Interacting galaxy

Big groups of stars can meet.

They pull on each other with a force. This pull can change their shape.

Small groups can pull on big ones. Sometimes, they even join to become one. This can make many new stars. Our own home may join another group later. It is a big dance in space.

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

They feel a pull from gravity. Gravity is a force that pulls objects together. This pull can change how a galaxy looks.

Some meetings are big. These happen between galaxies of a similar size. Other meetings are small. A small galaxy might pull on a big one. This can change the big galaxy's arms.

When galaxies collide, they might join together. This is called a merger. A merger can make a starburst. A starburst is a place where many new stars form. A 2024 study says mass matters here. If both galaxies are big, they make more stars.

Interactions can also help black holes. A black hole is a very heavy spot in a galaxy. The pull of the galaxies moves gas toward the center. This gas feeds the black hole. This can make the center of the galaxy very active. Even our Milky Way will collide with Andromeda one day. This will happen in about 4.5 billion years.

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.

Interacting galaxies, often called colliding galaxies, are systems where two or more galaxies influence one another through gravity.

Because galaxies are composed of a very tenuous distribution of matter, these encounters are not physical crashes like cars hitting each other. Instead, they are gravitational disturbances. These interactions can change a galaxy's shape, size, and how many stars it creates. Understanding these movements helps astronomers understand how galaxies evolve over billions of years.

The process of interaction begins when the gravitational fields of nearby galaxies start to pull on one another. This pull can distort the structure of the galaxies involved. If the galaxies do not have enough momentum to keep traveling past each other, they may eventually merge. During a merger, the galaxies fall back into each other after several passes. This can create a starburst, which is a region with a very high rate of new star formation.

A 2024 study suggests that the mass of the galaxies determines the strength of this effect. If the two galaxies have similar masses, the increase in the star formation rate will be greater.

Astronomers categorize these encounters into different types based on the mass of the participants. Major interactions occur between galaxies that have similar amounts of mass. Minor interactions happen when the masses of the galaxies vary significantly. In a minor interaction, a small satellite galaxy might disturb the spiral arms of a much larger primary galaxy.

Sometimes, a satellite galaxy like the Sagittarius Dwarf Elliptical Galaxy can dive directly into a primary galaxy like the Milky Way. This can trigger small amounts of star formation, creating star clusters that were once called "blue blobs."

Another specific type of interaction is known as galaxy harassment. This occurs within rich galaxy clusters, such as the Virgo or Coma clusters. In these dense areas, galaxies move at high relative speeds and encounter many other systems. This frequent contact can transform a low-luminosity spiral galaxy into a dwarf spheroidal or dwarf elliptical galaxy. Computer simulations show that these interactions convert galaxy disks into disturbed barred spiral galaxies. These encounters produce starbursts and can cause a galaxy to lose angular momentum and heat its gas.

Scientists study these complex movements using advanced computer simulations. These models use realistic physics to simulate gravitational forces, gas dissipation, and star formation. One such resource is the GALMER library of simulations found on the Paris Observatory website. Simulations also help explain the role of dynamical friction, which slows the relative motion of galaxy pairs. This friction determines whether a pair of galaxies will eventually merge based on the energy of their orbits.

Interactions also have a profound effect on the centers of galaxies. Most galaxies contain supermassive black holes. When galaxies interact, gravity drives gas toward the central regions. This inflow of gas fuels supermassive black hole accretion and triggers active galactic nucleus (AGN) activity.

Research from 2014 suggests that the type of AGN activity depends on the host galaxy. Blue, star-forming galaxies host radiatively efficient AGN fueled by cold gas. In contrast, more massive, passive galaxies host radio AGN fueled by mechanical accretion.

There are many notable examples of these cosmic dances in our sky. The Whirlpool Galaxy (M51) is a famous example of a satellite interacting with a primary galaxy. The Mice Galaxies (NGC 4676A/B) are currently in the second phase of a galactic collision. The Antennae Galaxies (NGC 4038/9) are in the third phase of a collision.

Even our own Milky Way is part of this story. Astronomers using the Hubble Space Telescope have determined that the Milky Way will collide with the Andromeda Galaxy in about 4.5 billion years. This collision may eventually result in the creation of a large disc galaxy or an elliptical galaxy.