What two things are needed to create an emission nebulae?

Question

Quizplus · Accepted Answer

Hot stars and interstellar gas, particularly hydrogen, are needed to create an emission nebula, which is a cloud of ionized gas that emits light of various colors. The gas is ionized by the energetic radiation from hot stars, which causes the electrons to be stripped from the atoms and molecules, creating a plasma that emits light as it recombines with the charged particles. The hydrogen gas is particularly important because it emits a characteristic red color at a wavelength of 656.3 nm, which is commonly seen in emission nebulae.

What Two Things Are Needed to Create an Emission Nebulae