Feeding on Neighbors: Galactic Cannibalism Explained Quiz

Galaxies are mostly empty space. While the galaxies themselves collide and change shape due to gravity, the distances between individual stars are so vast that they pass by one another like swarms of bees, rarely ever making physical contact.

By looking at the "kinematics" (how they move) and "metallicity" (what they are made of) of stars in the outer edges of a galaxy, astronomers can identify distinct groups of stars that were born in a different galaxy and later "cannibalized."

The Local Group is our home "neighborhood." Studying the interactions between the Milky Way, Andromeda, and their dozens of dwarf satellites allows us to observe the laws of gravity and motion in action on a cosmic scale.

Small dwarf stars can live for over 10 billion years. Because they were born early in the universe's history, their atmospheres contain the original "recipe" of elements (mostly hydrogen and helium) created during the Big Bang.

As galaxies collide, their gas clouds are compressed by gravity and shockwaves. This compression causes the gas to collapse into thousands of new stars at once, creating a "starburst" galaxy that glows brightly in infrared light.

The Sagittarius Dwarf is currently passing through the disk of the Milky Way. Its stars are being stripped away, contributing to the "halo" of stars that surrounds our galaxy and providing a real-time look at galactic cannibalism.

By observing how the remains of dwarf galaxies are scattered, scientists can "weigh" our galaxy and map the invisible dark matter halo that surrounds us, helping to verify our current models of how the universe is structured.

When two spirals like the Milky Way and Andromeda merge, the organized rotation of their disks is destroyed by gravity, resulting in a randomized, "blob-like" elliptical galaxy with very little gas left for new star formation.

Tidal forces are responsible for stretching a sphere-shaped dwarf galaxy into a long stellar stream. This is the same physics that causes tides on Earth, but on a massive, galactic scale.

Some dwarf galaxies are "fossils" because they contain the same chemical ratios of hydrogen and helium predicted by Big Bang nucleosynthesis, having avoided the star-birth cycles and mergers that change the composition of larger galaxies.

Galactic cannibalism is a primary driver of galaxy growth. According to the Big Bang model, structure forms "bottom-up," where small clumps of matter and dwarf galaxies merge over billions of years to create the massive spiral and elliptical galaxies we see today.

The hierarchical model suggests that large-scale structures were built from the merger of smaller ones. Observing this process provides evidence of how the universe evolved from smooth gas to complex galaxies.

By analyzing the Doppler shift in the light from stars within these streams, scientists can calculate their three-dimensional motion. This allows them to map out the "crime scene" of a past galactic merger and calculate the mass of the host galaxy.

Dark matter halos are much larger and more massive than the visible parts of galaxies. These massive "dark wells" are what first interact and collide, dragging the visible stars and gas along with them into the merger process.

In about 4 billion years, the two largest members of the Local Group will collide. This is the ultimate example of galactic interaction, driven by their mutual gravitational attraction overriding the general expansion of the universe.

As a dwarf galaxy falls into a larger one, it plows through the hot gas of the host's halo. This "ram pressure" strips away the dwarf's cold gas, stopping all star formation. This explains why most satellite galaxies are "red and dead" (composed only of old stars).

Astronomers find groups of stars moving together that have different chemical "fingerprints" than the rest of the Milky Way. These "alien" stars are the remains of devoured dwarf galaxies, providing empirical evidence of the galaxy's evolutionary history.

As a dwarf galaxy moves through the dark matter halo of a larger galaxy, its gravity creates a "wake" behind it. This wake pulls back on the dwarf galaxy, slowing it down and causing its orbit to spiral inward until it is eventually merged with the host.

As a larger galaxy’s gravity pulls on a dwarf galaxy, it strips away its stars, stretching them into long, thin arches called stellar streams. These streams act as "fossils" that allow astronomers to reconstruct the history of how our galaxy was built.

Galactic cannibalism is an ongoing process. The Milky Way is currently consuming several small dwarf galaxies, such as the Sagittarius Dwarf Spheroidal. This constant interaction provides evidence for the dynamic, evolving nature of matter distribution in the universe.