Introastro Edited Lecture Transcripts Edited Lecture Transcripts Week01

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Introduction to Astronomy, Week 1Edited Lecture Transcriptsprovided by Derek Grainge during session 002, 2013-2014

1 Positional Astronomy Introduction

Today we're going to start our class, and the best way to start a class on astronomy, is to just go outside and take a look at the night sky. See some of the objects we're going to talk about in their actual natural habitat so to speak. There's far too many of us to fit in any one place and we're scattered too widely over the globe, but through the wonders of technology I am going to invite you, in a minute, to do a, an, virtual night sky tour with me, and I encourage all of you to take either a virtual tour or even better, if you can, to go outside and take a look up at the sky. Familiarity with the night sky is not the main object of this class. But it's a very pleasant side effect and I hope you'll enjoy it. So, without further ado let me invite you to join me virtually in field outside of Athens, Greece for definiteness, and we'll look up and see some of what we can see in the sky. Welcome to our observatory. We've chosen to locate ourselves in the outskirts of Athens, Greece. It is November 27th at 9 p.m. And when our software is simulating the sky, and we're looking to the south, we have a very broad feel of the view of a full 180 degrees. So we can see all the way from the eastern horizon to our left, to the western horizon to our right. But because of the light pollution of a nearby city we see very few stars in the sky. This might be what can see, or it's certainly what I can see when I step outdoors, but the brightest stars in the sky are bright enough to shine against the glow. So we might see over here in the east, Betelgeuse and Rigel, and perhaps Capella and Aldebaran. And over here on the western horizon we'd see three very bright stars. Vega over here, Altair down there and Denad over here, and these make up what is normally called the summer triangle. Let's step a little farther away from the city where more stars are visible, and try to get a better look. There that's better. With software it's much easier to turn off light pollution than it is in real life, and now we see the same view of the sky as it must have appeared before the invention of electric lights. And let's take a look and name some of the objects we see. Over here to the east we saw the two bright stars Betelgeuse and Rigel, which are now shown to be, seen to be, part of the famous constellation of Orion the Hunter. There's his chest and his legs, his belt from which hangs his dagger. His club or bow over here, his outstretched arm, he even has a head up there. And that would be Orion the Hunter, and Aldebaran will be part of Taurus the Bull. There's his head and his body and his long outstretched horns. Looking farther to the right we can identify some other famous groups. This w shaped object is associated with Cassiopeia the vain queen. And not far from here, this great square of Pegasus. And from the great square Pegasus towards Cassiopeia, these two streaming lines of stars associated to Andromeda, Cassiopeia's daughter. And up here Vega, the bright star we saw in the western sky, is part in fact of Orpheus Lyre. This is the constellation Lyra, and finally my favorite constellation - we talked about Deneb over here, and Deneb is just the tail of Cygnus the Swan. There's his tail and southern his heart. And a long, swan neck that ends at Albireo's head. And then his pretty, swept back, swan wings. You can certainly see a swan. There are many other constellations but I don't have time to show them all. Let's allow the software to do that for us. Here I let the software designate for us the most famous of the constellations, and the brightest stars that comprise the shape or the asterism of the constellation. And we see Orion the Hunter and Taurus the Bull, and Pegasus the Flying Horse, and so on, as well as many others. And what we recognize is that many of them have their origin in Greek Mythology, at least in the northern hemisphere. The reason for this is. And the point I'm trying to make with this image is that if you show this picture, without the helpful blue lines, to someone who lived in Athens thousands of years ago, I don't know the, this, the philosopher Aristotle. He would not only recognize the stars he would name the constellations and he'd use the same names that we use. Because the pattern of the sky as Aristotle saw it on a fall day in Athens 2000 years ago would be extraordinarily similar, essentially identical to what we see today absent light pollution. There is this beautiful permanence in the sky. The stars that we see today are essentially the same as they've always been, and this is something that has made astronomy one of the oldest sciences. It's also something we can use to establish a geography of the sky. So that if I want to describe or if an astronomer wants to describe