Substellar objects are celestial bodies that are smaller than stars and do not undergo the nuclear fusion processes that define true stars. This category includes brown dwarfs, which are often referred to as "failed stars" because they have insufficient mass to sustain hydrogen fusion. They typically have masses between that of the largest planets and the smallest stars. Other types of substellar objects include planets, moons, and asteroids. These objects can vary widely in composition and size, and they orbit stars or exist in isolation in space. Substellar objects play a crucial role in our understanding of planetary formation and the diversity of celestial bodies in the universe.