Anne’s Image of the Day: Dark Cloud Barnard 68

December 9, 2013

Barnard 68, a dark cloud in Ophiuchus 

Image Credit: ESO

Barnard 68 (named after the American astronomer Edward Emerson Barnard) is a molecular cloud, dark absorption nebula or Bok globule of about half a light-year across and a mass about twice that of the Sun, located within our Milky Way galaxy, only 400 – 500 light-years away in the southern constellation of Ophiuchus (the Serpent-bearer). It is so close that not a single star can be seen between it and the Sun.

Barnard 68 is classified as a Bok globule, a dark cloud of dense cosmic dust and gas that could collapse and form a dense and hot, low-mass star. The cloud is still starless, giving astronomers the chance to study the first stage of star birth.

Because of its opacity, its interior is extremely cold, about 16 K (−257 °C). It’s so cold that most of the carbon monoxide and nitrogen gases have frozen onto the cloud’s dust grains. The extreme cold is vital to the black cloud’s starry future. To create a star, the cloud must collapse. When it contracts, the interior temperature rises. If the cloud is sufficiently heavy, it will become so hot at the center that energy-producing nuclear processes ignite and a new star is born.

The cloud is obviously in a state where the inward force of gravity, caused by its mass, more or less balances that of the outward pressure due to its temperature. Nevertheless, it is also evident that Barnard 68 is only marginally stable. Its 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.

Barnard 68 (and its neighbouring brethren, the dark clouds Barnard 69, 70 and 72) probably constitute the few resistent remains of a much larger cloud that has disappeared due to the influence of strong stellar winds and ultraviolet radiation from young and heavy stars as well as supernova explosions. These dark clouds may also be the precursors of an isolated and sparsely populated association of low-mass Sun-like stars.

Doppler shifts show the cloud’s central parts to be moving inward while its outer layers are moving outward. This behavior suggests that Barnard 68 is vibrating once every 250,000 years. If so, it’s the first Bok globule found to quiver. Or perhaps the cloud’s collapse has already begun.

Despite being opaque at visible-light wavelengths, use of the Very Large Telescope has revealed the presence of about 3,700 blocked background Milky Way stars, some 1,000 of which are visible at infrared wavelengths.

This image shows a colour composite of visible and near-infrared images of Barnard 68. It was obtained with the 8.2-m VLT ANTU telescope and the multimode FORS1 instrument in March 1999. North is up and East is left.