viewing an object that lies right here, even if it's not part of the asterism of Orion, he would say the star lies in the constellation Orion. Astronomers have formalized this. By, in recent memory, actually dividing the sky into 88 regions. In the northern hemisphere they're associated ancient Greek constellations for the most part. And so now we precisely can distinguish a star that lies in the constellation, Orion from some nearby star. That lies in Taurus, and when we talk about stars lying in one constellation or the other, I will be referring to this treaty that divides the sky into the 88 constellations. The point that this again is trying to remind us of, is that a star that is in Orion now will be in Orion a century hence, and so there's this geography that we set up, and we can identify positions in the sky. Removing for a moment all the distracting lines that the software was drawing, let's take a closer look at some of the objects we're seeing and try to pay attention again to what we're seeing. So we see large numbers of stars, we notice that they were different colors. Some of them are more reddish or orangeish or yellow, and some are blue. The colors of stars are in themselves an interesting topic, we'll talk about that. The distribution of stars is interesting. If you look you'll see that there are less stars here or here, than there are along this region of the sky. This stripe through the sky, and in fact, if we look closely we'll see that even in between the sky, stars, the sky here's brighter than elsewhere. This is of course, the famous stripe of The Milky Way. We'll want to take a close look at that. Here's what part of The Milky Way looks when you take a slightly long exposure photograph. The exposure brings out many, many stars, and we see that this bright stripe in the sky that I call the Milky Way has a lot more structure. It's not just a white stripe. There are these darker areas and brighter areas, different colors, lots of stars. We'll take a close look at that and try to understand what the Milky Way is. There are some other objects visible in the sky that we'll justify a closer look of the form that Aristotle would not have been able to take advantage of. If we look at Orion here, and if we look at stars, the three stars that form the dagger dangling from Orion's belt, and certainly the middle one. Its pretty clear, it does not really look right. There's something fuzzy about that star. Well take out a telescope and get a closer look at that. The slightly fuzzy, suspicious star in the center of Orion's dagger that we mentioned, turns out in a moderate telescope, to be a lot more interesting than you'd think. In fact all of this stuff, all of these stars, all of this nebulous scattered light, blue and pink, these darker regions. All of this is what our eye sees as a slightly fuzzy star. And understanding what is going on in the Orion Nebula, is going to be something that'll be very exciting for us. Slightly farther south over here just past Taurus, and in the Constellation Taurus in particular, is this weird collection of stars, a whole bunch of stars, clustered together very close, the collection of seven bright stars in the Constellation Taurus, call, traditionally called Pleiades in some cultures, The Seven Sisters different names for this obvious bright, collection. Here are the seven bright stars we see that there are many others near them. Just not bright enough for us to see, and even again, as in Orion, we see that there is this nebula stuff, there's this blue, shiny stuff that is not a star that is producing light. So some of the light we see is not coming from the stars at all. And, if we look at this collection of stars then perhaps it's worth looking just below Pegasus over here. If we looked very closely there would be a faint and slightly fuzzy object, probably better seen with a telescope than the naked eye. In a moderate telescope the group, slightly faint fuzzy object in the constellation Pegasus that I mentioned turns out to be this beautiful globular cluster called M13. Again we see many, many, many stars far more in fact than in the Pleiades. We also see that they're all dimmer. The stars in the Pleiades were bright like this. These are dimmer objects, and they also appear to be very much more tightly clustered. In the constellation of Andromeda up here in between Pegasus and Cassiopeia, if we look around here there's another one of those fuzzy objects, suspiciously not quite star-like. If you magnify it a little bit and collect light for a long time and process carefully, turns out to be this beautiful richly structured object, the Great Spiral Galaxy of Andromeda. I should point out that the fuzziness that we see is only the brightest central part of this. This object itself is about five times the size of the full moon in the sky. It's very large and very, very faint and requires careful image processing and light collection to get all this structure out. But nevertheless, our eyes were not misleading us, t