Bok globule

In astronomy, Bok globules are isolated and relatively small dark nebulae, containing dense cosmic dust and gas from which star formation may take 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 m3).[2] They contain molecular hydrogen (H2), carbon oxides and helium, and around 1% (by mass) 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 an article published in 1947, he and Edith 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 provided evidence for the infall of material onto an accreting protostar.[8] It is now thought that a typical Bok globule contains about 10 solar masses of material in a region about a light-year or so across, and that Bok globules most commonly result in the formation of double- or multiple-star systems.[5][9][10]

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 irradiated by ultraviolet light from hot nearby stars exhibit stripping of materials to produce a tail. These types are called "cometary globules" (CG).[11]

Image gallery

Bok globules in IC2944
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
NGC 281HSTFull
Bok globules located within the NGC 281 nebula (IC 1590 cluster)
Barnard 68
Barnard 68, at a distance of only 410 light-years, is one of the nearest Bok globules. Its size is about 12,500 AU (0.198 ly) (≈ 2 trillion km)

See also


  1. ^ Michael Szpir (May–June 2001). "Bart Bok's Black Blobs". American Scientist. 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, João 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.; Sargent, A. I.; Henning, T.; Zylka, R.; Zinnecker, H. (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. Bibcode:2000IAUS..200P.103L.
  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. ^ a b Yun, João Lin; Clemens, Dan P. (December 20, 1990). "Star formation in small globules – Bart Bok was correct". Astrophysical Journal 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 (1): 225. Bibcode:1992A&A...256..225R.
  7. ^ Yun, João Lin; Clemens, Dan P. (January 1992). "Discovery of outflows from young stellar objects in BOK globules". Astrophysical Journal 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 (1): 232. Bibcode:1993ApJ...404..232Z. doi:10.1086/172271.
  9. ^ 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. 75: 877–904. Bibcode:1991ApJS...75..877C. doi:10.1086/191552.
  10. ^ Launhardt, R.; Sargent, A. I.; Henning, T.; Zylka, R.; Zinnecker, H. (2000). "Binary and multiple star formation in Bok globules". In Reipurth, B.; Zinnecker, H. (eds.). Birth and Evolution of Binary Stars, Poster Proceedings of IAU Symposium No. 200 on The Formation of Binary Stars. p. 103. Bibcode:2000IAUS..200P.103L.
  11. ^ Cometary globules. 1 Formation, evolution and morphology, B. Lefloch and B. Lazareff, 1994.

External links

Barnard 68

Barnard 68 is a molecular cloud, dark absorption nebula or Bok globule, towards the southern constellation Ophiuchus and well within our own galaxy at a distance of about 400 light-years, so close that not a single star can be seen between it and the Sun. American astronomer Edward Emerson Barnard added this nebula to his catalog of dark nebulae in 1919. His catalog was published in 1927, at which stage it included some 350 objects. Because of its opacity, its interior is extremely cold, its temperature being about 16 K (−257 °C). Its mass is about twice that of the Sun and it measures about half a light-year across.

Bright giant

The luminosity class II in the Yerkes spectral classification is given to bright giants. These are stars which straddle the boundary between ordinary giants and supergiants, based on the appearance of their spectra.

CN star

A CN star is a star with strong cyanogen bands in its spectrum. Cyanogen is a simple molecule of one carbon atom and one nitrogen atom, with absorption bands around 388.9 and 421.6 nm. This group of stars was first noticed by Nancy G. Roman who called them 4150 stars.

Dark nebula

A dark nebula or absorption nebula is a type of interstellar cloud that is so dense that it obscures the light from objects behind it, such as background stars and emission or reflection nebulae. The extinction of the light is caused by interstellar dust grains located in the coldest, densest parts of larger molecular clouds. Clusters and large complexes of dark nebulae are associated with Giant Molecular Clouds. Isolated small dark nebulae are called Bok globules. Like other interstellar dust or material, things it obscures are only visible using radio waves in radio astronomy or infrared in infrared astronomy.

Dark clouds appear so because of sub-micrometre-sized dust particles, coated with frozen carbon monoxide and nitrogen, which effectively block the passage of light at visible wavelengths. Also present are molecular hydrogen, atomic helium, C18O (CO with oxygen as the 18O isotope), CS, NH3 (ammonia), H2CO (formaldehyde), c-C3H2 (cyclopropenylidene) and a molecular ion N2H+ (diazenylium), all of which are relatively transparent. These clouds are the spawning grounds of stars and planets, and understanding their development is essential to understanding star formation.The form of such dark clouds is very irregular: they have no clearly defined outer boundaries and sometimes take on convoluted serpentine shapes. The largest dark nebulae are visible to the naked eye, appearing as dark patches against the brighter background of the Milky Way like the Coalsack Nebula and the Great Rift. These naked-eye objects are sometimes known as dark cloud constellations and take on a variety of names.

In the inner outer molecular regions of dark nebulae, important events take place, such as the formation of stars and masers.

Finger of God (Carina)

The Finger of God, also known as God's Birdie, is a Bok globule located in the constellation Carina approximately 8,000 light-years (2,500 pc) from Earth. It is a mass of dust and gas approximately two light-years (0.6 pc) long which has broken off from the main body of the Carina Nebula. Light from nearby bright stars is expected to dissipate it within a few million years.While the Carina Nebula was first observed in 1752, dramatic changes of it were noted in 1841, and the Keyhole Nebula observed later that century, it was the Hubble Space Telescope that revealed details of the Finger in 1999.


