Editing Interstellar cloud (section)

==Chemical compositions==
The chemical composition of interstellar clouds is determined by studying [[electromagnetic radiation]] that they emanate, and we receive – from [[radio wave]]s through [[visible light]], to [[gamma rays]] on the [[electromagnetic spectrum]] – that we receive from them. Large [[radio telescope]]s scan the intensity in the sky of particular [[frequency|frequencies]] of electromagnetic radiation, which are characteristic of certain [[molecule]]s' [[electromagnetic spectroscopy|spectra]]. Some interstellar clouds are cold and tend to give out electromagnetic radiation of large [[wavelength]]s. A map of the abundance of these molecules can be made, enabling an understanding of the varying composition of the clouds. In hot clouds, there are often ions of many [[chemical element|elements]], whose spectra can be seen in visible and [[ultraviolet light]].

Radio telescopes can also scan over the frequencies from one point in the map, recording the intensities of each type of molecule. Peaks of frequencies mean that an abundance of that molecule or atom is present in the cloud. The height of the peak is proportional to the relative percentage that it makes up.<ref>{{Cite web |author=Project Leader Dr. Lochner |date=November 2009 |title=Spectra and What Scientists Can Learn From Them |url=http://imagine.gsfc.nasa.gov/docs/science/how_l1/spectra.html |archive-url=https://web.archive.org/web/20141109040237/http://imagine.gsfc.nasa.gov/docs/science/how_l1/spectra.html |archive-date=Nov 9, 2014 |access-date=12 February 2010 |publisher=Goddard Space Flight Center, NASA}}</ref>