Nebula: The nebula shown above are accumulations of dust and gas within our own Milky Way Galaxy. There are bright nebula and dark nebula. Dark nebula don't emit or reflect light, they are composed of interstellar dust and are seen only because they obscure the light from stars and emission nebula behind them. Bright nebula do emit or reflect light and are divided into four types based upon how they shine; reflection, emission, planetary nebulae, and supernova remnants. Reflection nebula are composed of dust and gas and reflect the light of stars near then or embedded within them. Emission nebula are glowing clouds of interstellar gas, often a dozen or more light years in size, that glow due to ionization effect of massive and hot nearby stars.(Click on thumbnail to view full size image and details.)
Messier 16
Messier 20
Messier 17
Messier 27
Veil Nebula
Planetary Nebula: The term "planetary nebula" derives from earlier times when the true nature of these objects had not yet been discovered. Observers with early telescopes could clearly see that these objects had a physical size, they were not the pinpoints of light that are stars. When observed through a small telescope, planetary nebula more resembled the observable discs of the planets, and thus the name. Although we now know the true nature of these objects, the name has remained.
Planetary nebula are "evolved" stars. When "low mass" stars expend the hydrogen fuel at their cores they enter one or more "giant" phases where the outer layers expand and the star ejects up to 25% of its total mass. Planetary nebula can be composed of several shells of expanding gas. These expanding shells of stellar material are set aglow by the central star which has since become a very strong source of ultraviolet radiation. The central stars of planetary nebula are among the hottest stars known.
It is known that the shells of planetary expand of 20 to 30 kilometers per second and that the diameter of the typical planetary nebula is approximately one light year. At a constant speed of expansion it can be calculated that the shells of nearly all of the planetary nebula visible were created within the last 50,000 years.
