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What is the Orion Nebula?

Enormous clouds of dust and gas are found throughout the galaxy. One of the closest is the Orion Nebula, which is 1500 light-years from Earth and measures several light-years across. It is visible to the human eye as a fuzzy patch in the constellation of Orion.

The galaxy contains tens of thousands of dark nebulae, so-called because the dust and gas obscure the light of stars behind them. Over time clumps of higher density gas form and grow within some of these, their gravitational attraction drawing matter from the surrounding cloud. As a clump grows, the weight of layer upon layer of gas builds up, increasing the pressure and temperature at the clump's core. The pressure continues to rise until hydrogen nuclei are packed so tightly together that they fuse, igniting a thermo-nuclear reaction that signals the birth of a star. We see this happening in the Orion Nebula - it is the birthplace of stars.

Hot young stars born within the nebula radiate their energy outward into the surrounding gas. High-energy photons from the stars ionize the atoms of the gas, knocking electrons from their orbits. As these electrons collide with other electrons and slowly return to their former orbits, they emit light. It is this light we see as the nebula's eerie glow. The Orion Nebula is an example of an emission nebula.

Since electrons can reside in atoms only in discrete energy levels, when electrons drop from outer to inner orbits they emit light at discrete wavelengths. 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. Ionization of these gases gives nebulae many of the colors we see in astronomical photographs.


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