Nebule are large masses of dust and gas inside a galaxy. They become visable when light shines off it, or the gas glows. New stars are created inside nebulas when the gasses contract, and gravity starts pulling the nebula in. Supernova remanents are the remains of a star after a supernova. These nebule are usually wispy. Planetary nebulas are the outer layers of a dying star that have been blown off the core. Reflection nebula reflect light, causing stars in front of it to appear brighter.Nebula can aslo obscure light from stars behind it, and can also glow. When emission nebula is stimulated by radiation from young stars, the gasses glow.
Some famous nebulas include the Horsehead Nebula, Orion Nebula, and the Helix Nebula.