The Trifid Nebula is a stellar nursery, where baby stars are born. In this infrared image, the green is cooler hydrogen gas, while the multitude of bright blue and cyan stars are hot, young stars between us and the distant nebula.
This image was captured by NASA's Wide-field Infrared Survey Explorer (WISE). The colours are all different wavelengths of infrared: red maps to the coolest end of the spectrum — red is light of 22-micron wavelength, moving up to green at 12-microns. Blues are the hotter end of the spectrum, with cyan representing 4.6 microns and blue is 3.4-microns wavelengths.
The green cloud is mostly hydrogen gas; within the cloud is a yellow-orange gas cloud that is the Trifid Nebula. Radiation and wind from young stars have blown a cavity into the nebula. Yellow bars appear to cut the cavity into three sections, which gives the nebula its name. These lines only glow in infrared, and are dark in visible-light views.
The blue stars are older, between us and the nebula 5,400 light years away.
Image credit: NASA/WISE