## Monday, March 19, 2012

### The Great Nebula in Orion.

Last Wednesday I was able to take out the imager at the Vis under good conditions for all of the second time this year.

Since it's still winter in the northern hemisphere, and the Earth's night side is still pointed more or less away from the galactic core, there aren't many of the objects I typically like to image (globular clusters) out at night. This led me to look around for another object to image, and I settled on the Great Nebula in Orion, AKA Messier 42, AKA the Orion Nebula. This nebula is, without a doubt, the most impressive nebula in the sky. It's also the brightest, and one of the few that can be seen with the naked eye (it's the middle "star" in the three stars that make up Orion's sword. No, not his belt, but his sword. I should post a picture...). I decided to devote an entire night to it and ended up getting almost two hours' worth of exposures. (Normally I like to image as many things as possible in a night, and for things like globular clusters that works alright, but the picture I got has taught me to reconsider that position when imaging nebulae...)

But enough words! You want to see the picture, and I want to show it to you:

 The Great Nebula in Orion, Messier 42.

Gorgeous, no?

The Orion nebula is the closest star-forming region (or "stellar nursery") to Earth, at 1,344 $$\pm$$ 20 light-years distant. It is probably the most photographed celestial object in history $$-$$ in fact, it was the very first nebula ever to be imaged, on September 30, 1880 by Henry Draper (who was a pretty amazing character, one of the pioneers of astrophotography, and the person in charge of the U.S. expedition to observe the transit of Venus in 1874).

The Orion nebula is a large ball or bubble of gas (which fluoresces red) and dust (which silhouettes as black, or reflects light as blue) about 24 light-years across. Several extremely young and massive stars much hotter and more luminous than our Sun have formed near the center and are steadily blowing a hole in the side of the bubble of gas and dust in which they are embedded. The four brightest are known as the Trapezium and are a little too bright to see in this picture (they're located in the center where it's brightest). This allows us to see into the central cavity.

Interestingly, it's quite possible that the Orion nebula doesn't look anywhere near as impressive from the other side. From that perspective it might appear as a mostly featureless dark nebula, with perhaps some small emission nebulae around the edges. This is because the stars in the Trapezium appear to be blowing open the side of the bubble asymmetrically, and we happen to be on the side where it's open. It does make you wonder about the other dark nebulae we can see in the sky, and whether they might not be visions of such cosmic grandeur from the other side...

One last note: on the left side of the image, you might be able to make out two faint straight lines. Those are satellite tracks that appeared in the luminance images while I was imaging. They're pretty faint, so you may or may not be able to see them.