 The Earth is constantly bombarded by radiation from outer space called cosmic rays. These cosmic rays are made up mostly of high energy protons and high energy photons called gamma rays. Cosmic rays collide with atoms in our atmosphere, generating particle chain reactions that continue all the way down to the surface. In our first segment on the Milky Way, in the How Far Away Is It? video book, we noted cosmic rays are produced in supernova, like the one that created the crab nebula. Gamma ray bursts are the most energetic and luminous electromagnetic events in the universe. They can release more energy in 10 seconds than our sun will emit in its entire 10 billion year expected lifetime. It is the very high energy of the deep space cosmic rays that have the power to smash into electrons, protons and neutrons in the atmosphere, creating a wide variety of previously unknown particles. Charles Thompson Reeves Wilson invented the cloud chamber in 1911 to detect these particles. So physicists climbed mountains like this one that Wilson used, and went up in balloons with their cloud chambers hunting for new particles. Here's what it looks like when charged particles pass through a cloud chamber. The chamber contains a vapor of alcohol placed inside a magnetic field. When charged particles move through the chamber, they cause little droplets to form. These are the cloud tracks that we see. It works on the same principle as tracks forming behind high flying jet aircraft. Here's what cosmic rays look like in a cloud chamber. These rays are penetrating multiple brass plates, each 13 millimeters thick. To understand these tracks, we'll start with the two charged particles we already know, the electron and the proton.