What best defines a galaxy?

• A. A cluster of planets orbiting a star.
• B. A single star surrounded by interstellar dust.
• C. A collection of stars, dust, and gas bound together by gravity.
• D. A region of space with no matter.

Answer: (C)

Think about what defines a galaxy: it’s a huge, gravitationally bound system that contains many stars along with gas and dust (and often dark matter) all held together as one structure. The key idea is gravity binding a large collection of material, not just a single star or an empty region. That’s why the description of a collection of stars, dust, and gas bound together by gravity is the best match for a galaxy.

The other scenarios describe smaller or non-existent structures: a planetary system is planets orbiting a star, not a galaxy; a lone star with a dusty envelope isn’t a galaxy; and empty space has no matter at all, so it can’t be a galaxy.