Stars, Galaxies, and the Universe

Beyond our solar system lies a universe almost too big to imagine — billions of stars gathered into galaxies, all part of an expanding universe. This lesson zooms out from the Sun to the largest scales and explains the leading idea for how it all began.

Stars and Galaxies

A star is a giant ball of hot gas that produces its own light and heat through nuclear reactions in its core. Our Sun is an average star; it only looks special because it is close. Stars are grouped into galaxies — enormous collections of billions of stars held together by gravity. Our galaxy is the Milky Way. Galaxies themselves cluster together, and the whole collection of all galaxies makes up the universe.

The Scale of Things

The sizes involved are staggering. Distances between stars are so large that we measure them in light-years — the distance light travels in a year. Light from distant stars can take thousands or millions of years to reach us, which means we see those stars as they were long ago. This scale is why space questions often deal with time as well as distance.

The Big Bang

The leading scientific explanation for the universe’s origin is the Big Bang theory. It says the universe began about 13.8 billion years ago from an extremely hot, dense state and has been expanding ever since. A key piece of evidence is that distant galaxies are moving away from us, which means the universe is still growing. The Big Bang describes how the universe expanded, not an explosion in space but an expansion of space itself.

Watch: A Short Video Lesson

Edu Sphere walks through this skill clearly in a few minutes. It is a helpful companion to the reading above:

A Routine for These Questions

1. A star makes its own light; the Sun is an average star.
2. Galaxies are huge groups of stars; ours is the Milky Way.
3. Distances are measured in light-years; distant light is old light.
4. The Big Bang theory says the universe began hot and dense and is expanding.

Practice

1. What is a star?
2. What is a galaxy?
3. What is the name of our galaxy?
4. What is a light-year a measure of?
5. What does the Big Bang theory say about the universe?
6. What evidence suggests the universe is expanding?

Answers

1. A giant ball of hot gas that makes its own light and heat.
2. A huge collection of billions of stars held together by gravity.
3. The Milky Way.
4. Distance (how far light travels in a year).
5. It began hot and dense about 13.8 billion years ago and is expanding.
6. Distant galaxies are moving away from us.

Where This Fits in Your Science Prep

This zooms out from the solar system and connects to stellar life cycles, which describe how stars are born and die. See all topics on the Science Topics Hub.