Barnard 68 is a

molecular cloud A molecular cloud—sometimes called a stellar nursery if star formation is occurring within—is a type of interstellar cloud of which the density and size permit absorption nebulae, the formation of molecules (most commonly molecular hydrogen, ...

, dark absorption nebula or

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 two to 50 sol ...

, towards the southern constellation

Ophiuchus Ophiuchus () is a large constellation straddling the celestial equator. Its name comes from the Ancient Greek (), meaning "serpent-bearer", and it is commonly represented as a man grasping a snake. The serpent is represented by the constellati ...

and well within the

Milky Way The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...

galaxy at a distance of about 125 parsecs (407 light-years). It is both close and dense enough that stars behind it cannot be seen from Earth. American astronomer

Edward Emerson Barnard Edward Emerson Barnard (December 16, 1857 – February 6, 1923) was an American astronomer. He was commonly known as E. E. Barnard, and was recognized as a gifted observational astronomer. He is best known for his discovery of the high proper m ...

added this nebula to his catalog of

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

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/−431 °F). Its mass is about twice that of the Sun, and it measures about half a light-year across.

Characteristics

Despite being opaque at visible-light wavelengths, use of the

Very Large Telescope The Very Large Telescope (VLT) is an astronomical facility operated since 1998 by the European Southern Observatory, located on Cerro Paranal in the Atacama Desert of northern Chile. It consists of four individual telescopes, each equipped with ...

Cerro Paranal Cerro Paranal is a mountain in the Atacama Desert of northern Chile and is the home of the Paranal Observatory. Prior to the construction of the observatory, the summit was a horizontal control point with an elevation of ; now it is above sea ...

has revealed the presence of about 3,700 obscured background Milky Way stars, some 1,000 of which are only visible at infrared wavelengths. Careful measurements of the degree of obscuration resulted in a finely sampled and accurate mapping of the dust distribution inside the cloud. Observations obtained with the

Herschel Space Observatory The Herschel Space Observatory was a space observatory built and operated by the European Space Agency (ESA). It was active from 2009 to 2013, and was the largest infrared telescope ever launched until the launch of the James Webb Space Telesco ...

were able to constrain the distribution of the dust component and its temperature even more. Having a dark cloud in the

solar neighborhood The Local Interstellar Cloud (LIC), also known as the Local Fluff, is an interstellar cloud roughly across, through which the Solar System is moving. This feature overlaps with a region around the Sun referred to as the solar neighborhood. It i ...

greatly facilitates observation and measurement. If not disrupted by external forces, the stability of dust clouds is a fine balance between outward pressure caused by the heat or pressure of the cloud's contents, and inward gravitational forces generated by the same particles (see

Jeans instability The Jeans instability is a concept in astrophysics that describes an instability that leads to the gravitational collapse of a cloud of gas or dust. It causes the collapse of interstellar gas clouds and subsequent star formation. It occurs when ...

and

Bonnor–Ebert mass In astrophysics, the Bonnor–Ebert mass is the largest mass that an isothermal gas sphere embedded in a pressurized medium can have while still remaining in hydrostatic equilibrium. Clouds of gas with masses greater than the Bonnor–Ebert mass mus ...

). This causes the cloud to wobble or oscillate in a manner like that of a large soap bubble or a water-filled balloon which is jiggled. In order for the cloud to become a star, gravity must gain the upper hand long enough to cause the cloud to collapse and reach a temperature and density where fusion can be sustained. When this happens, the much smaller size of the star's envelope signals a new balance between greatly increased gravity and radiation pressure. The cloud's mass is about twice that of the Sun, and it measures about half a light-year across. Barnard 68's well-defined edges and other features show that it is on the verge of gravitational collapse, followed by becoming a star within the next 200,000 years or so. The cloud is often confused with the

Boötes Void file:boovoid.png, 259x259px, A map of the Boötes Void The Boötes Void ( ) (colloquially referred to as the Great Nothing) is a roughly Sphere, spherical region of Interstellar medium, space in the vicinity of the constellation Boötes. It contain ...

, although the two have nothing in common. Pictures of Barnard 68 are often erroneously used to illustrate articles about the Boötes Void.

References

External links

The Black Cloud
- Ken Croswell
C18O abundance in the nearby globule Barnard 68
*

ESO The European Southern Observatory is an astronomical research organisation. ESO may also refer to: *Employee stock option (also: executive stock option) *'' Ether Saga Odyssey'', a fantasy massively multiplayer online role-playing game *''The Elde ...

How to Become a Star - ESO Telescopes Provide Most Detailed View Ever Into a Dark Cloud
10 January 2001 (accessed 1 March 2009) * ''

New Scientist ''New Scientist'' is a popular science magazine covering all aspects of science and technology. Based in London, it publishes weekly English-language editions in the United Kingdom, the United States and Australia. An editorially separate organ ...

''
Astrophile: Lucky strike turns a dark cloud into a star
6 September 2012 {{Portal bar, Astronomy, Stars, Outer space 068 Dark nebulae Ophiuchus

Characteristics

See also

References

External links