What best defines a galaxy?

• A. A large system of gas, dust, dark matter, stellar remnants, and stars held together by gravity.
• B. A single star with its planets.
• C. A cluster of dust in interstellar space.
• D. A region of empty space with few stars.

Answer: (A)

A galaxy is a gravitationally bound system composed of gas, dust, dark matter, stellar remnants, and stars. Gravity holds all these components together on enormous scales, with dark matter forming a dominant halo that provides the deep gravitational well needed to keep billions of stars and vast clouds of gas and dust bound as a single structure. Inside this bound system, stars move in orbits around the center, and fresh stars can form from the gas clouds. This combination of many stars, interstellar material, and dark matter distinguishes a galaxy from smaller collections like a single star with its planets or from diffuse, empty regions of space.