NGC 6334 is an emission nebula in Scorpius., also known as Cat's paw nebula

When an electron from an hydrogen atom falls from a higher to a lower energy level, the energy difference is emitted as a photon.

This image is taken through a filter that selects the Hydrogen Alpha photon, the smallest fall in the Balmer series ( from level 3 to level 2 of the different levels allowed for an electrron).