High Energy Particles From Space
Astronomers are popularly supposed to observe the cosmos using visible light. However, today they use the whole 70 octave electromagnetic spectrum from the longest wavelength - 20 meter - radio waves to the highest energy - 100 TeV - gamma rays. The gamma rays in particular are thought of as particles, called photons, because that is the way they are detected. Over the last fifty years it has become clear that sky is as rich and interesting in all regions of this spectrum. Many source that have been discovered by Fermi Gamma-ray Space Telescope and Cerenkov telescopes like HESS and VERITAS are quite undistinguished at optical wavelengths. In addition important astrophysical processes that control how the universe and its denizens change over time can only be studied in this way.
Even older is the study of cosmic rays which were discovered just one hundred years ago. Traditionally, cosmic rays comprise charged particles such as protons, electron and most of the nuclei found in the periodic table of elements. Those that come from the sun travel more slowly than photons, while the most energetic cosmic rays detected have the energy of a well-hit baseball! Understanding how cosmic rays are produced is a big challenge to astrophysics and particle physics and is important for seeking indirect evidence for dark matter. Some cosmic rays, called neutrinos, are uncharged, like photons. and propagate in straight lines from their sources. The have been seen from a recent supernova explosion and their high energy, cosmic counterparts are being sought.