Figure 11-2 A Reflection Nebula and Dark Nebula (a) This open cluster, called the Pleiades, can easily be seen with the naked eye in the constellation Taurus (the Bull). Pleiades lies about 440 light-years (134 pc) from Earth. The stars are not shedding mass, unlike the stars in Figures 11-13a and 11-21. The blue glow surrounding the stars of the Pleiades is a reflection nebula created as some of the stars’ radiation scatters off preexisting dust grains in their vicinity. (b) Each dot plotted on this H-R diagram represents a star in the Pleiades whose luminosity and surface temperature have been determined. Note that most of the cool, low-mass stars have arrived at the main sequence, indicating that hydrogen fusion has begun in their cores. The cluster has a diameter of about 5 light-years, is about 100 million years old, and contains about 500 stars. (c) The dark nebula Barnard 86 is located in Sagittarius. It is visible in this photograph simply because it blocks out light from the stars beyond it. The bluish stars to the left of the dark nebula are members of a star cluster called NGC 6520.