Long-exposure color photographs of the night sky often show regions that glow red, such as parts of the Orion Nebula. This distinctive red color is caused by:

Question

Quizplus · Accepted Answer

The red color in long-exposure color photographs of the night sky is caused by ionization and subsequent recombination of hydrogen atoms. As the gas in the nebula is ionized by nearby stars, the electrons in the hydrogen atoms are excited to higher energy levels. When these electrons return to their ground state, they release energy in the form of light, which appears as a bright red color in the photographs.

Long-Exposure Color Photographs of the Night Sky Often Show Regions