What is the primary characteristic of a galaxy?

The primary characteristic of a galaxy is that it contains an enormous collection of stars. Galaxies are vast systems that not only include stars but also gas, dust, dark matter, and other astronomical objects bound together by gravity. The number of stars in a galaxy can range from millions to trillions, which showcases the scale and complexity of these structures.

The existence of numerous stars is what fundamentally defines a galaxy, as it differentiates them from smaller groupings of stars or individual star systems. While other factors, such as rotation and size, can vary among different types of galaxies, the sheer number of stars is what captures the essence of what a galaxy fundamentally is.