Galaxies are best described as which of the following?

• A. A single star
• B. Large groups of stars, dust, and gas
• C. Planets orbiting a star
• D. A region of space with dark matter only

Answer: (B)

Galaxies are large, gravitationally bound systems that contain billions of stars, vast clouds of gas and dust, and a substantial amount of dark matter that influences their motion. This makes the description of galaxies as large groups of stars, dust, and gas the best fit. A single star by itself isn’t a galaxy, and planets orbiting a star are part of a star system contained within a galaxy, not a galaxy itself. A region of space with dark matter only ignores the visible matter—stars, gas, and dust—that galaxies also contain. Dark matter helps govern the structure, but galaxies are defined by the presence of many stars and interstellar material, not by dark matter alone.