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NASA Astronomy Picture of the Day 2015 November 23

A 212-Hour Exposure of Orion

The constellation of Orion is much more than three stars in a row. It is a direction in space that is rich with impressive nebulas. To better appreciate this well-known swath of sky, an extremely long exposure was taken over many clear nights in 2013 and 2014. After 212 hours of camera time and an additional year of processing, the featured 1400-exposure collage spanning over 40 times the angular diameter of the Moon emerged. Of the many interesting details that have become visible, one that particularly draws the eye is Barnard’s Loop, the bright red circular filament arcing down from the middle.

The Rosette Nebula is not the giant red nebula near the top of the image – that is a larger but lesser known nebula known as Lambda Orionis. The Rosette Nebula is visible, though: it is the red and white nebula on the upper left. The bright orange star just above the frame center is Betelgeuse, while the bright blue star on the lower right is Rigel. Other famous nebulas visible include the Witch Head Nebula, the Flame Nebula, the Fox Fur Nebula, and, if you know just where to look, the comparatively small Horsehead Nebula. About those famous three stars that cross the belt of Orion the Hunter – in this busy frame they can be hard to locate, but a discerning eye will find them just below and to the right of the image center.

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How does space become transparent? The Orion Nebula answers

“Until the gas is completely ionized, visible light can be reflected or absorbed, depending on the orientation of the stars and gas with respect to us. The only way to see through neutral gas is by looking in the infrared, which is sensitive to other features. Once the gas is 100% ionized, it’s 100% transparent, and the entire Universe is revealed.”

The distant nebulae might appear to illuminate the night sky, but this neutral gas is mostly only good for reflecting or absorbing starlight, which obscures the view of all the stars and galaxies lying in the background. But this light-blocking effect is only temporary, as over time, this neutral gas will give way to transparency. All it takes is the energy of the hot, blue stars forming inside, whose ultraviolet radiation will eventually ionize all of the material within it. The last gasps of the neutral gas will appear as Evaporating Gas Globules (EGGs), and when they’re all completely ionized, the starlight from everything beyond will be free to stream towards our eyes unimpeded.

Go get the full story in pictures, video and no more than 200 words on today’s Mostly Mute Monday!

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The Horse-head Nebula.

One of the most identifiable nebulae in the sky, the Horsehead Nebula in Orion, is part of a large, dark, molecular cloud. Also known as Barnard 33, the unusual shape was first discovered on a photographic plate in the late 1800s. The red glow originates from hydrogen gas predominantly behind the nebula, ionized by the nearby bright star Sigma Orionis.

 The darkness of the Horsehead is caused mostly by thick dust, although the lower part of the Horsehead’s neck casts a shadow to the left. Streams of gas leaving the nebula are funneled by a strong magnetic field. Bright spots in the Horsehead Nebula’s base are young stars just in the process of forming. 

Light takes about 1,500 years to reach us from the Horsehead Nebula.

Photo credit: NASA/Hubble.