We have retold 2 papers by this author. most often about: cosmic rays, supernova remnant, active galactic nucleus. 30 co-authors across our retellings.
Cosmic rays are not light but charged particles bombarding Earth from all directions. Most of them are protons (hydrogen nuclei), but heavier nuclei also show up: helium, nitrogen, iron.
The hot gas in galaxy clusters shines in X-rays, and a significant portion of that light comes from iron atoms that have lost many electrons. These atoms leave characteristic bright lines in the spectrum, like fingerprints. Astronomers use them to determine the temperature, density, and motion of the gas.