The nebula is the common name for accumulations of gas , plasma and dust in our and other galaxies . In other words, the nebula has a higher density than the average density of the interstellar medium. Depending on the density, size, and temperature of a given cloud, the hydrogen in it can be neutral ( region HI ), ionized (i.e., in the form of a plasma) ( region H II ), or molecular ( molecular cloud ). Neutral and ionized clouds are sometimes also called diffuse clouds, while molecular clouds are called dense clouds.
Chemical Composition
The analysis of the composition of interstellar clouds is carried out by studying their electromagnetic radiation using large radio telescopes . By examining the emission spectrum of the nebula and comparing it with the spectrum of specific chemical elements , we can determine the chemical composition of the cloud.
Typically, about 70% of the mass of the nebula is hydrogen , the remainder is mainly helium . Traces of heavy elements are also present in the clouds: metals such as calcium, neutral or in the form of cations Ca + (90%) and Ca ++ (9%), and inorganic compounds such as water , carbon monoxide , hydrogen sulfide , ammonia and cyanide hydrogen .
Until recently, it was believed that due to the low temperature and density of clouds, the rate of chemical reactions in nebulae is very low, which leads to the appearance of a very small number of complex compounds. However, the study of the spectra showed the presence of complex organic molecules. Typically, the reactions necessary to create them occur only at significantly higher temperatures and pressures. The fact of their discovery shows that chemical reactions in nebulae occur faster than anticipated. These reactions are studied in the CRESU experiment. Among the molecules, numerous organic compounds are unexpectedly found, such as formaldehyde , formic acid , ethanol, and free radicals ( HO Β° , CN Β° ).