A nebula is an interstellar cloud of dust, helium, hydrogen, and other ionized gases. The term "nebula" was once used for any large astronomical object. The Andromeda Galaxy, for example, was once referred to as the Andromeda Nebula. Most nebulae are incredibly large, with some reaching millions of light years in diameter. Nebulae are most often star-forming regions. In these regions the formations of gas, dust, and other materials "clump" together and form denser regions which attract further matter, and eventually become dense enough to form stars. It is believed that the remaining matter forms planets and other planetary system objects. In many science fiction movies it is depicted that starships will hide in nebulae so thick that the visibility is basically 0, but this is actually not the case. In reality a nebula that is barely visible to the naked eye from Earth would appear larger, but no brighter from close by.
