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  Nebula.jpg -  Ordinary matter, the stuff made of protons, neutrons and electrons, has an average density throughout the universe of just 1 atom per 4 cubic meters. A bit more than 73% of this, by mass, is hydrogen, 25% helium, with all the remaining elements accounting for less than 2%. Surprisingly just 10% of this is contained in stars and planets. The rest is in the form of gas or small dust particles in interstellar space. It's the stuff responsible for the cloud like figures we see in emission, reflection and dark nebula.  Emission nebula are heated by a nearby star sufficiently to ionize the gas causing it to glow. Since most of the gas is hydrogen, the primary color is red although green and blue are possible if the gas is heated sufficiently. There are many stunning emission nebula including the Lagoon, Eagle, North American, and Rosette nebula. The Great Orion nebula is probably the most widely recognized.  Reflection nebula also receive light from a nearby star but not enough to make the gas glow. Rather the light is reflected off small dust particles. The color is mostly blue for the same reason our sky is blue - blue light scatters more easily than red. The nebula surrounding the stars of the Pleiades open cluster is perhaps the most widely recognized reflection nebula.  Dark nebula don't receive enough light to either glow or reflect light - they are made of dust grains that block light from background stars or emission or reflection nebula. The Horsehead and nearby Flame nebula are beautiful examples of dark nebula  
Flame - Horsehead
Horsehead
Elephant Trunk
IC 63
IC 0410

Ordinary matter, the stuff made of protons, neutrons and electrons, has an average density throughout the universe of just 1 atom per 4 cubic meters. A bit more than 73% of this, by mass, is hydrogen, 25% helium, with all the remaining elements accounting for less than 2%. Surprisingly just 10% of this is contained in stars and planets. The rest is in the form of gas or small dust particles in interstellar space. It's the stuff responsible for the cloud like figures we see in emission, reflection and dark nebula.

Emission nebula are heated by a nearby star sufficiently to ionize the gas causing it to glow. Since most of the gas is hydrogen, the primary color is red although green and blue are possible if the gas is heated sufficiently. There are many stunning emission nebula including the Lagoon, Eagle, North American, and Rosette nebula. The Great Orion nebula is probably the most widely recognized.

Reflection nebula also receive light from a nearby star but not enough to make the gas glow. Rather the light is reflected off small dust particles. The color is mostly blue for the same reason our sky is blue - blue light scatters more easily than red. The nebula surrounding the stars of the Pleiades open cluster is perhaps the most widely recognized reflection nebula.

Dark nebula don't receive enough light to either glow or reflect light - they are made of dust grains that block light from background stars or emission or reflection nebula. The Horsehead and nearby Flame nebula are beautiful examples of dark nebula

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