What is a nebula?
The Milky Way is not just made of stars. The space between the stars has gas and dust, and in some places concentrations of this material can be found in clouds called nebulas. Some clouds that we see glowing are Emission Nebulae, glowing to fluorescence due the radiation of nearby stars. Some of those that glow may exhibit an obvious ring or circular shape, called Planetary Nebulae; these are the leftover remains of a dying star which has puffed away its outer atmosphere into a shell of gas. Other clouds, however, emit no light (Dark Nebulae) and can act as curtains hiding the more distant stars from our sight. In the summer Milky Way, an apparent dark lane can be seen dividing its faint band right down the middle. This is in fact enormous amounts of dark nebulosity hiding the thousands of stars beyond.
There are four basic types of nebulae: Emmission Nebulae Clouds of gas and dust which glow in the visible light wavelengths due to energy which is absorbed from nearby hot stars and then radiated as visible light. Reflection Nebulae Clouds of gas and dust which reflect visble light from nearby stars. Dark Nebulae Clouds of gas and dust which block the light from the stars beyond them, causing what appear to be black voids in the starfield. Planetary Nebulae Shock front phenomena which are caused by dying stars. A planetary nebula is a shell of expanding gas around a star that is close to the end of its life. Through the telescope it looks like a small green disc like a distant planet.
