Radius SurfaceBrightness (parsecs) (pc cm 6) 0.012 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 6.0 7.0 8.0 9.0 10.0 12.0 7.1 5.3 4.7 4.5 3.6 3.2 2.8 2.4 2.3 2.1 1.7 1.3 0.8 0,6 9. 1. If it were impossible to see a ring nebula with a surface brightness fainter than 1.6 pc cm6, how big would the nebula be when it becomes invisible? 625 parsecs 10. 2. What would the surface brightness of a ring nebula be if it has a radius of 3.25 parsecs? pc cm 6