Dark Nebulas are nebulas because they are made of gas and dust just like all the other nebulas. They stand out because they are dark, blocking the light from objects behind them such as the Horse Head. Dark Nebulas can also be seen standing out from a large area of bright stars. Another name for them is Absorption Nebula as they seem to absorb the light and not let it out.
One of the better known dark nebulas is the Horsehead Nebula in the constellation of Orion, the Hunter. Horsehead Nebula just refers to the dark patch in the middle rather than the whole thing. The whole thing is referred to as The Great Orion Molecular cloud.
The Dark Nebula is often an area with in another much larger area as mentioned above with the Horsehead Nebula. They may seem like void where there is nothing in space but in Dark Nebulas, star formation can occur. Oregon Uni
Some such as the Coalsack Nebula are visible with the naked eye. The Coalsack Nebula is located in Crux constellation in the southern hemisphere, too far south for those upper northern hemisphere to see.
The mass of these Dark Nebulas can contain over a million solar masses. These clouds have no standard shape and can extend to a region of at least 200 parsecs or over 650 Light Years. Temperatures in Dark Nebulas are cold, they are not far above absolute zero, the coldest ever temperature. Their temperature ranges from 10 to 100 Kelvin give or take. The Celsius equivalent of 100 Kelvin is -173.15 so it pretty cold. Swinburne
Smaller Dark Nebulas are called Bok Globules and are only about 3 light years across and have far less matter. 3 Light Years is 3/4 the distance between the Sun and Proxima Centauri, our nearest star.
The matter in side these Dark Nebula can over time contract as gravity pulls them together. The Dark Nebula can lighten up over time when the forming star turns into a protostar, the first step in becoming a fully fledged star. It is quite possible that billions of years ago, the area we are in today was once a Dark Nebula but an event caused the matter to contract.
The opposite are not known as Light Nebula but are referred to as Reflective Nebula, planetary Nebula's etc as they are visible and calculated by the absence of light. Dark Nebulas are not related to Dark Energy or Matter. We know and can see Dark Nebula's, we can't see Dark Matter or know it is exists.
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