Figure 11-2 RIVUXG A Dark Nebula and a Reflection Nebula (a) When first discovered in the late 1700s, dark nebulae were thought to be “holes in the heavens” where very few stars were present. In fact, they are opaque regions of dust grains that block out light from the stars beyond them. The few stars that appear to be within Barnard 86 lie between us and the nebula. Barnard 86 is in the constellation Sagittarius and has an angular diameter of 4 arcmin, about