What is galaxy? Plain-English meaning

Short definition: A galaxy is a massive system of stars, gas, dust, and dark matter bound together by gravity.

Plain-English explanation

Galaxies are enormous collections of stars, along with gas and dust, all held together by gravity. They come in many shapes and sizes, from spiral ones like our Milky Way to elliptical and irregular forms. The stars within a galaxy orbit a common center, and the entire system moves through space as a unit. Galaxies can contain billions of stars, and they are the basic building blocks of the universe on a large scale.

Why it matters

Understanding galaxies helps you grasp the scale of the universe and where Earth fits in. When you look at the night sky, the stars you see are mostly part of our own galaxy. Learning about galaxies also connects to broader questions about how the universe formed and whether other solar systems exist.

Concrete example

The Milky Way is the galaxy that contains our Sun and the solar system. It is a spiral galaxy with billions of stars, and from Earth we see it as a faint band of light across the night sky.

Common confusion

A galaxy is often confused with a solar system. A solar system is a star and the planets and other objects orbiting it, while a galaxy contains billions of stars and their solar systems, along with gas and dust, all bound together by gravity.

Related terms

star, solar system, black hole, planet, universe

How people actually use it

Astronomers study galaxies to learn about star formation, black holes, and the large-scale distribution of matter. Observations of distant galaxies help measure the expansion of the universe and the nature of dark energy.

Related terms explained

Milky Way

The Milky Way is the galaxy that contains our solar system, with a barred spiral shape. It has hundreds of billions of stars and spans about 100,000 light-years in diameter.

Example: On a clear night, the Milky Way appears as a band of light across the sky, which is the combined glow of its many stars.

spiral galaxy

A spiral galaxy is a type of galaxy with a flat, rotating disk containing spiral arms, a central bulge, and a halo of stars. Spiral arms are regions of active star formation, often with prominent gas and dust.

Example: The Andromeda Galaxy is a spiral galaxy and the nearest large galaxy to the Milky Way.

black hole

A black hole is a region of spacetime where gravity is so strong that nothing, not even light, can escape. They form from the collapse of massive stars or can be supermassive at the centers of galaxies.

Example: The supermassive black hole at the center of the Milky Way is called Sagittarius A*.

light-year

A light-year is the distance light travels in one year, approximately 9.46 trillion kilometers (5.88 trillion miles). It is used to measure vast distances in space.

Example: The nearest star to our Sun, Proxima Centauri, is about 4.24 light-years away.

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More context

The study of galaxies is central to cosmology, as they are the building blocks of the universe's large-scale structure. Galaxies are not static; they rotate, collide, and merge over billions of years. The Milky Way is on a collision course with Andromeda, expected to merge in about 4.5 billion years, though the solar system is unlikely to be affected directly.

Additional background

Galaxies are the fundamental building blocks of the universe on large scales. The first galaxies formed about 13 billion years ago, only a few hundred million years after the Big Bang. The Milky Way is on a collision course with Andromeda, and they will merge in about 4.5 billion years, forming a giant elliptical galaxy. Astronomers use galaxy rotation curves to infer the presence of dark matter, which outweighs visible matter by a factor of about five. Galaxies also contain interstellar gas and dust, which are the raw materials for star formation. The study of galaxy formation and evolution is a major field in cosmology, with telescopes like Hubble and James Webb providing deep images of early galaxies. Understanding galaxies helps us trace the history of the universe from its birth to the present day.