What is a nebula in astronomical terms?

A nebula is accurately described as a star cloud, which is a vast region in space that is filled with gas and dust, primarily hydrogen and helium, along with other ionized gases. This material can serve as the birthplace of stars—when the gas and dust in a nebula condense under gravitational forces, they can form new stars. Furthermore, some nebulae, such as emission nebulae, are regions where new stars are currently being formed and emit light due to the hot gases. Other types, like planetary nebulae, are formed from the remnants of dying stars, showcasing a variety of beautiful shapes and colors that contribute to the intricate structures observed in the universe.

The other options refer to different astronomical concepts: a star explosion specifically pertains to events like supernovae, a star cluster involves a group of stars that are physically close to one another, and a star system generally refers to a system that includes one or more stars and their associated celestial bodies. Thus, the definition of a nebula as a star cloud captures its fundamental nature as a region rich in the building blocks of stars.