Bok globules


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Bok globules

(bok) Small dark cool (10 K) clouds of gas and dust seen as near-circular objects against a background of stars or of an H II region; they are named after the American astronomer Bart J. Bok. They are believed to represent a late phase in the contraction of some dense cores when the material has become sufficiently dense to be opaque at optical wavelengths. The main sites for star formation are now known to be giant molecular clouds, but Bok globules give rise to some of the Galaxy's lower-mass stars. Most globules have diameters between 0.2 and 0.6 parsecs, and absorb between one and five magnitudes of light. Their mass varies from 20 to 200 solar masses. IRAS and other infrared telescopes have located protostars within some Bok globules; bipolar outflows from these produce Herbig–Haro objects seen at the globule's edge.
References in periodicals archive ?
It cannot come from the Einstein's mass-energy equivalence, considering the low gas temperature within the Bok globules.
The problem of correctly defining the source of the gravitational power heating the Bok globules remained unsolved in absence of a theory of the gravitational interaction able to specify the rate at which the gravitational waves hit the particles.
Assuming the [H.sub.2] molecules of the Bok globules, quick calculations can be made recognising that Eq.
The observed Bok globules denounced an inner hot core.
In fact the photon mean free path results [10.sup.11]-[10.sup.12] metres in the periphery of a cold large globule (R = [10.sup.15]) whereas takes a figure of [10.sup.2]-[10.sup.4] metres within the observed Bok globules (R = 2 x [10.sup.12]).
The most important event in the life of Bok globules is the ignition of the nuclear reactions which takes place when the inner core attains a temperature of the order of [10.sup.7] [degrees]K.
Bok globules and small molecular clouds: Deep IRAS Photometry and CO Spectroscopy.
where [T.sub.in] is the average temperature of the Bok globule at the initial stage t = 0.
"As a result," Hester explains, "the lifetimes of the EGGs subjected to photo-evaporation are fairly short as compared with most objects normally referred to as Bok globules." Photoevaporation in stellar nurseries is the process by which intense ultraviolet radiation from young, hot O- and B-type stars gradually blows away the surrounding placental clouds of gas and dust.
Bok globules have a fascinating human history that goes back to 1947, when Edith Reilly, a young technical assistant, walked into Bok's office at Harvard College Observatory and said that she wanted to work with him.
Close examination of the newly completed Palomar Observatory Sky Survey revealed more than 17,000 Bok globules, as these objects came to be known.
It was Fred Hoyle's 1957 science fiction tale, The Black Cloud, that really catapulted Bok globules into the public's awareness.