Which of the following best defines a galaxy?

• A. A galaxy is a large system containing stars, dust, and gas
• B. A galaxy is a collection of planets and moons
• C. A galaxy is a single star with its surrounding planets
• D. A galaxy is a cloud of gas with no stars

Answer: (A)

Galaxies are enormous systems bound by gravity that include billions or trillions of stars, plus clouds of gas and dust. This combination of many stars with interstellar material allows galaxies to form new stars and display a variety of shapes, from spirals to ellipticals. That scale and composition distinguish them from other cosmic structures: a planetary system is just a star and its orbiting planets, a single star with its planets is a star system, and a cloud of gas with no stars is a nebula. So the description that fits best is a large system containing stars, dust, and gas.