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The nebulae we see in the night sky are forming new stars.

The stars are eating up the nebulae and there is no obvious process in which those nebulae are being created to compensate for that.

Shouldn't the universe have run out of those nebulae a long time ago?

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I'm sorry but there are inconsistencies in your question: First: Nebulae are not responsible for creation of stars. Nebulae are created when the light is reprocessed in the interstellar environment. Second: The nebulae that we see in the sky are of different types and they are not all associated with the sites of star formation. Please check the en.wikipedia.org/wiki/Nebula for more information especially the different types of Nebulae. –  Tigran Khanzadyan Jul 17 '11 at 16:32
@Tigran - the wikipedia link you reference does explain, in the first paragraph, that indeed some nebulae are in the process of creating stars. –  Rory Alsop Jul 17 '11 at 19:20
@Rory - In contrary to that, the Nebulae are not forming stars they do reflect or reprocess the light coming from the stars. The Nebulae which are referred are Diffuse or Reflection ones. Often Nebulae are used to pin-point the star formation since they "show" the dense interstellar matter. You do not create a nebula in sense of a new entity like a star, but in mots cases it is simply "lights-up the room" full of dense matter from which stars form. –  Tigran Khanzadyan Jul 17 '11 at 19:55
@Tigran - to quote the link you posted: In these regions the formations of gas, dust, and other materials "clump" together to form larger masses, which attract further matter, and eventually will become massive enough to form stars. Obviously Wikipedia is not the Oracle, but it seems to hold up. –  Rory Alsop Jul 17 '11 at 23:04
@Rory Your quote of Wikipedia is correct in principle. Those regions you mention are molecular clouds which are reflected by light from nearby stars. The difference might seam to be subtle but it is substantial in nature. Stars are forming in molecular clouds by gravitational collapse into a denser clump. Nebula is just a light which is reflected or reprocessed by the wall or dense area. Nebula does show where the molecular cloud is but to say Nebula forms star would be inconsistent with the real process of star formation. –  Tigran Khanzadyan Jul 18 '11 at 5:59
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There is a well known process for taking mass away from nebulae (star formation) but there are many processes for putting mass back into nebulae as well. Stellar winds (which are very strong for extremely young, old, or massive stars) and supernovae being the most prominent examples. Star formation is not a very efficient process, a lot of the mass of a collapsing proto-stellar nebula does not end up in a star, it goes back into interstellar space where later it may become part of another nebula and participate in the formation of another start. Also, the amount of gas and dust in a galaxy is very significant, around 10-15% of the mass of all stars within our own galaxy, for example. Between that and the processes that distribute mass back to the interstellar medium star formation can continue for a very, very long time.

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