H-II Regions: The Red Nebulae

H-II Regions: The Red Nebulae

Most gaseous nebulae are glowing clouds of hydrogen gas excited by hot stars. These hot stars are young, bright and blue, and emit copious quantities of ultraviolet light. The ultraviolet light ionizes atoms in the surrounding gas; that is, it violently strips electrons from their parent hydrogen atoms, giving rise to a gas in which electrons and protons move freely about. Volumes where this has happened are known as H-II regions, or “gasbags”.

The liberated electrons can recombine with the protons to re-form hydrogen atoms; as they do so, they free themselves of excess energy by emitting light. But unlike the ultraviolet light that gave the electrons the energy to break from their parent atoms, this light from the recombination is at a series of special wavelengths. Astronomers call these wavelengths H-alpha, H-beta, H-gamma, etc. The process of re-emission of light at a different wavelength is called fluorescence, and in many ways a gaseous nebula is similar to a fluorescent light. The H-alpha line is the brightest emission line of gaseous nebulae and accounts for their characteristic red color. However, the nebular also glow in H-beta at 486nm, with a blue line; if you can find a suitable filter, it should be possible to take H-beta photographs. But because this line if far weaker, much longer exposures would be needed.

Lagoon Nebula. Photo: Bob’s Color Astrophotos

Astronomers are interested in H-II regions not only because they make pretty pictures, but because in many cases they are regions in which stars have recently formed. The complexes of nebulosity and young stars associated with the belt and sword of Orion are being studied intensively to learn about the process of star formation.

Other H-II regions are an entirely different kind of object – the expanding remnants of a supernova explosion. Here the energy to ionize the hydrogen debris from the supernova comes from its collision with interstellar gas and dust. Examples of this type of object are the Cygnus loop nebula and IC-443.

The nebulae are distributed in the plane of the Milky Way for the simple reason that the young stars and explosions which illuminate the nebulae are mostly in the plane of the Milky Way. A few nebulae, such as Zeta Ophiuchi, appear to be rather far from the plane, but this is because they are nearby. Young stars are formed mostly in the spiral arms of our galaxy, so it is not surprising that most of the H-II regions are also found in spiral arms; in fact, astronomers have used H-II regions to map the spiral arms near the sun. It appears there are at least three distinct arms in the sun’s neighbourhood. When you look toward the North America nebula, you are looking along the Cygnus arm which extends past the sun and connects with the stars of Orion. Looking toward the center of the Galaxy, you see an arm inside the Cygnus arm – the Sagittarius arm. There also appears to be an arm farther out than the Cygnus arm, called the Perseus arm.

(By John Davis, William Tobin and Joel Eaton, Astronomy Magazine, August 1976)