Bok globule

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 10–50 solar masses contained within a region about a light year or so across.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.] 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. [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.]

Bok globules were first observed by astronomer Bart Bok in the 1940s. In a paper published in 1947, Bok and E.F. Really 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. [Bok, B. J., Reilly, E. F. (1947). "Small Dark Nebulae", Astrophysical Journal, v.105, p.255.] 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. [Yun J.L., Clemens D.P. (1990). "Star formation in small globules – Bart Bok was correct", Astrophysical Journal, v.365, p.L73.] Further observations have revealed that some Bok globules contain embedded warm sources, some contain Herbig-Haro objects, [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.] and some show outflows of molecular gas. [Yun, J. L., & Clemens, D. P. (1992). "Discovery of outflows from young stellar objects in BOK globules", Astrophysical Journal, v.385, p.L21.] Millimetre-wave emission line studies have also provided evidence for the infall of material onto an accreting protostar. [Zhou, S., Evans, N. J., II, Kömpe, C., & Walmsley, C. M. (1993). "Evidence for protostellar collapse in B335", Astrophysical Journal, v.404, p.232.]

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.

ee also

* Molecular cloud
* Thackeray's Globules

References

External links

* [http://www.americanscientist.org/template/AssetDetail/assetid/14678 Article from "American Scientist"] (May-June 2001)
* [http://www.universetoday.com/am/publish/star_in_the_making.html A Star in the Making]