Bok globule

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An image of Bok globules in the H II region IC 2944, taken with the WFPC2 instrument on the Hubble Space Telescope
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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 is sometimes taking place. Bok globules are found within H II regions, and typically have a mass of about 10–50 solar masses contained within a region about a light year or so across[1]. 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[2].

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[3]. 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[4]. Further observations have revealed that some Bok globules contain embedded warm sources[1], some contain Herbig-Haro objects[5], and some show outflows of molecular gas[6]. Millimetre-wave emission line studies have also provided evidence for the infall of material onto an accreting protostar[7].

Bok globules are still a subject of intense research. Known to be some of the coldest objects in the natural universe (as cold as 3 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.

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  1. Clemens D.P., Yun, J.L., Heyer M.H. (1991). Bok globules and small molecular clouds – Deep IRAS photometry and (C-12)O spectroscopy, Astrophysical Journal Supplement, v.75, p.877
  2. Launhardt R., Sargent A.I., Henning T., et al. (2002). Binary and multiple star formation in Bok globules, Proceedings of IAU Symposium No. 200 on The Formation of Binary Stars. Eds Reipurth & Zinnecker, p.103
  3. Bok, B. J., Reilly, E. F. (1947). Small Dark Nebulae, Astrophysical Journal, v.105, p.255
  4. Yun J.L., Clemens D.P. (1990). Star formation in small globules – Bart Bok was correct, Astrophysical Journal, v.365, p.L73
  5. Reipurth, B., Heathcote, S., & Vrba, F. (1992), Star formation in Bok globules and low-mass clouds. IV - Herbig-Haro objects in B335, Astronomy & Astrophysics, v.256, p.225
  6. Yun, J. L., & Clemens, D. P. (1992). Discovery of outflows from young stellar objects in BOK globules, Astrophysical Journal, v.385, p.L21
  7. Zhou, S., Evans, N. J., II, Kömpe, C., & Walmsley, C. M. (1993). Evidence for protostellar collapse in B335, Astrophysical Journal, v.404, p.232
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