What is a nebula?

A nebula is a huge cloud of gas and dust in space. Nebulae can be places where new stars are born or remnants of dead stars. Some nebulae look bright and colorful because the gas glows or reflects light from nearby stars. Others may look dark because they block the light behind them. Nebulae can be very large—spanning many light-years across. They are important objects in astronomy because they help scientists learn about star formation, the life cycle of stars, and the makeup of the universe.