Interstellar clouds, also known as molecular clouds, are vast regions in space filled with gas and dust. These clouds are primarily composed of hydrogen molecules, along with other elements and compounds. They play a crucial role in the formation of new stars and planetary systems, as they provide the necessary material for star formation. These clouds can vary in size and density, with some being dense enough to collapse under their own gravity, leading to the birth of stars. Nebulae are a type of interstellar cloud that can be observed from Earth, often glowing due to the light from nearby stars or reflecting starlight.