From the November 2010 issue

I read that ultraviolet light is the cause of HII regions, but this light is invisible. So why are these objects the color red?

David Siegel, Gloucester City, New Jersey
By | Published: November 19, 2010 | Last updated on May 18, 2023
The Lagoon Nebula (M8) is a beautiful example of a specific type of emission nebula called an HII region. It appears red because it emits radiation at the Hydrogen-alpha wavelength, which appears red to human vision.
Gerald Rhemann

HII regions are large clouds of mostly ionized hydrogen (unionized hydrogen is called HI). As you hint at, these areas form when hot young stars emit ultraviolet radiation that knocks electrons off nearby hydrogen atoms in the surrounding gas. When you observe an HII region, you’re looking at a stellar nursery.

Often, one of those unbound electrons will meet an electron-free hydrogen atom (which is just a proton). Opposites attract, and thus they recombine. However, the electron is energized and needs to fall to a lower energy state. As it does this, it emits radiation of specific wavelengths depending on what energy level it started at and how far it’s falling. One of the most common drops is from the third to the second level. This process emits a photon with a wavelength of 656.3 nanometers, which is firmly in the red part of the visible spectrum. Scientists call this observed emission the Hydrogen-alpha line.

— Liz Kruesi, Associate Editor