Final answer:
The pink glow of the Great Nebula in Orion is caused by the ionization of hydrogen, particularly the first line in the Balmer series.
Step-by-step explanation:
The visual pink glow of the Great Nebula in Orion is due to the ionization of hydrogen. This emission indicates the presence of hot young stars nearby that ionize the clouds of gas. Specifically, the red glow is produced by the first line in the Balmer series of hydrogen. When electrons recombine with protons and transition back to lower energy orbits, they emit this characteristic red light. The Orion Nebula is an example of an H II region, which refers to a cloud of ionized hydrogen that has lost its one electron.