The red glow of an emission nebula
a) is a reflection of light from stars near the nebula.
b) is the unresolved light of many faint red stars within the nebula.
c) is emitted by warm gas falling onto the stars at the center.
d) is produced by hydrogen gas heated to high temperatures by massive stars within the nebula.
Answer: d) is produced by hydrogen gas heated to high temperatures by massive stars within the nebula.