Question

What is the primary visible color of an emission nebula?

A) black, because of dust
B) blue from the scattering of light off the tiny molecules
C) blue due to ionized helium atoms
D) red due to ionized hydrogen atoms
E) red because of reddening by dust

Answer 1

Answer: D) red due to ionized hydrogen atoms

Step-by-step explanation:

The emission nebulae emit their own light because the hydrogen atoms present in the gas that compose it are excited by the powerful ultraviolet light of nearby hot stars. This causes the hydrogen to ionize, generating the characteristic red light of the nebula.

It should be noted that this red color is due to the emission of H-alpha (
`H-\alpha`) lines from hydrogen after ionization, where the
`H-\alpha` line is centered on a wavelength of approximately 656 nm, which corresponds to red.