Molecular cloud

Molecular clouds are interstellar gas clouds, the size, density and temperature enables the formation of molecules.

A main component of such a cloud is molecular hydrogen (H2), according to the frequency of the elements. This is very difficult to observe, so its frequency only using other molecules is often determined. The easiest with observed molecule is carbon monoxide (CO ), whose spectral lines can be detected with radio telescopes. The frequency of other molecules, however, is at least a factor of 1000 lower than the frequency of H2. When the cloud is dense enough, many types of molecules can form, to complex amino acids. A certain density is necessary in order to protect the molecules against radiation, as this would destroy them again. Very dense and cold molecular clouds are known as dark clouds. These are as it were the birthplaces of stars.

From the mid- 1960s, a variety of molecules in the interstellar medium was detected with radio telescopes, including the hydroxyl radical (OH ) and cyan (CN). These molecules emit radiation in the millimeter wavelength range. Meanwhile, more than 150 different molecules have been detected in molecular clouds, such as Water (H2O ), but also toxic substances such as hydrogen cyanide ( HCN) and even alcohol. The carbon monoxide is the most important molecule for the study of molecular clouds because you can due to the CO/H2-Verhältnisses determine the mass of such a cloud. More than half of the ( baryonic ) mass of the Milky Way is in the molecular clouds.

GMCs

Very large molecular clouds are giant molecular clouds (GMC - Giant Molecular clouds ) called. The mass of these GMCs is about 104-107 solar masses (M ☉ ) and they have a typical dimension of 50 to several hundred parsecs. About 80 % of molecular hydrogen in the Milky Way can be found in GMCs. The star formation in the Milky Way and nearby galaxies takes place almost exclusively in GMCs. The majority of GMCs in our Galaxy is located in the spiral arms. It is assumed that GMCs form by gravitational instabilities of even greater molecular gas reservoirs in the spiral arms, known as the GMA ( Giant Molecular Associations).

Parts of such a molecular cloud collapse by self-gravity to form new stars so continuously, often in larger groupings, called clusters. The most famous of them is likely to be the Orion molecular cloud, whose visible part is the Orion Nebula and the dark cloud of the most famous Horsehead Nebula. As we look into our own galaxy directly into the galactic plane, and part of the GMCs ( just in front of and behind the galactic core ) are not observable or determinable, in their removal, one must rely on observations of nearby galaxies for a better understanding of GMCs. Observations of GMCs in the Large Magellanic Cloud suggest a life cycle for GMCs. Due to the distance and lack of sensitivity to luminosity, only the formation of O- stars can be observed. During the first 6 million years after their formation, there is no star formation in the GMC, takes place in the following 13 million years, the first signs of star formation - H II regions are forming. In the last 7 million years can be observed star clusters with very young stars. The generated radiation field of young stars destroys the entire GMC during this final phase. While in the Large Magellanic Cloud and M33 in approximately 25-33 % of all GMCs still shows no signs of massive star formation, there are in the Milky Way very few GMCs without massive star formation.