What happens when galaxies collide depends on their sizes and their speeds. A galaxy merger requires only two conditions: the galaxies must be close enough and be moving at a slow enough relative speed to be captured by their mutual gravities. If they pass too quickly, they cannot bind their gravities together. They interact by stripping stars and heating up.

If one galaxy is larger than the other, the smaller galaxy is cannibalized or incorporated. The larger galaxy’s strong gravity draws the smaller galaxy into a long arc and absorbs it.

Galaxies of comparable sizes have a great power struggle. Their gravities fling outer portions of their galaxies into space, creating enormous tidal tails. As they merge, starbursts occur as gas clouds mix and create millions of new stars. As their gravitational fields change, spiral galaxies lose energy and angular momentum. Hydrogen gas flows toward the central regions. When half the gas reaches the center, the gas forms a dense ring around the nucleus and can trigger massive star formation. It can provide fuel for the supermassive black hole lurking in the galaxy’s center. The loosely bound gas in the outer disk ejects into tidal tails.

“Peculiar galaxies” are what astronomers call these odd-looking galaxies. Eventually, they balance and form new galaxies. Two spirals mix by merging as the nuclei swirl around each other. The flat disks cannot retain their shapes and form a large ellipsoid, or elliptical galaxy. Two examples of merging galaxies are the Antennae (NGC 4038/9) and the Mice (NGC 4676), named for the shapes they resemble with arcing tails and plumes of material.

Our Milky Way Galaxy has been involved in 5 to 11 minor mergers, incorporating dwarf galaxies. Evidence for smaller galaxy mergers is stars and globular clusters in the young halo of our galaxy. The Sagittarius dwarf spheroidal galaxy is in the late stages of merging. The trail of stars can still be seen encircling the Milky Way. Our Milky Way Galaxy will collide with the Andromeda Galaxy in 3 to 4 billion years. Since both galaxies are enormous spiral galaxies, it will be a long and extremely violent process of merging. Eventually, they will form a new elliptical galaxy.