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Galaxies & Cosmic Structures

Galaxy Mergers and Galactic Cannibalism

Galaxies grow by colliding and merging, and large galaxies can absorb smaller ones over billions of years.

Stars rarely hit each other, but gravity reshapes both galaxies and can trigger bursts of star formation.

  1. Observe

    Galaxies with tidal tails, bridges or shells and bursts of star formation.

  2. Measure

    Positions and motions of stars and gas in each galaxy.

  3. Model

    N-body simulations of how galaxies gravitationally interact.

  4. Explain

    Galaxies are built up through repeated mergers.

What is it?

Gravity pulls galaxies toward each other. When they pass close or merge, their shapes are distorted into long tails and bridges, gas is compressed, and star formation can surge. When one galaxy is much smaller, the larger one can slowly tear it apart and absorb it, sometimes called galactic cannibalism.

Individual stars are so far apart that direct star-to-star collisions are extremely rare. The Milky Way is currently absorbing the Sagittarius dwarf galaxy, and Gaia data indicate a large merger happened roughly 8 to 11 billion years ago. Predictions that the Milky Way will merge with Andromeda in a few billion years are now thought to be less certain than once believed.

How does it happen?

  1. Stage 1
    Two galaxies approach
  2. Stage 2
    Tidal forces distort them
    Tails and bridges form
  3. Stage 3
    Gas is compressed
    Starbursts, possible active nucleus
  4. Stage 4
    They settle into one galaxy

What do we actually observe?

Tidal tails
Long streams of stars pulled out during a close pass, as in the Antennae Galaxies.
Stellar streams
Remnants of absorbed dwarf galaxies wrap around the Milky Way.
Starbursts
Compressed gas forms stars rapidly, making merging galaxies bright in infrared.

How do scientists detect it?

  • Hubble and JWST. Image merging galaxies and their star-forming regions.
  • Gaia. Maps positions and motions of over a billion stars, revealing ancient merger debris.
  • Radio and infrared telescopes. Trace gas and dust.

Why does it matter?

  • Mergers are a major way galaxies grow and change shape.
  • Merging supermassive black holes should emit low-frequency gravitational waves.
  • The Milky Way's history is written in the debris of its past mergers.

The SpaceWeatherTracker connection

Our Sun orbits inside a galaxy that has itself been shaped by mergers. Understanding the galactic setting is part of understanding the environment the heliosphere moves through.

This is an explainer. SpaceWeatherTracker does not show live data for this phenomenon; see the “Space Phenomena Now” section on the hub for what is currently monitored.

Related phenomena

Primary family: Galaxies & Cosmic Structures

Galaxy Mergers and Galactic Cannibalism: common questions

Will the Milky Way collide with Andromeda?

The two galaxies are approaching each other. Earlier estimates predicted a merger in about 4 to 5 billion years, but recent analyses suggest the outcome is less certain.

Do stars collide when galaxies merge?

Very rarely, because the distances between stars are enormous compared to their sizes.

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