hi everyone and welcome to astronomy 1 P1 at at Brock University I'm Professor Barak shashani and I will be your instructor for this course so this first lecture is just going to be an introduction we're going to learn some basic concepts in science and astronomy get an overview of some of the fascinating things we'll learn about and we'll also discuss scales what are the smallest and largest things in the universe so first of all what is astronomy astronomy studies celestial objects anything that exists outside of Earth and this includes objects such as planets moons asteroids comets Stars black holes galaxies and the universe as a whole astronomy is a science so to understand how astronomy works we must first understand how science Works science is not a body of knowledge it's a method for obtaining and verifying knowledge about our universe in science we make observations and experiments and use them to create hypothesis that try to explain how things work scientific hypotheses have predictions which need to be tested experimentally if an experiment disagrees with the hypothesis we need to modify or discard the hypothesis but if enough experiments agree with the predictions of a hypothesis then it eventually becomes an EST lished Theory now it's important to understand theory in everyday language means the same thing as hypothesis or speculation but in science theory means a hypothesis that was rigorously tested and verified so a theory is an accurate explanation of how things work now one once a hypothesis becomes a theory we can use it to understand the universe better predict the results of future experiments or events and create new technologies these by the way are basically the three types of scientists the theorists the experimentalists and the applied scientists or Engineers but it's always possible that other experiments in the future will contradict the theory and then we need to find an even better theory in this way science is self-correcting and always moves forward our understanding of nature becomes better and more precise with each new Theory this process of creating hypothesis and then testing them is called the scientific method one of the most important components of this method is skepticism scientists remain skeptical about any new hypothesis until there is enough evidence supporting it because of skepticism the scientific method is the only reliable and trustworthy method of obtaining knowledge scientists don't trust theories based on belief or faith We Trust theories because we test them and find evidence for them and it's very important to understand that no theory is sacred if you find any evidence that contradicts a theory then we don't trust it anymore and we try to find the better one so just so you know anything you learn in this course might turn out to be false or at least to be not 100% accurate sometime in the future but that's a good thing because if this happens if we discover that something that we thought was true is actually not true that means that we further improved our understanding of the universe like any other science astronomy changes constantly new theories attempt to explain things we could not explain before and the new instruments allow us to make more precise measurements this goes all the way back to the beginning of astronomy ancient astronomers had a model of the universe with the Earth at the center we can see it in this book here from 1550 and this model is called the geocentric model but with more precise measurements the predictions of this model failed so this model predicted certain things such as where each planet is going to be in the sky at any point in time and at first the predictions were accurate but after a while first of all the predictions became less and less accurate with time and also we made more precise PR measurements using more precise instruments and we discovered that this model doesn't really give 100% accurate predictions eventually astronomers realized that the sun is actually at the center this is now the accepted Theory because its predictions fit our observations now is this the final Theory in this case I would say probably yes because with the theory that the sun is at the center of the solar system we can predict the positions of all the planets in the solar system with extremely accurate Precision so it's very unlikely that this theory is ever going to be replaced with something else but there is one thing that we discovered later after we had this theory that assumes the sun is at the center of the solar system we discovered that the sun is actually just one of numerous stars and the universe has no Center so the theory that the sun is at the center of the universe proved to be incorrect but the theory that the sun is at the center of the solar system is correct now you may think that in the 21st century we already know everything we need to know about astronomy but in fact that is not the case there are many unanswered questions including what are dark matter and dark energy what is at the center of a black hole and and does life exist on other planets the main job of astronomers and astrophysicists is to answer such questions but it may take decades or even centuries to answer some of them astronomy is different from most other Sciences because astronomers can't do experiments in a lab they can only observe observe astronomical objects that are located incredibly far away and they do this using instruments such as telescopes as technology improves these instruments get better and better and allow us to make observations in Greater detail and in different ways here's an example as you may know