Barnard 68
Encyclopedia
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 500 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 catalogue of dark nebulae in 1919. He published his catalogue 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.
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.
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 within the next 100,000 years or so, and is on its way to becoming a star.
Molecular cloud
A molecular cloud, sometimes called a stellar nursery if star formation is occurring within, is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen ....
, dark absorption nebula or Bok globule
Bok globule
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 to 50 solar masses contained within a region about a light year or so across...
, towards the southern constellation Ophiuchus
Ophiuchus
Ophiuchus is a large constellation located around the celestial equator. Its name is from the Greek "serpent-bearer", and it is commonly represented as a man grasping the snake that is represented by the constellation Serpens. Ophiuchus was one of the 48 constellations listed by the 2nd-century...
and well within our own galaxy at a distance of about 500 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 catalogue of dark nebulae in 1919. He published his catalogue 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.
Characteristics
Despite being opaque at visible-light wavelengths, use of the Very Large TelescopeVery Large Telescope
The Very Large Telescope is a telescope operated by the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile. The VLT consists of four individual telescopes, each with a primary mirror 8.2m across, which are generally used separately but can be used together to...
at Cerro Paranal
Cerro Paranal
Cerro Paranal , also known as Paranal Mountain is a mountain in the Atacama desert of northern Chile that is home to the Paranal Observatory. It is famous for hosting the Very Large Telescope and the VLT Survey Telescope. It is located 120 km west of Antofagasta and 80 km north of Taltal,...
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
Hydrostatic equilibrium
Hydrostatic equilibrium or hydrostatic balance is the condition in fluid mechanics where a volume of a fluid is at rest or at constant velocity. This occurs when compression due to gravity is balanced by a pressure gradient force...
of the cloud's contents, and inward gravitational forces generated by the same particles (see Jeans instability
Jeans instability
In physics, the Jeans instability causes the collapse of interstellar gas clouds and subsequent star formation. It occurs when the internal gas pressure is not strong enough to prevent gravitational collapse of a region filled with matter...
and Bonnor-Ebert mass
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....
). 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
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
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 within the next 100,000 years or so, and is on its way to becoming a star.
External links
- http://kencroswell.com/theblackcloud.htmlThe Black Cloud - Ken CroswellKen CroswellKen Croswell is an astronomer and author living in Berkeley, California. His first degree mixed science and wider interests, majoring in physics and minoring in English literature. He also got a PhD in astronomy for studying the Milky Way's halo....
] - C18O abundance in the nearby globule Barnard 68
- ESO, How to Become a Star - ESO Telescopes Provide Most Detailed View Ever Into a Dark Cloud, 10 January 2001 (accessed 1 March 2009)