds get squeezed during the encounter. A starburst galaxy is what you get when star formation spikes well above the normal rate. That makes this term a common aftermath of a merger, not a separate process. ### Active Galactic Nucleus (AGN) Collisions can funnel gas toward a galaxy's center, where it can feed the central black hole. That inflow can power an AGN, so a merger can be one pathway to nuclear activity. This connection is why astronomers often link disturbed galaxies with bright central emissions. ### [Elliptical Galaxy](/intro-astronomy/key-terms/elliptical-galaxy) A major merger between spiral galaxies can erase much of the ordered disk structure and leave a more rounded, smooth remnant. That is one reason elliptical galaxies are often tied to past mergers. If a question asks about a likely collision outcome, elliptical structure is a common answer. ## On the AP Exam A quiz image or short-answer prompt may show a warped galaxy with tidal tails and ask you to identify the process. The move is to connect those visual clues to gravity-driven interaction, then explain what it does to gas, star formation, and structure. In a problem set or discussion, you might trace the future of the Milky Way and Andromeda or compare a spiral galaxy before and after a merger. If the question mentions a bright central region, think about whether gas inflow could be feeding an AGN. The best answers use cause and effect, not just the label "collision." ## galaxy collisions vs Gravitational Interaction Gravitational interaction is the broader force at work whenever galaxies influence each other, even at a distance. Galaxy collisions are the specific event or outcome when that interaction becomes strong enough to distort, merge, or dramatically reshape the galaxies. In other words, interaction is the mechanism, collision is the encounter you can point to. ## Key Takeaways - Galaxy collisions are gravitational encounters, not star-on-star smashups. - The biggest changes usually come from gas compression, tidal forces, and orbital disruption. - Collisions can trigger starbursts, feed supermassive black holes, and create AGN activity. - A major merger can turn spiral structure into a more elliptical-looking remnant. - The Milky Way and Andromeda are expected to merge far in the future, which makes this a real example, not just theory. ## FAQs ### What is galaxy collisions in Intro to Astronomy? Galaxy collisions are events where two galaxies pass through, distort, or merge because of gravity. In Intro to Astronomy, the term usually comes up when you study how galaxies grow, how starbursts start, and why some galaxies look warped or disturbed. ### Do stars collide when galaxies collide? Usually, no. Galaxies are mostly empty space, so stars are very unlikely to hit each other directly. The big effect is gravitational, which changes orbits and squeezes gas into new regions. ### What happens after two galaxies collide? The galaxies can merge into a single system, lose some of their original structure, and form new stars from compressed gas. If the encounter is strong enough, the result can look more like an elliptical galaxy than a spiral galaxy. ### How are galaxy collisions related to AGN? A collision can push gas inward toward the center of a galaxy. That gas may feed the central supermassive black hole, which can power an active galactic nucleus. Not every merger makes an AGN, but the connection is common in astronomy. ## Related Study Guides - [28.2 Galaxy Mergers and Active Galactic Nuclei](/intro-astronomy/unit-28/2-galaxy-mergers-active-galactic-nuclei/study-guide/fAB8S2ySCPFsqRwy) ## About This Document Canonical Fiveable pages are available as Markdown at the same path plus `.md`. - [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns - [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing - [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`) - [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`) ## Structured Data ```json {"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/intro-astronomy/key-terms/galaxy-collisions#resource","name":"Galaxy Collisions | Intro to Astronomy","url":"https://fiveable.me/intro-astronomy/key-terms/galaxy-collisions","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/intro-astronomy/key-terms/galaxy-collisions#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:22:19.663Z","isPartOf":{"@type":"Collection","name":"Intro to Astronomy Key Terms","url":"https://fiveable.me/intro-astronomy/key-terms"},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/intro-astronomy/key-terms/galaxy-collisions#term","name":"galaxy collisions","description":"Galaxy collisions are gravitational encounters in which two or more galaxies pass through, distort, or merge with each other. 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