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A molecular cloud—sometimes called a stellar nursery if star formation is occurring within—is a type of interstellar cloud of which the density and size permit absorption nebulae, the formation of molecules (most commonly molecular hydrogen, H 2), and the formation of H II regions.
By definition, these clouds must have a v lsr greater than 90 km s −1, where v lsr is the local standard rest velocity. They are detected primarily in the 21 cm line of neutral hydrogen, [6] and typically have a lower portion of heavy elements than is normal for interstellar clouds in the Milky Way.
Westerhout 51 nebula in Aquila - one of the largest star factories in the Milky Way (August 25, 2020). Star formation is the process by which dense regions within molecular clouds in interstellar space—sometimes referred to as "stellar nurseries" or "star-forming regions"—collapse and form stars. [1]
molecular cloud An interstellar cloud in which the prevailing physical conditions allow molecules to form, including molecular hydrogen. moment of inertia factor. Also normalized polar moment of inertia. A dimensionless quantity that characterizes the radial distribution of mass inside a planet or moon. moon See natural satellite. Moon
Examples of the former case are giant molecular clouds, the coldest, densest phase of interstellar gas, which can form by the cooling and condensation of more diffuse gas. Examples of the latter case are planetary nebulae formed from material shed by a star in late stages of its stellar evolution .
A total of as many as 159 distinct clouds have been identified overall, of which various characteristics such as density, size, and mass are known; in addition, there are seven large H II regions, three supernova remnants, 45 T Tauri stars, 18 molecular jets, and as many as 215 infrared radiation sources, coincident with young stellar objects ...
Observations and measurements of cosmic dust in different regions provide an important insight into the Universe's recycling processes; in the clouds of the diffuse interstellar medium, in molecular clouds, in the circumstellar dust of young stellar objects, and in planetary systems such as the Solar System, where astronomers consider dust as ...
The densest molecular clouds have significantly higher pressure than the interstellar average, since they are bound together by their own gravity. When stars form in such clouds, especially OB stars, they convert the surrounding gas into the warm ionized phase, a temperature increase of several hundred.