It is basically a cloud of dust, helium, hydrogen, and other gases. The Andromeda Galaxy is referred to as the Andromeda Nebula (and spiral galaxies in general as "spiral nebulae"). Most nebulae are of vast size and they are hundreds of light-years in diameter. A nebula that is visible to the human eye from Earth would appear larger, but not brighter. The Orion Nebula, the brightest nebula in the sky and occupying an area twice the angular diameter of the Full Moon, can be viewed with the naked eye. Although denser than the space surrounding them, most nebulae are far less dense than any vacuum created on Earth. Many nebulae are visible due to fluorescence caused by embedded hot stars, while others are so diffused that they can be detected only with long exposures and special filters.