 Greetings and welcome to the introduction to astronomy. In this lecture, we are going to discuss radio telescopes. So optical telescopes use visible light to study the universe, and in this one we are going to look at how radio telescopes can be used to get another view on the universe. So let's take a quick look at radio telescopes. And what we have here is, first of all, what is a radio telescope? Well, we have to recall that radio waves will also penetrate the atmosphere and we can observe them from the ground. So radio telescopes can be based on the ground just like optical telescopes. The interesting thing is that they give us an entirely new view on the universe. So it is a completely different wavelength of light. These are very long wavelengths compared to optical light and very low energy photons. So a very different view of the universe. We are going to see some very different things and get some very different views when we look at things with radio telescopes. There are objects that are visible in radio telescopes that are completely invisible to optical telescopes. And of course there are objects visible in optical telescopes that cannot be seen by radio telescopes. So it gives us a completely new view of the universe. But to qualify here, before we get too far in, radio waves are not sound waves. Sound waves cannot travel through the vacuum of space. So you cannot hear radio waves. It's not that stars and galaxies are making noise. That would be a completely different type of wave. Remember, radio waves are electromagnetic radiation and can travel through the vacuum of space. We use radio waves here on Earth to encode sounds in them and then re-decipher those codes out afterwards. So we use them to transmit information here on Earth by encoding it in the radio wave, but they are not directly sound waves themselves. So let's look at the beginnings of radio astronomy here and what we have was starting back in the 1930s, Carl Jansky built some of the very early radio telescopes. Now you can see here that this does not look anything like any telescope that we've looked at before. It's essentially a big antenna. And that was a way that he had used to be able to detect radio waves from space. Now that wasn't what was intended. He was studying radio communication here on Earth and found radio waves from some unknown source that was not known of. What he was finding, what we eventually figured out is that because he was detecting them four minutes earlier each day, that they were coming around every 23 hours and 56 minutes. And that is the rotation period of the Earth relative to the stars. So that was what we called the sidereal rotation period. Now because of that, that means that these objects were not associated with the Earth and they could not be associated with the Sun. They had to be something else out in space. And what he was detecting was the galactic center. The center of our galaxy is invisible. In visible light, you cannot see it with looking at visible light, but in radio waves it is one of the strongest radio sources in the sky and therefore Carl Jansky was able to detect it with his early instrumentation. Now when we look at various objects, they can tell that they are quite different when we look at them in optical or radio. Now this one shows what was called the Crab Nebula. The Crab Nebula is a supernova remnant. So a supernova that occurred back in the back in about a thousand years ago. Now when we look at it in visible light, this is the kind of image we will get. However, if we look at it in radio waves, we get somewhat similar, but different intensities. The different intensities here the red shows the highest level of radio intensity, which is not necessarily where the brightest visible light is coming from. And when we also look at it in other wavelengths like x-rays or gamma rays or ultraviolet or infrared, we get a different picture. So what kind of things can we study? We study a couple of different things in radio astronomy. We can look at very cool objects, which would be molecular clouds, and those are very cold, so they're not giving off much visible light, but they are giving off radio waves, and we can detect those. However, we can also detect very energetic objects, and that is something like the Crab Nebula. In the Crab Nebula, when you accelerate electrons, you can give off another type of radiation that is radio radiation called synchrotron radiation. And that can then be detected by a radio telescope. So not only do you detect cool objects, but you can detect very energetic objects as well. And again, the big key is that I like to point out is that you are getting a completely different picture of the universe. And if you recall, just 100 years ago, all we had was visible light. That was the only way we could observe anything. Now we can observe objects across the spectrum, and it gives us a much more complete view of the universe. So let's look at some of the advantages and disadvantages of using radio telescopes. And some of the advantages here, first of all, you can observe 24-7. Radio waves are not affected by daylight and not much affected by weather, so you can actually observe during the day. So radio telescopes, you can see them working all day long. You don't have to wait for the sun to go down. Now honestly, you cannot point close to the sun because the sun is a radio source as well. So as long as you're not looking in the general direction of the sun, you can still be able to observe even during the day. Weather is not much of an issue either. If it's raining or snowing, you can still observe with a radio telescope. Now if you have a thunderstorm where you're getting electrical discharges in the atmosphere, then that is something different. And in those cases, you would not be able to get accurate observations. You would not be getting problems because of the interference of the electrical discharges from the radio. But the key point, one of the things I