Nebula. Optical image of a star-forming nebula. Nebular are interstellar clouds of dust, hydrogen, helium and other ionised gases. They are often star-forming regions where the formations of gas, dust, and other materials 'clump' together to form larger masses, which attract further matter, and eventually become massive enough to form stars. The remaining materials are then believed to form planets, and other planetary system objects. Imaged by NASA's Chandra X-ray Observatory.