The Trifid Nebula(Click for larger rendering)
The HII regions (emission nebulae) are so named because they are composed mostly of a plasma of ionized hydrogen (HII) and free electrons. The hydrogen atoms of the interstellar medium are ionized by the ultraviolet radiation from a nearby star or stars. Only very hot stars, typically young stars, have enough radiation in the ultraviolet region at wavelengths necessary to ionize the hydrogen. The excess energy beyond that needed to ionize the hydrogen goes to kinetic energy of the ejected electrons. Eventually, by collision, this energy is shared by other particles in the gas. An equilibrium is established in a typical emission nebula when the temperature equivalent of this kinetic motion is between 7000 K and 20,000 K. For a typical emission nebulae, the density of ions (and electrons) is 1.0E8 to 1.0E10 particles per m^3.
As the ions de-excite to lower energy levels, in most cases after recombination of ions with electrons, they emit their characteristic spectral lines. The most prominent of these in the visible spectrum is the red line of hydrogen, giving most emission nebulae a characteristic red glow. There also exist "forbidden lines" (ones not normally seen in earth-bound laboratories) in the spectra from nebulae. The most prominent are green lines from doubly ionized oxygen, giving some nebula a green shading.
Interspersed within the glowing gas of nebulae are lanes of dark dust which can give nebulae their dramatic appearance. Some of the most famous and beautiful of the emission nebulae visible from the Northern Hemisphere are the Orion Nebula (M42, the Messier catalog number), the Lagoon Nebula (M8), and the Trifid Nebula (M20). The Orion Nebula, located in the sword of Orion, is illuminated by the stars of the Trapezium. The Lagoon Nebula is a very large nebula which has distinct bright rims and small dark clouds projected onto its brightest parts. The Trifid Nebula in Sagittarius is characterized by dust lanes that divide it into three distinct parts.
Page contributed by G. Samuel Lightner.
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