Astronomy Picture of the Day Index - Nebulae: Emission Nebulae

APOD: 2000 January 11 - The Rosette Nebula in Hydrogen, Oxygen, and Sulfur
Explanation: The Rosette Nebula is a large emission nebula located 3000 light-years away. The great abundance of hydrogen gas gives NGC 2237 its red color in most photographs. The wind from the open cluster of stars known as NGC 2244 has cleared a hole in the nebula's center. The above photograph, however, was taken in the light emitted by three elements of the gas ionized by the energetic central stars. Here green light originating from oxygen and blue light originating from sulfur supplements the red from hydrogen. Filaments of dark dust lace run through the nebula's gases. The origin of recently observed fast-moving molecular knots in the Rosette Nebula remains under investigation.

APOD: 1996 June 6 - The North America Nebula
Explanation: Here's a familiar shape in an unfamiliar location! This emission nebula is famous partly because it resembles Earth's North American Continent. To the right of the North America Nebula is a less luminous Pelican Nebula. Let's be grateful that pelicans aren't really that large! The two emission nebula are located about 1500 light years away and are separated by a dark absorption cloud. The nebulae can be seen without a telescope from a dark location. Look for a small nebular patch north-east of Deneb in the constellation of Cygnus. It is still unknown which star or stars ionize the red-glowing hydrogen gas.

APOD: 1998 August 28 - Hydrogen Trifid
Explanation: Clouds of glowing hydrogen gas mingle with dark dust lanes in the Trifid Nebula, a star forming region in the constellation Sagittarius. In this and other similar emission nebulae, energetic ultraviolet light from an embedded hot young star strips electrons from the surrounding hydrogen atoms. As the electrons and atoms recombine they emit longer wavelength, lower energy light in a well known characteristic pattern of bright spectral lines. At visible wavelengths, the strongest emission line in this pattern is in the red part of the spectrum and is known as "Hydrogen-alpha" or just H-alpha. This image of the nebula was taken using a filter to select only light near the H-alpha wavelength. It shows those regions with substantial emission from atomic hydrogen. The relative strength of this emission can trace the densities of atoms within the gas cloud.