Elephant's Trunk (APOD 18 Oct 2007)

[brbear1]

This nebula, like others I have seen here, has light emanating from the fringes of the dust-gas cloud. I have assumed that that light comes from stars that are hidden by the nebula but their light shines at the edges of the cloud. My question is, can light be emitted from a celestial body that has not coalesced into a star?

[Chris Peterson]

This nebula, like others I have seen here, has light emanating from the fringes of the dust-gas cloud. I have assumed that that light comes from stars that are hidden by the nebula but their light shines at the edges of the cloud. My question is, can light be emitted from a celestial body that has not coalesced into a star?

Yes. Most of the light you are seeing from this object is emitted. The radiation from stars stimulates gases in the nebula, which re-radiate at wavelengths that depend on the elements involved. The filters used for this image were tuned to hydrogen, oxygen, and sulfur. This is an emission nebula, which distinguishes it from a reflection nebula which glows only by reflecting starlight. Most of the colorful nebulas you see on APOD and elsewhere are emission nebulas, glowing by their own light.

[brbear1]

Thanks, Chris, for the explanation. I thought light could only be generated by stars as they undergo nuclear fusion.

[iamlucky13]

Electromagnetic radiation (including light) is emitted at some rate by any object warmer than absolute zero. The intensity of the emission depends on the temperature. Interstellar gas is generally very faint, but if it's warmed by a nearby star, it can be brighter, which we often see when a star is hidden behind the gas cloud (so light from the star can't reflect off the gas).

There is much, much more to it, but that's a start. Also, it tends to be specific wavelengths, which is why they talk about the colors assigned to hydrogen, sulfure, and oxygen colors. More about it:

http://csep10.phys.utk.edu/astr162/lect ... ption.html