Bok globule

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An image of Thackeray's Globules, a set of Bok globules in the H II region IC 2944, taken with the WFPC2 instrument on the Hubble Space Telescope

Bok globules are dark clouds of dense cosmic 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[1] to 50 solar masses contained within a region about a light year or so across (about 4.5 × 1047, see Orders of magnitude (volume)).[2] 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.[3]

Bok globules were first observed by astronomer Bart Bok in the 1940s. In a paper published in 1947, Bok and E.F. Reilly hypothesized 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.[4] 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.[5] Further observations have revealed that some Bok globules contain embedded warm sources,[2] some contain Herbig–Haro objects,[6] and some show outflows of molecular gas.[7] Millimeter-wave emission line studies have also provided evidence for the infall of material onto an accreting protostar.[8]

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

Bok globules that are blasted by nearby stars exhibit fragmentation of materials to produce a tail. These types are called "cometary globules" (CG).[9]

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References[edit]

  1. ^ Michael Szpir (May–June 2001). "Bart Bok's Black Blobs". American Scientist. Archived from the original on 2003-06-29. Retrieved 2008-11-19. "Bok globules such as Barnard 68 are only about half a light-year across and weigh in at about two solar masses" 
  2. ^ a b Clemens, Dan P.; Yun, Joao Lin; Meyer, Mark H. (March 1991), BOK globules and small molecular clouds – Deep IRAS photometry and (C-12)O spectroscopy, Astrophysical Journal Supplement 75: 877, Bibcode:1991ApJS...75..877C, doi:10.1086/191552 
  3. ^ Launhardt, R.; A.I. Sargent, T. Henning, et al. (10–15 April 2000). "Binary and multiple star formation in Bok globules". "Birth and Evolution of Binary Stars, Poster Proceedings of IAU Symposium No. 200 on The Formation of Binary Stars". Potsdam, Germany: Bo Reipurth and Hans Zinnecker. p. 103. Retrieved 2010-03-29. 
  4. ^ Bok, Bart J.; Reilly, Edith F. (March 1947), Small Dark Nebulae, Astrophysical Journal 105: 255, Bibcode:1947ApJ...105..255B, doi:10.1086/144901 
  5. ^ Yun, Joao Lin; Clemens, Dan P. (December 1990), Star formation in small globules – Bart Bok was correct, Astrophysical Journal, Part 2 – Letters 365: L73, Bibcode:1990ApJ...365L..73Y, doi:10.1086/185891 
  6. ^ Reipurth, Bo; Heathcote, Steve; Vrba, Frederick (March 1992), Star formation in Bok globules and low-mass clouds. IV – Herbig-Haro objects in B335, Astronomy & Astrophysics 256: 225, Bibcode:1992A&A...256..225R 
  7. ^ Yun, Joao Lin; Clemens, Dan P. (January 1992), Discovery of outflows from young stellar objects in BOK globules, Astrophysical Journal, Part 2 – Letters 385: L21, Bibcode:1992ApJ...385L..21Y, doi:10.1086/186268 
  8. ^ Zhou, Shudong; Evans, Neal J., II; Koempe, Carsten; Walmsley, C.M. (March 1993), Evidence for protostellar collapse in B335, Astrophysical Journal, Part 1 404: 232, Bibcode:1993ApJ...404..232Z, doi:10.1086/172271 
  9. ^ Cometary globules. 1 Formation, evolution and morphology, B. Lefloch and B. Lazareff, 1994

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