 Greetings and welcome to the Introduction to Astronomy. In this lesson, we are going to talk about cosmic rays. So this is a different type, a different way of being able to study the universe, not through light rays, as we're used to and as we've talked about in the past, using different types of electromagnetic radiation, such as x-rays and visible light and radio waves. But cosmic rays are actually particles from space that we can study as well. And these have been known for over 100 years now and we have studied them since 1911 when they were discovered by Victor Hess, who was using balloons to try to get up higher in the atmosphere. So you can see the gondola here in the balloon structure right up above and then the balloon would have been up above that, and he would use those to get higher up in the atmosphere. Now cosmic rays cannot be detected from the surface of the earth because they are blocked by the earth's atmosphere, so our atmosphere protects us from being bombarded by these high energy particles. And as I specified, they are particles, not light rays, that as the other types of electromagnetic radiation that we have studied in the past. They are essentially the same composition as the regular interstellar material or anything else that we look at in astronomy. They are about 90% hydrogen, nuclei, protons, about 9% helium or heavier nuclei, and about 1% are electrons and positrons, and if you recall the positron is the antiparticle of the electron, so it is an antiparticle of the electron. Now when we look at these, there are some differences that we see, so overall the rough composition is similar, but there are some differences compared to other things that we've seen. And when we look at the abundances of things in cosmic rays, we said that they're similar to them, but they have a higher proportion of the light elements, so not heavier elements but the lighter elements, and not light as in hydrogen and helium. Hydrogen and helium make up the vast majority of everything that we see. But the three elements in the periodic table that come after hydrogen and helium that are relatively rare, lithium, beryllium, and boron are very rare elements in the universe. And what happens is when we form helium, helium fusion in stars, you have three helium atoms fusing to form one carbon atom. So three helium atoms combine together and we jump over these elements. And that's because if we try to fuse two helium atoms together, the product is unstable and immediately breaks apart. So the only way to form heavier elements in stars is to skip over these. So where do they come from? Well, we believe that they are formed by these cosmic rays and collisions within cosmic rays. So much of the lithium, beryllium, and boron that we see is formed not in stars as other elements are, but are actually formed in cosmic rays, these high energy particles streaming through the universe and occasionally colliding with each other. And those helium nuclei and those protons could then form these slightly heavier elements. Now fortunately for us, as I've mentioned, cosmic rays are blocked by the atmosphere of the Earth. So they are unable to get through the Earth's atmosphere. How do we detect them from the surface of the Earth? Well, that's hard, but there are ways to do it. We can observe their interaction with particles in the atmosphere. So when they collide in the upper atmosphere, they produce a stream of high energy particles and we can use those to work backwards and figure out something about the cosmic rays. So we take these interactions, use them to learn more about the cosmic rays itself, even though from the Earth's surface we cannot see them directly. Now other ways would of course be to get up above the Earth's atmosphere to observe these. Up above the Earth's atmosphere, then you would be able to detect the cosmic rays directly. Now where do these come from? Some of these cosmic rays are produced by the Sun, but the vast majority come from outside of the solar system. They are not a part of our solar system. And the difficulty with them as compared to light rays is that they are very hard to track. They do not travel in straight lines. So if we have a source over here of the particles and we're over here on the Earth, light electromagnetic radiation will travel in straight lines pretty much and come to us right here on Earth. However these other ones can travel and will travel along the magnetic field so they can follow long winding paths to get to us here on Earth and therefore how do we know where they came from? When it comes from this direction we see it as coming from out here, whereas the real source was over here. So magnetic fields can change these paths of particles that they cannot do to electromagnetic radiation. They do travel at high speeds, they travel at about 90% of the speed of light, so a little bit slower than the light and the electromagnetic radiation itself, but they do travel still at very, very high speeds. Most of them come from outside our solar system and they do come from within our galaxy. We think that perhaps supernova explosions cause, give us a lot of these cosmic rays. And that would be a massive explosion would send out streams of these particles that could eventually just travel, would be traveling through space and remember as they travel through space there is nothing to slow them down. They just keep traveling so if they are emitted at very high energies they continue to move at those very high energies. So they could travel there for thousands or millions of years at those very high speeds until they eventually strike something like the Earth. But the very highest energy ones that we see could actually come from other galaxies so they could come from outside our galaxy and stream, and stream into the Earth as well. So when we look at our image here we do have some that are coming from the sun, from perhaps a coronal mass ejection, charged particles coming out, those would strike the Earth's magnetic field. There could be galactic cosmic rays coming from all other directions and there could be those very high ones that are actually coming from other galaxies and maybe even more energetic events. So how do we go about detecting these? Well there's a couple ways that we can do this. One is the veritas array and that detects not the cosmic rays themselves. You can see the image here it's actually on the surface of the Earth so the cosmic rays are not going to make it down to the surface of the Earth. But when those cosmic rays strike particles in the Earth's atmosphere so they strike a particle here that will then produce gamma rays which can then come down to the Earth and be detected. So the particle itself does not make it to the Earth, this is the particle, but it does not make it to the surface of the Earth however other gamma rays can be detected and these can detect those flashes of gamma radiation that come down to the Earth from these particles and can then trace back and try to learn a little bit more about these cosmic rays. We can also use balloons, put balloons up in atmosphere and use sheets of plastics that will then interact with the cosmic rays, detect them, catch them and then use those again to work backwards to try to learn a little bit more about what these cosmic rays are. But again as I've said they're very difficult because they don't travel in straight lines so we don't know exactly where they're coming from and that makes it much more difficult than studying ordinary types of electromagnetic radiation and they're also hard to detect directly because of their location and that they do not make it down through the Earth's atmosphere. So let's finish up this section as we do with our summary and what we talked about with cosmic rays and what they are they are very high velocity, 90% of the speed of light, an atomic nuclei that travel through space. They are difficult to track down because magnetic fields will cause their paths to deviate making them difficult to trace back to their source of origin so we don't know for sure where they come from but we do believe based on studies that most of them originate within our galaxy and probably from 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.