Do stars collide when galaxies merge?

Almost never. Stars are so tiny compared with the distances between them that direct collisions are extremely rare. What changes during a merger is the gravitational arrangement of the stars and the behavior of the gas, not the stars themselves.

Will the Milky Way merge with another galaxy?

Yes. The Milky Way and the Andromeda galaxy are approaching each other and are expected to undergo a major merger billions of years from now, likely producing a single, more elliptical-like galaxy.

Galaxy Mergers and Interactions

Gravitational encounters and mergers between galaxies reshape their structure, trigger bursts of star formation, and drive the buildup of massive galaxies.

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Definition

Galaxy mergers and interactions are gravitational encounters between galaxies that distort and intermingle their stars and gas, ranging from mild tidal disturbances to complete coalescence, and are a principal mechanism of galaxy transformation and growth.

Scope

This topic covers tidal interactions that produce bridges and tails, the dynamics of major and minor mergers, the role of dynamical friction in bringing galaxies together, merger-induced starbursts and the formation of luminous infrared galaxies, and the production of elliptical galaxies through the merging of disks.

Core questions

How do tidal forces produce the bridges and tails seen in interacting galaxies?
What distinguishes major from minor mergers in their effects?
How do mergers trigger starbursts and feed central black holes?
Can the merging of spirals produce elliptical galaxies?

Key theories

Tidal interactions: Toomre and Toomre showed with simple simulations that gravitational tides during close encounters naturally generate the bridges and tails observed in interacting galaxies.
Merger-driven elliptical formation: Simulations demonstrate that the merger of two disk galaxies can produce a pressure-supported remnant resembling an elliptical, linking mergers to galaxy morphology.
Merger-induced starbursts: Mergers funnel gas to galactic centers, igniting intense starbursts that can make systems shine as luminous infrared galaxies and may also fuel nuclear activity.

Clinical relevance

Mergers are central to hierarchical galaxy assembly, transforming morphology, triggering star formation and black hole growth, and explaining peculiar systems; the predicted future merger of the Milky Way and Andromeda makes the process directly relevant to our own Galaxy.

History

Toomre and Toomre's 1972 simulations explained interacting galaxies as tidal phenomena and suggested mergers build ellipticals. The development of self-consistent N-body and hydrodynamic simulations in the 1980s and 1990s, together with infrared observations of merging starbursts, established mergers as a key evolutionary process.

Key figures

Alar Toomre
Juri Toomre
Joshua Barnes
Lars Hernquist

Seminal works

toomre1972
barnes1992
sanders1996

Frequently asked questions

Do stars collide when galaxies merge?: Almost never. Stars are so tiny compared with the distances between them that direct collisions are extremely rare. What changes during a merger is the gravitational arrangement of the stars and the behavior of the gas, not the stars themselves.
Will the Milky Way merge with another galaxy?: Yes. The Milky Way and the Andromeda galaxy are approaching each other and are expected to undergo a major merger billions of years from now, likely producing a single, more elliptical-like galaxy.