Cosmic Car Crashes: Galaxy Collisions Quiz

Although galaxies contain billions of stars, the space between them is enormous. If a star were the size of a marble, the nearest star would be hundreds of miles away. Consequently, during a collision, the galaxies pass through each other like ghosts; however, their gravitational fields and gas clouds interact violently.

When galaxies interact, the interstellar gas clouds are compressed by shockwaves and gravitational tidal forces. This compression causes the gas to collapse rapidly, leading to a massive increase in the birth rate of new stars.

As galaxies approach each other, mutual gravity pulls long streamers of stars and gas into space, known as tidal tails. A "bridge" can also form if matter is pulled from one galaxy toward the other. These features are the visual signatures of the gravitational "tug-of-war."

Computer simulations show that when two large spiral galaxies collide, the organized rotation of their disks is destroyed. The stars eventually settle into a randomized, three-dimensional swarm characteristic of an elliptical galaxy.

As the galaxies merge, the two central black holes sink toward the new common center due to dynamical friction. They form a binary pair, spiraling closer until they merge into a single, even more massive black hole, releasing gravitational waves.

In high-speed collisions or when a galaxy falls into a dense cluster, its gas can be "stripped" away by the pressure of the surrounding medium. Once the gas is gone, the galaxy can no longer create new stars and is considered "quenched."

The dynamics of a merger depend on whether it is a "minor merger" or a "major merger." The speed determines if they merge or simply "fly by," while gas content determines how much star formation the collision will produce.

Galactic cannibalism is common; the Milky Way is currently "eating" several small dwarf galaxies. The smaller systems are torn apart by tidal forces, and their stars are incorporated into the halo or disk of the larger galaxy.

As a massive object moves through a sea of smaller stars, its gravity creates a "wake" behind it. The pull of this wake acts as a drag force, slowing the object down and causing it to sink toward the center of the gravity well.

The Antennae (NGC 4038/4039) provide a spectacular look at a mid-stage merger. Their long, curved tidal tails resemble the antennae of an insect, helping astronomers verify mathematical models of how gravity reshapes galaxies.

If a smaller galaxy passes directly through the center of a larger disk galaxy, it creates a ripple effect. This gravitational wave pushes gas and dust outward in a circle, triggering a ring of intense star formation moving away from the impact site.

Predicted by Einstein, gravitational waves are ripples in spacetime. As the central black holes of two colliding galaxies spiral together, they emit these waves, carrying away orbital energy until they finally join.

Before a collision, stars in a spiral galaxy move in organized, circular orbits. The chaotic gravitational changes during a merger "scatter" these stars into randomized, three-dimensional paths, which is why mergers often produce puffy elliptical galaxies.

In the early universe, galaxies were closer together and collided much more frequently. This "hierarchical" model suggests large galaxies like ours are built from the pieces of many smaller ones through constant interaction.

Even after a merger is complete, evidence remains. Some elliptical galaxies show "shells" of stars. If the central black holes haven't merged yet, a galaxy might appear to have two distinct centers or "nuclei."

Andromeda (M31) is currently moving toward the Milky Way. In about 4 to 5 billion years, the two will begin a merger that will fundamentally change the structure of both galaxies.

The intense star formation triggered by a collision leads to many supernovae. The collective energy of these explosions can create a "galactic wind" that pushes remaining gas into intergalactic space, halting further star formation.

While some shapes are determined at birth, many are the result of environment. Elliptical galaxies are most common in dense clusters where collisions are frequent, suggesting the Hubble Sequence is partly a map of historical interactions.

Collisions stir up dust that blocks visible light. Infrared telescopes (like JWST) peer through the dust, radio telescopes track cold gas, and X-ray telescopes detect super-heated gas and central black hole activity.

Galaxy collisions happen on a truly cosmic timescale. Because galaxies are so large and move relatively slowly on that scale, the process from first interaction to final settling into a single galaxy takes a massive amount of time.