Bok globules


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Related to Bok globules: planetary nebula, Bipolar outflow, Herbig-Haro objects, T Tauri star

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.