Aninterstellar cloud is an accumulation ofgas,plasma, andcosmic dust ingalaxies. Put differently, an interstellar cloud is a denser-than-average region of theinterstellar medium, thematter and radiation that exists in thespace between thestar systems in a galaxy. Depending on thedensity,size, andtemperature of a given cloud, itshydrogen can be neutral, making anH I region; ionized, or plasma making it anH II region; or molecular, which are referred to simply asmolecular clouds, or sometime dense clouds. Neutral andionized clouds are sometimes also calleddiffuse clouds. An interstellar cloud is formed by the gas and dust particles from ared giant in its later life.
The chemical composition of interstellar clouds is determined by studyingelectromagnetic radiation that they emanate, and we receive – fromradio waves throughvisible light, across theelectromagnetic spectrum togamma rays. Largeradio telescopes scan the intensity in the sky of particularfrequencies of electromagnetic radiation, which are characteristic of certainmolecules'spectra. Some interstellar clouds are cold and tend to give out electromagnetic radiation of largewavelengths. 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 manyelements, whose spectra can be seen in visible andultraviolet 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.[2]
Until recently,[when?] 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,organic molecules were observed in the spectra thatscientists would not have expected to find under these conditions, such asformaldehyde,methanol, andvinyl alcohol. The reactions needed to create such substances are familiar to scientists only at the much higher temperatures and pressures of earth and earth-based laboratories. The fact that they were found indicates that thesechemical reactions in interstellar clouds take place faster than suspected, likely in gas-phase reactions unfamiliar to organic chemistry as observed on earth.[3] These reactions are studied in theCRESU experiment.
Interstellar clouds also provide a medium to study the presence and proportions of metals in space. The presence and ratios of these elements may help develop theories on the means of their production, especially when their proportions are inconsistent with those expected to arise from stars as a result offusion and thereby suggest alternate means, such ascosmic ray spallation.[4]
These interstellar clouds possess a velocity higher than can be explained by the rotation of theMilky Way.[5] By definition, these clouds must have a vlsr greater than 90 km s−1, where vlsr is the local standard rest velocity. They are detected primarily in the21 cm line of neutralhydrogen,[6] and typically have a lower portion of heavy elements than is normal for interstellar clouds in the Milky Way.
Theories intended to explain these unusual clouds include materials left over from the formation of the galaxy, ortidally-displacedmatter drawn away from other galaxies or members of theLocal Group. An example of the latter is theMagellanic Stream. To narrow down the origin of these clouds, a better understanding of their distances andmetallicity is needed.
High-velocity clouds are identified with an HVC prefix, as withHVC 127-41-330.
