A molecular cloud , sometimes also called a star cradle (if stars are born in it), is a type of interstellar cloud whose density and size allow molecules to form in it, usually hydrogen (H 2 ).
Molecular hydrogen is difficult to detect using infrared or radio surveillance, so another molecule, CO ( carbon monoxide ), is used to determine the presence of H 2 . The relationship between CO luminosity and H 2 mass is believed to remain constant, although there are reasons to doubt the veracity of this in some galaxies [1] [2] .
The significant size and mass of the molecular cloud leads to the effect of gravitational instability , due to which the density of matter inside the cloud becomes uneven. In areas with increased density, under certain conditions, the substance begins to converge. Rapprochement can acquire such force and speed that a gravitational collapse occurs, which may result in the formation of a new star [3] .
Observations
Within our galaxy, the amount of molecular gas is less than one percent of the volume of the interstellar medium . At the same time, it is its most dense component, including about half of the total gas mass within the galactic orbit of the Sun. Most of the molecular gas is contained in the molecular ring between 3.5 and 7.5 kilos parsecs from the center of the galaxy (the Sun is 8.5 kiloparsecs from the center). [four]
Large-scale carbon monoxide distribution maps in our galaxy show that the position of this gas correlates with its spiral arms. [5] The fact that molecular gas is located mainly in spiral arms is not consistent with the fact that molecular clouds must form and disintegrate in a short period of time - less than 10 million years - the time it takes for a substance to pass through the region of the sleeve. [6]
If we take a vertical section, the molecular gas occupies the narrow middle plane of the galactic disk with a characteristic height scale , Z , approximately 50–75 parsecs, much thinner than the warm atomic ( Z = 130–400 pc) and warm ionized ( Z = 1000 pc) gas components interstellar medium . [7] The H II regions are exceptions for ionized gas distribution, since they themselves are bubbles of hot ionized gas created in molecular clouds by intense radiation emitted by young massive stars and therefore have approximately the same vertical distribution as molecular gas.
This smooth molecular gas distribution is averaged over large distances, but the small-scale gas distribution is very irregular and for the most part it is concentrated in discrete clouds and cloud complexes. [four]
Molecular Cloud Types
Giant Molecular Clouds
Vast regions of molecular gas with masses of 10 4 —10 6 solar masses are called giant molecular clouds (GMOs). Clouds can reach tens of parsecs in diameter and have an average density of 10² — 10³ particles per cubic centimeter (the average density near the Sun is one particle per cubic centimeter). The substructure within these clouds consists of complex weaves of threads, sheets, bubbles, and irregular boulders. [6]
The densest parts of filaments and blocks are called “molecular nuclei”, and molecular nuclei with a maximum density (more than 10 4-10 6 particles per cubic centimeter), respectively, “dense molecular nuclei”. In observations, molecular nuclei are associated with carbon monoxide, and dense nuclei are associated with ammonia. The concentration of dust within the molecular nuclei is usually sufficient to absorb light from distant stars so that they look like dark nebulae . [eight]
GMOs are so huge that locally they can cover a significant part of the constellation, in connection with which they are referred to with reference to this constellation, for example, the Orion Cloud or the Taurus Cloud . These local GMOs line up around the sun, called the Gould Belt . [9] The most massive collection of molecular clouds in the galaxy, the Sagittarius B2 complex, forms a ring around the galactic center within a radius of 120 parsecs. The area of the Sagittarius constellation is rich in chemical elements and is often used by astronomers looking for new molecules in interstellar space as a sample. [ten]
Small Molecular Clouds
Isolated gravitationally bound small molecular clouds with masses of less than a few hundred solar masses are called the Bock globule. The densest parts of small molecular clouds are equivalent to the molecular nuclei found in giant molecular clouds and are often included in the same studies.
High Latitude Diffuse Molecular Clouds
In 1984, the IRAS identified a new type of diffuse molecular cloud. [11] They were diffuse fibrous clouds that are visible at high galactic latitude (looking out from the plane of the galactic disk). These clouds had a typical density of 30 particles per cubic centimeter. [12]
See also
- Cloud of orion