Interstellar cloud is the generic name given to accumulations of gas and dust in our galaxy. Depending on the density, size and temperature of a given cloud, the hydrogen in it can be neutral (H I regions), ionized, (H II regions), or molecular (molecular clouds). When insufficiently dense to be called a cloud, it may be referred to as interstellar dust or simply dust. Dust in an astronomical context typically has a different composition from the household meaning of dust.
Over 200 newly formed stars are scattered within a cavern-like, gaseous, interstellar cloud (NGC 604). The stars irradiate the gas with energetic ultraviolet light stripping electrons from atoms and exciting them - producing a characteristic nebular glow.
Chemical compositions
Analysing the composition of interstellar clouds is achieved by studying electromagnetic radiation that we receive from them. Large radio telescopes scan the intensity in the sky of particular frequencies of electromagnetic radiation which are characteristic of certain molecules' spectra. Some interstellar clouds are cold and tend to give out EM radiation of large wavelengths. We can produce a map of the abundance of these molecules to produce an understanding of the varying composition of the clouds. In hot clouds, there are often ions of many 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.
Unexpected chemicals detected in interstellar clouds
Until recently the rates of reactions in interstellar clouds were expected to be very slow, with minimal products being produced due to the low temperature and density of the clouds. However, large organic molecules were observed in the spectra that scientists would not have expected to find under these conditions. The reactions needed to create them normally only occur at much higher temperatures and pressures. The fact that they were found indicates that these chemical reactions in interstellar clouds take place faster than suspected. These reactions are studied in the CRESU experiment.
The dust particles, that form only a small part of the cloud, consist mainly of silicates (sand like matter) and graphite which are often surrounded by a small layer of ice.
The dark interstellar clouds are the most dense and are cold concentrations of interstellar gas.
The interstellar material forms a dynamic reservoir from which stars are formed and to which they return their remnants.
Most of the dust in interstellar space comes from stars that have moved off the main sequence and entered the red giant phase of their evolution (see stars, evolution).
Dust grains that have drifted into the general interstellar medium find their way into denser clouds and, eventually, into molecular clouds where the density is sufficiently high for more complex organic synthesis to take place (see interstellar molecules).
Interstellardust particles strongly absorb, scatter, and polarize visible light at wavelengths comparable to their size, reemitting the light in the far-infrared region of the spectrum.
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