What light energy is specifically used to pinpoint the hottest, youngest stellar populations and very hot gas?

Ultraviolet (UV) light, which is higher energy than visible blue light.

Different wavelengths of light provide unique insights into the various components of a spiral galaxy. While blue light indicates hot, young stars, ultraviolet (UV) light offers an even more sensitive probe of the highest energy emissions. UV radiation is inherently higher energy than visible blue light. Consequently, observations mapped in the UV spectrum closely mirror the distribution of the very brightest, youngest stellar populations, often showing even greater intensity right at the shock front where the compression is most immediate and energetic. This specific wavelength range is critical for identifying the hottest stellar environments and associated hot gas.