The term for a large group of stars, gas, and dust held together by gravity is a galaxy. Galaxies can vary significantly in size and structure, containing billions to trillions of stars along with their planetary systems, stellar remnants, and interstellar matter. The Milky Way, for example, is the galaxy that contains our solar system, and it is just one of billions in the observable universe. Galaxies can be classified into several types, including spiral, elliptical, and irregular, based on their shape and formation processes. They play a crucial role in the study of cosmology and the universe’s structure and evolution.