| Revision as of 21:22, 26 February 2009 editDebresser (talk | contribs)Autopatrolled, Extended confirmed users, Pending changes reviewers, Rollbackers, Template editors110,467 editsm 2 reffixes (my mistakes)← Previous edit | Revision as of 07:21, 27 February 2009 edit undoRotational (talk | contribs)9,031 edits Paragraph two gives background information on dark clouds, something which the average reader may find helpful. Please don't make major changes without discussionNext edit → | ||
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| '''Barnard 68''' is a ], dark ] or ], towards the southern constellation ] and well within our own galaxy at a distance of only 400 or so light-years, so close that not a single star can be seen between it and the Sun |
'''Barnard 68''' is a ], dark ] or ], towards the southern constellation ] and well within our own galaxy at a distance of only 400 or so light-years, so close that not a single star can be seen between it and the Sun. 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. <ref></ref> | ||
| Dark clouds appear so because of submicrometre-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, C<sup>18</sup>O, CS, NH<sub>3</sub> (]), H<sub>2</sub>CO (]), c-C<sub>3</sub>H<sub>2</sub> (]) and a molecular ion N<sub>2</sub>H<sup>+</sup> (]), all of which are relatively transparent. These clouds are the spawning grounds of stars and planets, and understanding their development is essential to understanding ]. <ref></ref><ref></ref> | |||
| ==Characteristics== | |||
| ⚫ | Despite being opaque at visible-light wavelengths, use of the ] at ] has revealed the presence of about 3 700 blocked background Milky Way stars, some |
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| ⚫ | Despite being opaque at visible-light wavelengths, use of the ] at ] has revealed the presence of about 3 700 blocked background Milky Way stars, some . Careful measurements of the degree of obscuration resulted in a finely sampled and accurate mapping of the dust distribution inside the cloud. Having a dark cloud in the solar neighbourhood 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 ] of the cloud's contents, and inward gravitational forces generated by the same particles (see ] and ]). This causes the cloud to wobble or oscillate in a manner not unlike 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 collapse of the cloud and reach a temperature and density where ] 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. <ref></ref> Barnard 68's well-defined edges and other features, show that it is on the verge of gravitational collapse within the next 100 000 years or so and is on its way to becoming a star. | ||
| ⚫ | ==External links== | ||
| American astronomer ] added this nebula to his catalogue of ] in 1919. He published his catalogue in 1927, at which stage it included some ]. | |||
| ⚫ | ====External links==== | ||
| *]] | *]] | ||
| * | * | ||
| ==References== | ====References==== | ||
| {{reflist}} | {{reflist}} | ||
Revision as of 07:21, 27 February 2009
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 only 400 or so light-years, so close that not a single star can be seen between it and the Sun. 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.
Dark clouds appear so because of submicrometre-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, CO, 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.
Despite being opaque at visible-light wavelengths, use of the Very Large Telescope at Cerro Paranal has revealed the presence of about 3 700 blocked background Milky Way stars, some 1 000 of which are 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. Having a dark cloud in the solar neighbourhood 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 and Bonnor-Ebert mass). This causes the cloud to wobble or oscillate in a manner not unlike 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 collapse of the cloud 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. Barnard 68's well-defined edges and other features, show that it is on the verge of gravitational collapse within the next 100 000 years or so and is on its way to becoming a star.
American astronomer Edward Emerson Barnard added this nebula to his catalogue of dark nebulae in 1919. He published his catalogue in 1927, at which stage it included some 350 objects.