light is a type of electromagnetic radiation that humans can see and we're going to talk more about that later but there are other types of radiation such as infrared X-rays and radio waves which we cannot see but can be detected by instruments we can observe the sky not only with light telescopes but also with radio telescopes that see things that we could not see using just VIs ible light we can even place telescopes in outter space this allows us to observe without being obstructed by the Earth's atmosphere now this might be a good time to plug in my own research so one of my main field of scientific research is general relativity this Theory provides a precise description of the force of gravity and explains how space and time walk and how they get combined into space time general relativity completely revolutionized astronomy it introduced new celestial objects such as black holes by the way this is not a real black hole this is just a simulation of how a black hole would look like but we do have photos of actual black holes that I'm going to show you later later general relativity also introduced new tools to prove the sky such as gravitational lensing and gravitational waves and it helped us understand the origin of our universe in the form of the big bang and here you can see a timeline of the universe starting from the Big Bang at the left at essentially the beginning of time and then advancing in time until we get to today which is 13.8 billion years after the Big Bang on the far right I'll talk much more about all these things and my own research in astronomy 1 P2 next STM now in astronomy we often measure distances in a unit called a light TI this is the distance light travels during one year now it's important to understand that even though it has the word year in it a light ear is not a unit of time and to see why that is remember that when you say I'm 5 minutes away you're actually saying how far away you are five minutes away is the average distance a human walks in five minutes or maybe drives if you're in a car so we see that even though you're specifying a duration of time you're actually measuring a distance now no human walks at exactly the same speed all the time but light always travels at the same speed the speed of light that's one of the most important lessons of my field of research the theory of relativity also the speed of light is the fastest possible speed so light always moves the same distance at the same amount of time and it gets there faster than anything else so it makes sense to use light ear as a unit of distance so just to be clear we use light as a unit of distance because we know that in one year light anywhere in the universe and any kind of light will always move the exact same distance so that lets us Define a precise distance that is constant and call it a lie now the speed of light which like I said before is always the same is approximately 300,000 km/ second or if you prefer 1 billion kilom per hour now remember this basic formula distance is speed multiplied by time so to calculate how many kilometers are in a li here we know the speed we just need to know the time so first we need to calculate how many seconds are in a year now in astronomy we use a Julian year which is defined to be exactly 365.25 days long multiply that by 24 hours a day to get the total number of hours in a year multiply it by 60 minutes per hour to get the total number of minutes in a year and finally multiply that by 60 seconds per minute to get the number of seconds in a year and you'll get approximately 31.6 million seconds so remember distance is equal to speed multiplied by time so the distance of one light here is equal to the speed of 300,000 km/s times the time time of 31,600 th000 seconds and that equals approximately 9 and5 trillion kilometers now just in case you're not sure what a trillion is let me just review some common large numbers here so a thousand is one followed by three zeros a million is one followed by by 6 zeros a billion is one followed by 9 zeros a trillion is one followed by 12 zeros and we also have larger numbers than that we have quadrillion which is one followed by 15 zeros and quintilian which is one followed by 18 zeros and there are even larger numbers that I'm not going to mention right now so 9 and half trillion kilometers means 9 and halftimes 1 followed by 12 zeros this is a very long distance if you drive a car at 120 km per hour it will take you about 9 million years to drive a distance of just one light TI well this is all the time we have for today so we'll continue next time Time by talking more about astronomical distances so welcome to our second lecture last time we ended by talking about some common large numbers and try to understand how long a light here is basically so now I want to give you some idea of astronomical distances so one light till like we saw is a lot it's N9 and a half trillion kilometers and if you drive a car at 120 kilm per hour it's going to take you 9 million years to drive just one light here so that's already quite a lot but actually most of the astronomical distances of Interest are much larger than one light year so here's an example the Orion Nebula is, 1300 light years away from Earth in kilometers that's 12 and a 12 quadrillion kilometers away which is this number over here with all these zeros and if you're wondering what the nebula is well it is a cloud of gas and dust that new stars and planets are born from now the speed of light is really fast which means that it reverses short distances almost instantaneously it seems like light travels at an infinite speed to us in our daily EXP experience