Why Are Emission Nebulae (Mostly) Colored Red?
Most of us know that radiation from a hot star causes hydrogen gas in a nebula to glow with a pinkish-red color, plus there are often other colors due to different elements. But mostly they're red. That's the simple answer. If you'd prefer to stop reading now, that's fine. If you have a taste for technical details, read on. I came across an article that explains this phenomenon on the atomic level, and what follows is an effort to put the explanation in reasonably simple terms in accord with my own limited grasp of atomic physics.
The pinkish-red color of nebulae, such as M42 in Orion or the Lagoon Nebula in Sagittarius, is really a combination of four different bright spectral lines of hydrogen gas. A hydrogen atom has a single proton at its center and a single electron in its outer region. A proton is a positively charged subatomic particle, while an electron is a negatively charged and less massive particle. The electron can exist in a variety of energy states. The ground state (lowest energy) is denoted as n=1. The electron can jump to higher energy states, provided that there is some external source to supply the additional energy needed. This increase in an electron's energy state means it's excited.
Once it reaches a higher energy state, the electron will fall back to a lower energy state. This constitutes a loss of energy, and this lost energy is emitted as a photon, which is a particle of light. This drop to a lower energy state is called electron decay. The color of the photon emitted is determined by its energy, which is based on the level of increased energy to which the electron had jumped. The higher the energy, the bluer will be the photon; the lower the energy, the redder it will be. The least energetic of the excited states is identified as n=2, the more excited states as n=3, n=4, etc. There are four electron decays that emit light of a frequency visible to the human eye; these are known as Balmer decays. Here are the colors emitted when decaying to the n=2 state:
|From||Balmer Decay||Color Emitted|
In many nebulae, hydrogen atoms emit all four of the above colors, which causes them to look pinkish-red.
But why don't the electrons decay all the way down to the lowest energy state (n=1)? That's because the temperature of the nebulosity is usually about 10,000 degrees, which keeps the atoms somewhat excited.
Excitation to the higher energy states (n=3, n=4, etc.) is the result of stronger radiation in the form of ultraviolet high-energy photons emitted by 25,000 degree blue-white giant stars. As an example, in M42 a single one of the young stars in the Trapezium is sufficiently hot to cause the entire nebula to glow.
Once the electron decays back to the n=2 state, it is immediately excited up to a higher energy state again by the constant bombardment of ultraviolet photons. This turnaround takes less than .000,000,01 second. However, most of the ultraviolet photons end up completely removing the electrons from the atoms, which creates a region dominated by positively charged hydrogen ions (H+1 protons), rather than neutral hydrogen atoms. Neutral hydrogen areas are called HI regions and areas of positively charged hydrogen ions are called HII regions. You probably recall that emission nebulae are commonly referred to as "HII regions". Electrons that have been stripped from the hydrogen atoms can rejoin atoms and resume the process of Balmer decays all over again. This ongoing process is what gives us the reddish-colored nebulae. So that's the technical explanation. You were warned!