Astronomy Picture of the Day Index - Nebulae: Emission Nebulae

APOD: 2005 September 30 - IC 1396 H-Alpha Close-Up
Explanation: Clouds of glowing hydrogen gas mingle ominously with dark dust lanes in this close-up of IC 1396, an active star forming region some 2,000 light years away in the constellation Cepheus. In this and other similar emission nebulae, energetic ultraviolet light from a 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. Part of IPHAS, a survey of H-alpha emission in our Milky Way Galaxy, this image spans about 20 light-years and highlights bright, dense regions within IC 1396, likely sites where massive new stars are born.

APOD: 2003 October 22 - The Heart and Soul Nebulae
Explanation: Is the heart and soul of our Galaxy located in Cassiopeia? Possibly not, but that is where two bright emission nebulae nicknamed Heart and Soul can be found. The Heart Nebula, officially dubbed IC 1805 and visible above on the right, has a shape reminiscent of a classical heart symbol. Both nebulae shine brightly in the red light of energized hydrogen. Several young open clusters of stars populate the image and are visible above in blue, including the nebula centres. Light takes about 6,000 years to reach us from these nebulae, which together span roughly 300 light years. Studies of stars and clusters like those found in the Heart and Soul Nebulae have focussed on how massive stars form and how they affect their environment.

APOD: 2004 January 15 - An Orion Deep Field
Explanation: Adrift 1,500 light-years away in one of the night sky's most recognizable constellations, the glowing Orion Nebula and the dark Horsehead Nebula are contrasting cosmic vistas. They both appear in this stunning composite digital image assembled from over 20 hours of data that includes exposures filtered to record emission from hydrogen atoms. The view reveals extensive nebulosities associated with the giant Orion Molecular Cloud complex, itself hundreds of light-years across. The magnificent emission region, the Orion Nebula (aka M42), lies at the upper right of the picture. Immediately to its left are a cluster of of prominent bluish reflection nebulae sometimes called the Running Man. The Horsehead nebula appears as a dark cloud, a small silhouette notched against the long red glow at the lower left. Alnitak is the easternmost star in Orion's belt and is seen as the brightest star to the left of the Horsehead. Below Alnitak is the Flame Nebula, with clouds of bright emission and dramatic dark dust lanes. Fainter tendrils of glowing hydrogen gas are easily traced throughout the region in this Orion deep field.