Kerr, F. J., et al. 1986, Astron. Most of the emission is thermal, from interstellar dust warmed by absorbed starlight, including that in star-forming regions embedded in interstellar clouds. Wheelock, S. L., et al. Owing to the strong obscuration by interstellar dust the light is primarily from stars within a few thousand light-years of the Sun, nearby on the scale of the Milky Way, which has a diameter on the order of 100,000 light years. Nebulosity from hot, low-density gas is widespread in the image.
Dark patches are due to absorbing dust clouds.
In the Milky Way, extended soft X-ray emission is detected from hot, shocked gas. Color variations indicate variations of absorption or of the temperatures of emitting regions. Snowden, S. L., et al. At these extreme energies, most of the celestial gamma-rays originate in collisions of cosmic rays with hydrogen nuclei in interstellar clouds. The bright, compact sources indicate high-energy phenomena associated with pulsars.