This famous Messier object, also known as Sh2-45, is an emission nebula located 5000 light years away in the constellation of Sagittarius at a declination of -16 degrees.  The nebula extends almost 1 degree across.

Of all of the sky objects, I believe that this one has the highest number of commonly accepted nicknames. All of these nicknames are easier to see in an eyepiece than in an image – too many other things get in the way here.  Focusing on the bright central portion of the nebula leads to the names of the Swan Nebula as well as the Checkmark Nebula. Focusing on the broader nebula leads to the names of the Omega Nebula as well as the Horseshoe Nebula.

The most commonly used nickname (determined by Google search numbers) surprised me – the Omega Nebula.  The Omega Nebula name originated with the famous astronomer John Herschel, who stated that "The figure of this nebula is nearly that of the Greek capital Omega, somewhat distorted and very unequally bright."

Regardless of the name chosen, this nebula is beautiful and fascinating. Thick dust clouds are both obscuring the central bright stars of the nebula and also reflecting and spreading light of other stars. The object spans a big section of the sky — its gas and dust clouds measure about 80 light-years across. This nebula is considered one of the brightest and most massive star-forming regions of our galaxy.

Having said of all that, I believe that this is one of the most difficult famous deep sky objects to process.  I tried many different versions and palettes before landing on anything that I thought looked decent, and even this image (which used the HOS palette) is not that appealing to me.  I attached a SHO version in Revision F.

The wide field of this region, shown in the mouseover and in Revision E, includes the Messier open star cluster M18, two other Sharpless objects (Sh2-44 & Sh2-47), and two vdB reflection nebula (vdB 120 & vdB 121).