Emmision Nebula

> Emission Nebula

An emission nebula is a cloud of ionized gas (i.e. a plasma) emitting light of various colors. The most common source for ionization are high-energy photons emitted from a nearby hot star. Among the several different types of emission nebula are H II regions, in which star formation is taking place and young, massive stars are the source of the ionising photons; and planetary nebulae, in which a dying star has thrown off its outer layers, with the exposed hot core then ionizing them.

Usually, a young star will ionize part of the same cloud from which it was born. Only big, hot stars can release the amount of energy required to ionize a significant part of a cloud. Often, an entire cluster of young stars is doing the work.

Eagle Nebula

In the image of the Eagle Nebula, also known as M16, one can see three distinct "pillars" of gas. It took the Hubble Space Telescope to see these pillars, imaged in 1995, and they would not be visible to back-yard astronomers. Inside the pillars are newly-formed stars, whose solar winds are literally blowing away the surrounding gas and dust. The most prominent pillar is about 10 light-years tall and one light-year thick. The full nebula was discovered in 1764, and lies about 7,000 light-years away.

The nebula's color depends on its chemical composition and amount of ionization. Due to the high prevalence of Hydrogen in interstellar gas, and its relative low energy requirement for ionization, many emission nebulae are red. If more energy is available, other elements can be ionized and green and blue nebulae are possible. By examining the spectra of nebulae, astronomers deduce their chemical content. Most emission nebulae are about 90% hydrogen, with the remainder helium, oxygen, nitrogen, and other elements.

Some of the most beautiful emission nebulae visible from the northern hemisphere are the Lagoon Nebula and the Orion Nebula.

The Lagoon Nebula

The image of the Lagoon Nebula (M8) is also taken by the Hubble Space Telescope. It lies approximately 5,200 light-years away, and was originally discovered in 1747. The full nebula spans approximately 140x60 light-years, but this is just a small section of it that Hubble imaged.

Emission nebulae often have dark spots in them which result from clouds of dust which block the light. The combination of emission nebula and dust cloud make for some interesting looking objects, and many of these nebulae bear the name of objects that they resemble, such as the North America Nebula or the Cone Nebula.

Some nebulae are made up of both reflection and emission components such as the Trifid Nebula.

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