 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 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 a hundred 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. 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 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 used 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 was the 300 meter Eroscebo telescope and to give you an idea of how large this telescope was if we went across it from edge to edge that was about 300 meters which would be about three football fields that would fit across the width of this telescope and this telescope was completed in 1963 and for many decades was the largest single dish radio telescope in existence and used for all sorts of studies of the sky. Now because it was such a large telescope it was not steerable as the Green Bank telescope was you couldn't point it to any direction in the sky but you could look pretty much at what was straight overhead and you could use the detector up above strung by cables up here that would move a little bit and allow you to look at slightly different regions of the sky but still it had to pass pretty much overhead at Eroscebo in Puerto Rico in order to be able to detect that object and over those decades it did a number of things including information on searching for extraterrestrial intelligence sending out messages that are still traveling out through space today now until 2016 this was the largest telescope in existence single dish in 2016 there was the fast telescope in China which is 500 meters and so slightly bigger than this and in fact not quite twice as big as this one now the Eroscebo telescope is no longer in operation in fact in 2020 it did collapse in part and here we can see part of that where cable fell and actually damaged some of the mechanism here so we can see lots of it that was damaged here and after studying it for a couple of months and a second partial collapse that actually caused the damage that we see here it was decided that the telescope would be decommissioned and taken apart but you can also see underneath here some of the support structures holding the telescope up so using actually the earth to hold it up as a strong support and a good way to make a much larger telescope than would otherwise be possible we looked at the green bank telescope but this is many times larger than that and needed a much stronger surface to be able to hold it and what could be stronger than actually using the earth itself as a lot of the support for this telescope so let's finish up as we do with our summary and 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 I'll see you back again next time for another topic in astronomy so until then, have a great day everyone and I will see you in class