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Interstellar cloud is an all inclusive name given to an accumulation of gas, plasma, and dust that exists in multiple galaxies. It can also be referred to as a denser than normal area of the interstellar medium. These interstellar clouds can be classified by state of the hydrogen that it contains: clouds with neutral hydrogen are called HI regions, ionized hydrogen clouds are H II regions and usually contain plasma, molecular hydrogen clouds are called molecular. H I and H II are also called diffuse clouds while molecular clouds can be called dense clouds.
The composition of an interstellar cloud is analyzed by the electromagnetic radiation that it emits. Radio telescopes are used to measure the intensity of the frequencies and those frequencies tell scientists which molecules are present in a clouds spectra. 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.
According to old theories, 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. Recently it has been discovered that certain molecules were observed in the spectra that should not be found under these conditions: formaldehyde, methanol, and vinyl alcohol. The reactions needed to create such substances are at the much higher temperatures and pressures of Earth and Earth-based laboratories. Their presence indicates that the reactions in interstellar clouds take place faster than suspected, likely in gas-phase reactions unfamiliar to organic chemistry as observed on Earth.
Interstellar clouds also provide a means to study the presence and proportions of metals in space. The presence and ratios of these elements may help develop theories about their production since their proportions are not in line with what is expected to arise as a result of fusion and may suggest an alternate means, such as cosmic ray spallation.
It seems that the more time that is spent studying an interstellar cloud, the more questions that arise. Perhaps, once the reactions within a cloud are understood, we will be better able to control reactions on this planet.
We’ve also recorded an episode of Astronomy Cast about galaxies. Listen here, Episode 97: Galaxies.