 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to talk about cosmic rays. So these are not electromagnetic rays like light and radio waves and x-rays, but are actually particles from space that strike Earth. So these were discovered in 1911 by Victor Hess, and in order to see them, they don't get down to the ground. They don't strike the Earth's surface. They get absorbed early up and high up in the Earth's atmosphere. So in order to study them, we need to get up higher in the Earth's atmosphere, not above the atmosphere. As shown here, it was done first in a balloon to get higher in the atmosphere where there's less atmosphere and better able to detect these particles. Now to emphasize, these are particles. They are not light rays. And they have a similar composition to interstellar material. They are mostly hydrogen, meaning protons, individual protons traveling through space. A little bit are helium and heavier nuclei, and a small fraction are electrons and positrons traveling through space. Now what do cosmic rays tell us? We learned since their composition is similar to the star's interstellar material, but they have a higher proportion of light elements. Now remember, there's hydrogen, helium, and metals, but three elements in between the hydrogen and helium, and then the carbon, nitrogen, and oxygen, which are also very common, are lithium, beryllium, and boron. These are elements that are not formed in stars. So they are more common in cosmic rays, and are actually formed through collisions of cosmic rays together, but they do not naturally form within stars. So when we find these elements, they are formed through cosmic rays. Cosmic rays are blocked by Earth's atmosphere. That's great for us. They don't come streaming down and striking Earth at very high speeds. However, we can observe their interaction with particles in the atmosphere, and that helps us to learn more about them. Now where do cosmic rays come from? Well, they have a couple of different sources. Some are produced by our sun. So here's the sun, particles coming from the sun streaming around Earth's magnetic field, but we do get some particles, things like coronal mass ejections, will eject particles towards Earth, and that can be a source of some cosmic rays. Most, however, come from outside the solar system, but they are hard if they're impossible to track. Why? They don't travel in straight lines. Light travels in essentially a straight line, so we can look at something and trace back where it is. However, these particles are changed by magnetic fields, and their directions are changed, so where they appear to come from has no relation to where they actually originated. So it's very difficult. They're deflected by the galactic magnetic field, by Earth's magnetic field, by the solar magnetic field. All sorts of magnetic fields will deviate their paths a little bit, so we can never tell exactly where they came from. We do know that they're traveling at high speeds, 90% of the speed of light, and we believe that most cosmic rays come from within our galaxy. What is a likely possibility of this? Supernova explosions. So a massive supernova explosion would expel out a lot of particles at a very high speed, which could then continue to travel through space until they end up striking something. Although we do believe that many of the highest energy rays may come from other galaxies, and we'll look at other galaxies and how they produce some energetic particles later on. Now, when we see the cause of the particles, the particles will travel along these magnetic fields, so they get deviated generally away from Earth, but some will get funneled down into the poles, and that's where we actually get our aurora. So the aurora will occur near the northern and southern magnetic poles. Now, we looked at detecting cosmic rays through balloons. How else can we detect them? Well, we could get up in space. We mentioned balloons previously, and we look at interaction with plastic sheets that will trap the cosmic rays and then allow us to be able to study the particles that were trapped. We also have, we could get up into space and observe them from satellites, but we can also observe them from Earth, and one thing is the veritas which detects gamma rays produced when the cosmic rays interact with the upper atmosphere. So these high energy particles strike atoms in the Earth's upper atmosphere, produce gamma rays which then are visible as a flash of light. So as those gamma rays travel through, they will interact and will give off a flash of light so we can detect and then interpret back what was seen there. So we have things like the veritas array shown here. One way to detect these cosmic rays, very high energy particles from within our solar system, within our galaxy, and even outside of our galaxy. So let's go ahead and finish up with our summary here. And what we looked at is that cosmic rays are high velocity atomic nuclei that are traveling through space. Magnetic fields will deviate their paths, meaning that we cannot find their direct source of origin, but we believe most of them come from our galaxy in supernova explosions. So that concludes this lecture on cosmic rays. We'll be back again next time for another topic in astronomy. So until then, have a great day everyone, and I will see you in class.