Astrophotography by Salvatore Grasso
   Nebulae are interstellar masses of ionized gasses and dust. There are different classes of Nebulae which can be split into four main categories; Emission, Reflection, Planetary, and Dark Nebulae. While all nebulae are made up of gas and dust, the processes that make them, are quite different.
   Emission Nebulae are glowing clouds of hot interstellar matter. The gas glows as a result of radiation from nearby hot young stars ionizing hydrogen gas. Since Hydrogen predominately glows red when ionized, Emission Nebulae are generally red in color and show up well in the Hydrogen Alpha Emission Line (656nm).
   Reflection Nebulae are named literally because they glow from scattered starlight on dust particles in dust clouds just off the line of sight between earth and the dust cloud. Reflection Nebulae appear to glow blue for the same reason the earth's daytime sky is blue; the wavelength blue light scatters back to earth more easily than other wavelengths.
   Planetary Nebulae are the remains of Sun like stars after they have completed their life cycles. Dying stars become red giants in their last stage of life. When not massive enough to become supernovae, they explode creating a planetary nebula and leave a white dwarf star at their core.
   Dark Nebulae are cold masses of dense interstellar dust. They are much colder than their surroundings which causes them to obscure regions behind them. Also the dust clouds obscure wavelengths of light passing through them. Wavelengths are red-shifted, which is why stars at the edge of dark nebulae appear to be red in color.
Nebulae
Natural Color
Narrowband Color
