A nebula is a vast cloud of gas and dust in space, often serving as a nursery for new stars. These celestial formations can be composed of hydrogen, helium, and other ionized gases. Nebulae can be categorized into different types, such as emission nebulae, which glow due to the ionization of their gas, and reflection nebulae, which shine by reflecting light from nearby stars. Some nebulae are remnants of dead stars, like supernova remnants, while others are regions where new stars are forming, known as stellar nurseries. Famous examples include the Orion Nebula and the Crab Nebula, each showcasing the beauty and complexity of these astronomical structures.