Ontario Grade 9 Science Class Practice Exam

A galaxy is indeed a huge group of single stars, star systems, star clusters, dust, and gas bound together by gravity. This definition encompasses the vast complexity and scale of galaxies, which can contain billions to trillions of stars along with various celestial objects and materials that compose them. The gravitational forces at play in a galaxy allow for the formation of structures such as spiral arms, bars, and other features, depending on the galaxy's type.

In contrast, a single star, while it is indeed a significant astronomical object, does not represent the collective nature or large scale that characterizes a galaxy. Similarly, a small group of stars lacks the extensive variety and quantity inherent in larger galaxies. A satellite of Earth, such as the Moon, refers specifically to an astronomical body that orbits a planet, rather than the vast structure of star systems and materials we find in a galaxy. Thus, the comprehensive nature of C accurately describes what galaxies are in the universe.