The Trifid Nebula (M20) is a rare combination of a red emission nebula, a blue reflection nebula, and dark dust lanes. An extremely hot, young multiple star at its center has so much energy that it causes hydrogen gas to glow (red emission nebula). The blue shining part is illuminated by slightly cooler stars that do not have enough energy to ionize the gas. Instead, their light is simply scattered by the fine dust particles (reflection nebula). The dark dust lanes that divide the red nebula into three parts lie in front of it and completely block the light from the red background. The blue reflection nebula and the red emission nebula are directly adjacent to one another and blend seamlessly together. Cosmologically speaking, they are part of the same massive cloud of gas and dust in the constellation Sagittarius. As a single system, they are approximately 4,100 to 5,200 light-years away from Earth.
At the bottom left of my image, you can see the star cluster M21. It is about 4,250 light-years away from us and is located in the same cosmic region as M20. Astronomers believe that the stars of M21 formed a few million years ago from the very outer reaches of that gas cloud, which we can still see glowing today in M20.