astronomy Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, g ...

, Bok globules are isolated and relatively small

dark nebula A dark nebula or absorption nebula is a type of interstellar cloud, particularly molecular clouds, that is so dense that it obscures the visible wavelengths of light from objects behind it, such as background stars and emission or reflection neb ...

e, containing dense

cosmic dust Cosmic dust, also called extraterrestrial dust, star dust or space dust, is dust which exists in outer space, or has fallen on Earth. Most cosmic dust particles measure between a few molecules and 0.1 mm (100 micrometers). Larger particles are c ...

and gas from which

star formation Star formation is the process by which dense regions within molecular clouds in The "medium" is present further soon.-->interstellar space

may take place. Bok globules are found within

H II region An H II region or HII region is a region of interstellar atomic hydrogen that is ionized. It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds ...

s, and typically have a

mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different ele ...

of about 2 to 50

solar mass The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass ...

es contained within a region about a

light year A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...

or so across (about ). They contain molecular hydrogen (H₂),

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...

oxides and

helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic ta ...

, and around 1% (by mass) silicate dust. Bok globules most commonly result in the formation of

double A double is a look-alike or doppelgänger; one person or being that resembles another. Double, The Double or Dubble may also refer to: Film and television * Double (filmmaking), someone who substitutes for the credited actor of a character * ...

- or multiple-star systems.

History

Bok globules were first observed by astronomer Bart Bok in the 1940s. In an article published in 1947, he and

Edith Reilly Edith is a feminine given name derived from the Old English words ēad, meaning 'riches or blessed', and is in common usage in this form in English, German, many Scandinavian languages and Dutch. Its French form is Édith. Contractions and var ...

hypothesized that these clouds were "similar to insect's cocoons" that were undergoing

gravitational collapse Gravitational collapse is the contraction of an astronomical object due to the influence of its own gravity, which tends to draw matter inward toward the center of gravity. Gravitational collapse is a fundamental mechanism for structure formatio ...

to form new stars, from which

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

s and

star cluster Star clusters are large groups of stars. Two main types of star clusters can be distinguished: globular clusters are tight groups of ten thousand to millions of old stars which are gravitationally bound, while open clusters are more loosely cl ...

s 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 Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of Light, visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from ...

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 object Herbig–Haro (HH) objects are bright patches of nebulosity associated with newborn stars. They are formed when narrow jets of partially ionised gas ejected by stars collide with nearby clouds of gas and dust at several hundred kilometres per s ...

s, and some show outflows of molecular gas. Millimeter-wave

emission line A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to identi ...

studies have provided evidence for the infall of material onto an accreting

protostar 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 50 ...

. It is now thought that a typical Bok globule contains about 10

es 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. 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 Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the Endling, last individual of the species, although the Functional ext ...

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).Cometary globules. 1 Formation, evolution and morphology
B. Lefloch and B. Lazareff, 1994.

Image gallery

File:Bok globules in IC2944.jpg, Thackeray's Globules, a set of Bok globules in the

IC 2944, taken with the WFPC2 instrument on the

Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most vers ...

File:NGC 281HSTFull.jpg, Bok globules located within the

NGC 281 NGC 281, IC 11 or Sh2-184 is a bright emission nebula and part of an H II region in the northern constellation of Cassiopeia and is part of the Milky Way's Perseus Spiral Arm. This 20×30 arcmin sized nebulosity is also associated with open clu ...

nebula (IC 1590 cluster) File:Barnard 68.jpg,

Barnard 68 Barnard 68 is a molecular cloud, dark absorption nebula or Bok globule, towards the southern constellation Ophiuchus and well within the Milky Way galaxy at a distance of about 500 light-years, so close that not a single star can be seen between i ...

, at a distance of only 410 light-years, is one of the nearest Bok globules. Its diameter is about (≈ 2 trillion km)

References

External links

A Star in the Making
{{Authority control *Bok globule 1940s in science

History

Image gallery

See also

References

External links