Bok globule

An image of Bok globules in the H II region IC 2944, taken with the WFPC2 instrument on the Hubble Space Telescope

A Bok globule is a dark cloud of dense dust and gas in which star formation sometimes takes place. Bok globules are found within H II regions, and typically have a mass of about 2 to 50 solar masses contained within a region about a light year or so across. They contain molecular hydrogen (H2), carbon oxides and helium, and around 1% (by mass) of silicate dust. Bok globules most commonly result in the formation of double or multiple star systems.

Bok globules were first observed by astronomer Bart Bok in the 1940s. In a paper published in 1947, Bok and E.F. Reilly hypothesised that these clouds were 'similar to insect's cocoons' that were undergoing gravitational collapse to form new stars from which stars and star clusters were born.This hypothesis was difficult to verify due to the observational difficulties of establishing what was happening inside a dense dark cloud that obscured all visible light emitted from within it. An analysis of near infrared observations published in 1990 confirmed that stars were being born inside Bok globules.Further observations have revealed that some Bok globules contain embedded warm sources,some contain Herbig-Haro objects,and some show outflows of molecular gas.Millimetre-wave emission line studies have also provided evidence for the infall of material onto an accreting protostar.

Bok globules are still a subject of intense research. Known to be some of the coldest objects in the natural universe (as cold as 8 kelvins) their structure and density remains somewhat a mystery. Methods applied so far have relied on column density derived from near infrared extinction and even star counting in a bid to probe these objects further.