What causes reddening of star light?
What causes extinction of star light?
Define and contrast reddening and extinction. Are these effects ever seen in the Earth's atmosphere?
What is the composition of a typical interstellar dust particle? How does it compare to material found on Earth?
What is the density and temperature of interstellar space?
What is a nebula?
What causes and emission nebula to emit light? What would the spectrum of an emission nebula look like?
What is an HI region? What is an HII region? What is the difference between these regions?
What is the composition of emission nebula?
What powers an HII region? Why are HII regions larger around O stars than those around B stars?