Interacting galaxy

Galaxies are huge groups of stars. Sometimes, they meet each other in space.

These meetings are called interacting galaxies. They happen because of gravity. Gravity is a force that pulls objects toward one another. When galaxies get close, their gravity pulls on each other. This can change the shape or size of a galaxy.

It is not like two cars hitting each other. Galaxies are mostly empty space, so they pass through each other. Instead, they feel a tugging force that disturbs their parts.

There are different ways galaxies can interact. A major interaction happens between two galaxies of similar mass. A minor interaction is when a small galaxy disturbs a larger one. For example, a small satellite galaxy might pull on the spiral arms of a big galaxy. Sometimes, a small galaxy dives right into a larger one. The Sagittarius Dwarf Elliptical Galaxy is doing this to our Milky Way. This can cause new stars to form in small groups.

People once called these star groups "blue blobs."

Scientists use computers to study these events. They use physics to simulate how gravity and gas work. These simulations show how galaxies might merge into one. A merger happens if the galaxies do not have enough energy to keep moving. They fall back into each other after passing through. If one galaxy is much bigger, it stays mostly the same. The smaller galaxy gets stripped apart and becomes part of the big one. The Paris Observatory has a library of these simulations called GALMER.

Many famous galaxies show these interactions. The Whirlpool Galaxy is a satellite interacting with its primary. The Mice Galaxies are in the second phase of a collision. The Antennae Galaxies are in the third phase of a collision.

A 2024 study looked at how many stars form during these events. It found that the mass of the galaxies is very important. If the two galaxies have similar masses, they create a larger starburst. A starburst is a place where many new stars are born at once.

Interactions also affect the centers of galaxies. Most galaxies have a supermassive black hole at the center. When galaxies interact, gas is driven toward these centers. This gas fuels the black hole and causes activity. This is called active galactic nucleus activity, or AGN activity.

Even our own Milky Way has a future collision. Astronomers say we will collide with the Andromeda Galaxy. This will happen in about 4.5 billion years. They used the Hubble Space Telescope to track Andromeda's movement.