said previously, is that you get a completely different view of the universe. You are looking at a new wavelength, and you're able to see things that you couldn't see before. For example, radio waves penetrate dust and allow people like Carl Jansky to be able to detect radio emission from the center of our galaxy. We cannot see visible light from the center of our galaxy. There is too much dust in the way, and it is completely blocked out. We can also detect cool hydrogen gas that gives off no visible light. It does give off radio wavelengths, so it can be detected by radio telescopes, even though it cannot by visible telescopes. Now some of the disadvantages of radio telescopes, one is the poor resolution. If you recall, the resolving power of a telescope depended on the size of the telescope, so bigger telescopes gave you better resolving power. However, we also look at, it also depends on the wavelength, and if you're looking at a very long wavelength, the resolution gets worse. Now radio wavelengths can be things in the centimeters or so in wavelength, and that means that their resolution is many, many times worse than those of optical telescopes. Therefore, we need to make very large telescopes for radio astronomy. And a small radio telescope would be something maybe in the 20 to 30 meter range, that would be a relatively small radio telescope, and that would be comparable to some of the very largest optical telescopes that are being planned right now. We also, like we get light pollution, we can also get interference with radio signals on Earth. So radio signals being transmitted on Earth can also be detected and cause a source of interference for radio telescopes. Now one way we can improve and increase the resolution is by using what we call interferometry. So interferometry up here is a method that astronomers use to increase the resolution of a radio telescope. It was developed back in the 1950s as a way of improving the resolution because the resolution of radio telescopes was so poor compared to others. It's now being expanded to use at other wavelengths as well, so it's not limited just to radio telescopes. And what is done in this is that you observe the same object at the same time with multiple telescopes and then combine the signals together. And what this does is to increase the resolution of the telescope. You're increasing the effective size of the telescope to the distance between them. So if you have telescopes that are kilometers apart, you then have an effective telescope with a diameter of several kilometers. And you can put those further and further apart. So we can use this in radio astronomy to match or even exceed the resolution of optical telescopes. So let's look at a few famous radio telescopes. This is the VLA here, the very large array, which is a set of 27 telescopes that are set out in the desert of New Mexico. And they can be used as a large interferometer. So they can be a number of kilometers apart, and you can set them all to observe the same object and then combine those signals together and with various mathematical methods, then be able to essentially simulate a much larger telescope. Now in terms of telescopes with just one dish, we have the 100 meter, the Green Bank Telescope, which was completed in 2001. So 100 meters across, that's about the size of a football field across, and that was completed in 2001 and is the largest fully steerable radio telescope. So when you think about that, you're actually steering something larger than a football field here, able to steer that and point at different parts of the sky. When you get a telescope like this, you can see that it has a curved dish much like a mirror. It does not need to be near as smooth as an astronomical telescope, as an optical telescope mirror, because the wavelengths are so much longer. It has to be smooth relative to the actual wavelengths that you're observing. So it does that, and then it reflects, and then there's a detector up here that then can detect the radio waves and send signals down to the control room. Now the largest, one of the largest radio telescopes is the Arecibo Telescope. This one is 300 meters across. So you can imagine then three football fields sticking across here. That is simply too large to be able to steer. The engineering to be able to support something like that would be tremendous. This was completed back in 1963 and is not supported by a structure, but is supported by the Earth itself. It is actually hollowed out into a valley in the hills of Puerto Rico here near Arecibo. So it uses the Earth as the support mechanism. That means it can only look straight overhead, and so objects that pass overhead are very close to overhead in Puerto Rico are things that can be observed by this. Now this was the largest single dish telescope until 2016 when another one with a similar structure was done and that was the fast telescope in China, which is actually 500 meters across or half of a kilometer across. So larger and larger telescopes continue to be built to be able to see much more detail. So let's finish up with our summary here. As to what we've looked at in this section, we've talked about radio waves, that they are also things that we can observe from the surface of the Earth because they do penetrate the atmosphere, so we can study them here from the surface. They do give us a completely different view of the universe, and that's very important that we're now getting a new perspective on the universe as we're able to observe further across the electromagnetic spectrum. And we talked about interferometry and how that is used to increase the resolution of radio telescopes, making them comparable to or even better than those of the optical telescopes that are used. So we can then compare objects across the different types of wavelengths and be able to get better studies of them. So that concludes this lecture on radio astronomy. 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.