Image Credit & Copyright: Adam Block/John and Christie Connor/Mount Lemmon SkyCenter/University of Arizona (http://skycenter.arizona.edu/gallery)
NGC 6559 is an emission nebula, a star forming region that lies about 5000 light-years away from Earth, just east-northeast of its much more famous neighbor, the Lagoon Nebula (Messier 8) – and has formed from the same molecular cloud – in the constellation of Sagittarius.
As energetic light from neighboring stars ionizes interstellar hydrogen, protons and electrons recombine to emit light of very specific colors, including the red hue observed. The soft red glow of fluorescent hydrogen is evidence that there are young hot stars associated with the dusty clouds. The very bright, red arc of gas seems to shield these luminous and hot stars beneath it. Presumably the birth and energetic emission of these stars causes the gas to glow very brightly.
The bright stars also illuminate the tiny solid dust particles, producing the blue reflection nebula seen near two of the bright stars. The dust is also evident in silhouette, both as sinuous dark lanes winding through the luminous gas (known as B 303) and as dark patches and filaments. These absorption nebulas of dust are obscuring parts of the blue reflection nebula.
The first massive stars formed from the dense gas will also emit fierce winds that erode, fragment, and sculpt their birthplace before they explode. After tens of millions of years, the dust boils away, the gas gets swept away, and all that is left is a naked open cluster of stars.