The image below is a composite of two separate shots. The large nebula on the right is Messier 8, the Lagoon Nebula. It is a bright star forming region, and is visible to the naked eye even in somewhat light polluted skies. The red glow comes from hydrogen gas that is being energized by ultraviolet radiation from the stars that are forming within the nebula. The process that causes this is similar to the process that causes fluorescent lights to glow.
The small nebula near the upper-left is Messier 20, the Trifid Nebula. The same process that causes the Lagoon Nebula to glow red is at work in the red portion of the Trifid. The blue color comes from starlight that is being reflected off of gas and dust. Trifid means "divided into three parts," in reference to the dark lanes that divide the red portion.
Messier 21 is the open cluster on the left side of the image. And, located in the lower-right corner is globular cluster NGC 6544.
|Messiers 8, 20, 21, and NGC 6544 in Sagittarius; ST80 on Vixen SP|