Wonderful world, wonderful universe
When we talk about space, we tend to think there is nothing, dark quiet vacuum. In fact, this is not entirely right. Between the stars may be a vast expanse of space are quiet, but far from being a truly "vacuum", but the existence of various substances. These substances include the interstellar gas, dust and particles, etc., people put them called "interstellar matter."
Interstellar matter and the evolution of celestial bodies are closely linked. Observation confirmed that the interstellar gas primarily hydrogen and helium by two elements, which is the composition of the star are the same. People even suspect that the stars are from the interstellar gas, "condensation" formed. Interstellar dust are a number of very small solid-state material, composition including carbon compounds, oxides and so on.
Interstellar matter in space is not evenly distributed. Gravitational effects at certain places of gas and dust may attract each other and intensive, so as to form Cloud Like. It is the image of them called the "Nebula." In accordance with the form, the Milky Way in the nebula can be divided into diffuse nebulae, planetary nebulae, such as several.
Diffuse nebula, as its name, as no clear boundaries, usually irregular shape. Their scores of light years in diameter, about the average density of atoms per cubic centimeter of 10-100 (in fact than the laboratory vacuum be much lower). They are mainly distributed in the galactic plane (HOTKEY) nearby. Comparison of the diffuse nebulae have a well-known Orion Nebula, Horsehead Nebula, etc..
Planetary nebulae look a bit like a spit of the smoke ring, the center is empty, and often have a very bright star. Stars continuously outward ejections of material to form a nebula. This shows that planetary nebulae are the result of the evolution of stars in their twilight years. There is relatively well-known constellation Aquarius antihelix and Lyra Nebula Ring Nebula.
Photo credits: Xjbxjhxm.