A molecular cloud dense with hydrogen atoms can form molecules, including diatomic hydrogen, and is called a giant or small molecular cloud. Star formation occurs exclusively within these clouds, which are nicknamed “stellar nurseries”. For a molecular cloud to be a stellar nursery, it must have pockets of sufficient density, be subject to shaking forces, and be illuminated and ionized by radiation from a nearby massive star. Bok globules are very dense nuclei found in stellar nurseries that contain a variety of molecules not usually found in typical interstellar space. Eventually, the new stars created in these nurseries destroy them by absorbing or blowing away the local material.
A “stellar nursery” is a romantic way of referring to a molecular cloud in the process of forming new stars. A molecular cloud is a region of space sufficiently dense with hydrogen atoms that molecules, most commonly H2, or diatomic hydrogen, can form. Molecular clouds can be giant, 1000 to 100,000 times the mass of the Sun, or smaller, less than a few hundred times the mass of the Sun. These are called giant molecular clouds and small molecular clouds, respectively.
As far as we know, star formation occurs exclusively within these molecular clouds, hence the nickname “stellar nursery”. For a molecular cloud to be a stellar nursery, several conditions must be understood. First, the molecular cloud must have enough pockets of sufficient density (“molecular nuclei”) to provide the raw material to produce stars. Second, the molecular cloud must be subject to shaking forces, such as large stars or nearby supernovae. When a portion of a molecular cloud is illuminated and ionized by radiation from a nearby massive star, it is called the HII region.
Since HII regions are the portions of molecular clouds most agitated by external sources, they are the most likely place to be a stellar nursery. External influences are necessary to create a star, because otherwise, a critical density is rarely reached in a molecular cloud. If the density is not enough, the gas particles in the cloud continue to orbit each other forever. Due to an external influence, such as a supernova shock wave, molecular clouds can condense into localized regions, becoming so-called Bok globules.
Bok globules are very dense nuclei found in stellar nurseries. Typically, they contain about 10-50 solar masses of material in an area of about one light-year. Bok globules are notable in astronomy because they contain a variety of molecules not usually found in typical sparse interstellar space: molecular hydrogen, oxides of carbon, helium, and silicate dust. Sooner or later, many Bok globules are thought to collapse to form stars or, more frequently, binary star systems or star clusters. Our Sun is actually thought to be an anomaly in that it has no binary pair.
The star nurseries are eventually destroyed by the stars that create them. The new stars either absorb much of the local material or blow it away with the solar wind. Eventually, these newborn stars could go supernova, triggering the formation of more stars in nearby stellar nurseries.
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