now on astronomical scales it can take light many years even millions and billions of years to travel from place to place now since the oran nebula is 1300 light years away light from the oran nebula takes 1300 years to reach us right this is what is a lie it's the distance that light travels in one year so to travel 1300 light years light has to travel for 1300 years so when we see the Orion Nebula in the sky we actually see it as it was 1300 years ago which was actually in the 8th century we're not seeing it as it is today and we'll only know how it looks like today when the light from the nebula reaches us which is going to be, 1300 years from today so as far as we know maybe the entire nebula disappeared 500 years ago but we won't know about it until we actually get the light from it in a few hundred years this is of course uh quite amazing but it's actually also very useful because it means when you look up into the sky it's kind of like having a time machine and the farther away you look the farther into the past you see so this allows us to see how the universe looks like millions or billions of years in the past if we can look far enough which means if we have instruments that are precise enough we can use that information to reconstruct the history and evolution of our universe okay so when I look at uh a Galaxy that is 1 billion light years away I see how that Galaxy looked like one billion years ago I don't have to do anything this is just what I see because this is when the light was emitted that is now reaching me okay so now that we understand a bit about Cosmic distances I want to take you on a brief tour of the universe we start of course with planet Earth which is where we are right now it is approximately spherical and has a diameter of around 13,000 kilom it is very special because not only is it where we are it is also the only astronomical object that we know contains life at least so far we might Discover Life on some other planet or moon in the solar system but that's not very likely there may be life on other planets in other solar systems but for now Earth is the only place we know that evolved life now the Moon is also special because it is the only celes Cal body that humans have visited in person as of 2022 now this is being recorded maybe like in 20 years someone is watching this video then we've already been to Ms by then but right now the Moon is the only place that humans have visited in person we've sent probes to other places but not humans the Moon is approximately spherical and has diameter of around 3,500 km it is located an average distance of 384,000 kilom from Earth which seems far but of course if you compare that to a light here which is 9 and a half trillion kilom it's really nothing so the Moon is Clos but it is actually far enough that light takes 1.3 seconds to travel that distance and this caused a noticeable delay when astronauts communicated with Earth the Sun is a star one of many stars and a star is a huge ball of gas that generates energy and Light by nuclear reactions and of course we'll learn more about how that works later the next closest star is proximal centor which is only 4.2 light which is about 40 trillion kilm away now between us and this star in these 40 trillion kilometers there is nothing there are no other stars this is the closest star to us now going back to the sun and the solar system including the Earth there are eight planets that revolve around the sun these planets are Mercury Venus Earth Mars Jupiter Saturn Uranus and Neptune now one thing to note about this illustration is the sizes of the planets are actually to scale so like this is how big Jupiter is is compared to Earth but uh the distances of course are not to scale right so there is a much much larger distance between all these planets but of course there just isn't enough space on the screen to draw the actual distance now planets and moons don't generate their own light but they reflect the Sun's light so you can see here the sun generating a lot of light using these nuclear reactions all these planets that you see you only see them because they reflect the light of the sun I mean that's basically how you see most things you see them because they reflect light now galaxies are enormous collections of between 100 million to 100 trillion Stars each one of these stars like our sun some bigger some smaller uh in different places along their lifespan but there's nothing really special about our sun between those stars there is Interstellar gas and dust also known as the interstellar medium so Interstellar means inter is between Stellar is star so between the Stars this by the way is the is called the Phantom Galaxy and it's a one of the new photos released by the uh James web Space Telescope uh just last month now we are located in the Milky Way galaxy which contains 100 to 400 billion stars including our sun this galaxy is about between 100,000 and 200,000 lights wide and 1,000 light is thick now it's important to understand since we're inside the milkway Galaxy we of course don't know how it looks like from the outside you know someone once asked me if if we have pictures of the Milky Way of course you have pictures of the Milky Way from inside the disk which is what you see when you look up in the sky but we don't have have photos of the Milky Way from above the disk it would take us a few million years just to get to a place where we can take such a photo we think the Milky Way is a b spiral galaxy which means it might look something like this galaxy which is called the NGC 173 so you can see that it is spiral maybe it's kind of hard to see in this uh