Globule can refer to:

Bok globule, dark clouds of dense cosmic dust

Drop (liquid), small column of liquid

Antibubbles of liquid on top of a surface of liquid

Globule (CDN), content delivery network

Molten globule, protein state

HH 46/47

HH 46/47 is a complex of Herbig–Haro objects (HH objects), located around 450 parsecs (about 1,470 light-years) away in a Bok globule near the Gum nebula. Jets of partially ionized gas emerging from a young star produce visible shocks upon impact with the ambient medium. Discovered in 1977, it is one of the most studied HH objects and the first jet to be associated with young stars was found in HH 46/47. Four emission nebula, HH 46, HH 47A, HH 47C and HH 47D and a jet, HH 47B, have been identified in the complex. It also contains a mostly unipolar molecular outflow, and two large bow shocks on opposite sides of the source star. The overall size of the complex is about 3 parsecs (10 light years).

Infrared dark cloud

An infrared dark cloud (IRDC) is a cold, dense region of a giant molecular cloud. They can be seen in silhouette against the bright diffuse mid-infrared emission from the galactic plane.

Kelvin–Helmholtz mechanism

The Kelvin–Helmholtz mechanism is an astronomical process that occurs when the surface of a star or a planet cools. The cooling causes the pressure to drop, and the star or planet shrinks as a result. This compression, in turn, heats the core of the star/planet. This mechanism is evident on Jupiter and Saturn and on brown dwarfs whose central temperatures are not high enough to undergo nuclear fusion. It is estimated that Jupiter radiates more energy through this mechanism than it receives from the Sun, but Saturn might not. The latter process causes Jupiter to shrink at a rate of two centimetres each year.The mechanism was originally proposed by Kelvin and Helmholtz in the late nineteenth century to explain the source of energy of the Sun. By the mid-nineteenth century, conservation of energy had been accepted, and one consequence of this law of physics is that the Sun must have some energy source to continue to shine. Because nuclear reactions were unknown, the main candidate for the source of solar energy was gravitational contraction.

However, it soon was recognized by Sir Arthur Eddington and others that the total amount of energy available through this mechanism only allowed the Sun to shine for millions of years rather than the billions of years that the geological and biological evidence suggested for the age of the Earth. (Kelvin himself had argued that the Earth was millions, not billions, of years old.) The true source of the Sun's energy remained uncertain until the 1930s, when it was shown by Hans Bethe to be nuclear fusion.

Lambda Boötis star

A Lambda Boötis star is a type of peculiar star which has an unusually low abundance of iron peak elements in its surface layers. One possible explanation for this is that it is the result of accretion of metal-poor gas from a circumstellar disc, a second possibility is the accretion of material from a hot Jupiter suffering from mass loss. The prototype is Lambda Boötis.

Lead star

A lead star is a low-metallicity star with an overabundance of lead and bismuth as compared to other products of the S-process.

Photometric-standard star

Photometric-standard stars are a series of stars that have had their light output in various passbands of photometric system measured very carefully. Other objects can be observed using CCD cameras or photoelectric photometers connected to a telescope, and the flux, or amount of light received, can be compared to a photometric-standard star to determine the exact brightness, or stellar magnitude, of the object.A current set of photometric-standard stars for UBVRI photometry was published by Arlo U. Landolt in 1992 in the Astronomical Journal.


A protostar is a very young star that is still gathering mass from its parent molecular cloud. The protostellar phase is the earliest one in the process of stellar evolution. For a low mass star (i.e. that of the Sun or lower), it lasts about 500,000 years The phase begins when a molecular cloud fragment first collapses under the force of self-gravity and an opaque, pressure supported core forms inside the collapsing fragment. It ends when the infalling gas is depleted, leaving a pre-main-sequence star, which contracts to later become a main-sequence star at the onset of Hydrogen fusion.

Q star

A Q-Star, also known as a grey hole, is a hypothetical type of a compact, heavy neutron star with an exotic state of matter. The Q stands for a conserved particle number. A Q-Star may be mistaken for a stellar black hole.

Starfield (astronomy)

A starfield refers to a set of stars visible in an arbitrarily-sized field of view, usually in the context of some region of interest within the celestial sphere. For example: the starfield surrounding the stars Betelgeuse and Rigel could be defined as encompassing some or all of the Orion constellation.

Stellar atmosphere

The stellar atmosphere is the outer region of the volume of a star, lying above the stellar core, radiation zone and convection zone.

Supernova impostor

Supernova impostors are stellar explosions that appear at first to be a supernova but do not destroy their progenitor stars. As such, they are a class of extra-powerful novae. They are also known as Type V supernovae, Eta Carinae analogs, and giant eruptions of luminous blue variables (LBV).

Yellow giant

A yellow giant is a luminous giant star of low or intermediate mass (roughly 0.5–11 solar masses (M)) in a late phase of its stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature as low as 5,200-7500 K. The appearance of the yellow giant is from white to yellow, including the spectral types F and G. About 10.6 percent of all giant stars are yellow giants.

Young stellar object

Young stellar object (YSO) denotes a star in its early stage of evolution. This class consists of two groups of objects: protostars and pre-main-sequence stars.

Luminosity class
Star systems
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