Diffuse
Nebulae
Diffuse nebulae are clouds of interstellar matter, namely
thin but widespread agglomerations of gas and dust. If they are large
and massive enough they are frequently places of star formation, thus
generating big associations or clusters of stars.
Some of the young stars are often very massive and so hot
that their high energy radiation can excite the gas of the nebula (mostly
hydrogene) to shine; such nebula is called EMISSION nebula. Emission nebulae are usually the sites of recent and ongoing
star formation (like M 42 Orion Nebula).
If the stars are not hot enough, their light is reflected
by the dust and can be seen as white or bluish - this is REFLECTIONnebula.
DARK nebulae are clouds of dust which are simply
blocking the light from stars behind. They are physically very similar
to reflection nebulae; they look different only because of the geometry
of the light source, the cloud and the Earth. Dark nebulae are also often
seen in conjunction with reflection and emission nebulae. A typical diffuse
nebula is a few hundred light-years across ( Horshead Nebula).
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