What describes a galaxy?

• A. A single planet with rings.
• B. A small group of comets.
• C. A star that emits much more light than others.
• D. A grouping of millions or billions of stars, gas, and dust held together by gravity.

Answer: (D)

Galaxies are large gravitationally bound systems of stars, gas, and dust. This is why the description of a grouping of millions or billions of stars, gas, and dust held together by gravity best fits what a galaxy is. A single planet with rings is just a planetary body, not a vast assembly of stars. A small group of comets is a tiny collection that doesn’t form the massive, star-filled structure of a galaxy. A star that emits much more light than others is simply a bright star, not a collection of many stars and interstellar material.