A nebula is a vast cloud of gas and dust in space, often serving as a nursery for new stars. These clouds can be composed of hydrogen, helium, and other ionized gases. Nebulae come in various types, including 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, such as planetary nebulae formed when a star sheds its outer layers. Others, like supernova remnants, are created from the explosive death of massive stars. Nebulae play a crucial role in the life cycle of stars and the evolution of galaxies.