Dark nebula. Optical image of Barnard 68, a dark nebula of dense dust and gas. The dust is so thick that it obscures the rich star field of the Milky Way behind it. Such nebulae are thought to be the precursors of star systems. Barnard 68 is about 500 light years from Earth in the constellation Ophiuchus. This image was made using the CFH12K camera on the Canada-France-Hawaii Telescope at Mauna Kea, Hawaii.