picture but if you look carefully you'll see that there are spirals here's one spiral here's one here's one here's one so there are these Spirals and also there is this Long Bar in the middle just a straight bar like this so this is more or less how the Milky Way would look like if aliens were observing it like we're observing this galaxy now where exactly are we in this galaxy so the sun is between 25,000 and 29,000 light years from the center of the Galaxy it is located inside a spiral arm called the Orion arm okay so here is a sketch of course not an actual photo of the Milky Way galaxy and you can see here the Orion arm sometimes also called the or signus arm uh and again you see these spirals so each of these spirals has a name the solar system over here inside this arm and this yellow um this yellow circle illustrates the orbit of the sun around the center of the Galaxy so the sun doesn't stay in place the sun orbits the center of the Galaxy just like the Earth orbits the Sun and the Moon orbits the earth it takes the sun between 220 and 250 million years to orbit the center of the Galaxy and it does that at around 230 kilm per second now you may wonder why don't we feel this enormous speed and I'll explain that um in the coming weeks by the way any questions so far feel free to raise your hand if you have any questions yeah we were have say a telescope strong enough to look at another planet far out there would that technically looking back in time yes so even now when you look at me you're seeing me in the past right you're seeing me like one nond in the past so whenever you look at anything you always see it in the past it's just that when you look at stuff that is Cosmic distances away you see it in the far past not just one nond in the past but possibly thousands or millions of years in the past yes and that happens like the light literally just can't travel to us fast enough so even if we had like a telescope that could look that far out like just wouldn't hit it for like years exactly so the telescope all it does is it captures light that gets to it but the light still has to get to it so the light has to start here in some star or planet or Galaxy and then slowly travel slow of course in Cosmic terms until finally it gets into your telescope and that can be billions of years later um yeah watching the light of the star then it might have been destroyed already when we watching it very good so yeah if if you see a star right now there's no guarantee that star actually exists it could have been destroyed destroyed and you just haven't seen it yet yeah so like if we look at a planet there could be life like now but like we wouldn't see it at the P right so um if we could detect life on a planet which is hard to do from afar but I guess you can detect for example certain um chemical elements in the atmosphere and so on so yeah it's possible that we see a planet right now in some other solar system and we don't detect any life but by the time we are seeing it it has actually already detected life it's just that we don't know it yet because we haven't seen the light from that period yet yes the G yes I'll get to that in the one of the next slides any other questions yeah so even when we see like a meteor shower that's already happen that's see well uh a meteor shower happens in the Earth's atmosphere so you're seeing it as it was like a few micros seconds in the past I mean of course it's still in the past whenever you see anything it's always in the past but usually not in a noticeable way but when you look Earth that's when things start to have a delay yeah does this also change the way that we perceive time as well do people experience time different distances they perceive um not exactly but people do perceive time differently uh in different frames of reference and I'm going to talk about that uh when we talk about about relativity a bit later all right so at the center of our galaxy there is a super massive black hole called Sagittarius A star a black hole is a region of space where gravity is so strong that nothing can ES Escape that region not even light which is why it's black because light from it cannot reach us this black hole at the center of our galaxy so a star has the mass of four Million Suns and that's why it's called a super massive black hole most Galaxies have similar super massive black holes at those Cent now this image was taken by the Event Horizon telescope and you're probably wondering first of all why is it so blurry and why is it orange because supposed to be black right um and you may be wondering how exactly we could get this image and so on so if you are interested in that I will talk about that when we uh talk about black holes in astronomy 1 P2 any questions yeah are a million trillion years are we going to getu that black well um that that's that's a good question so there there is this misconception that a black hole sucks things into it a black hole is just something that has mass just like the Sun or the Earth so it attracts things rotationally but you can orbit it so yeah if you went directly into the black hole you would be sucked in by it just like if you went directly into the sun you will be sucked by it but uh since we are actually let me go back a bit since we are actually orbiting the center of the Galaxy we're just going to continue orbiting for the foreseeable future we're never going to get sucked in at some point the Galaxy might be destroyed in some way like collision with another galaxy or something that's going to take that might be billions or trillions of years in the future so you know we're safe we're not going to get sucked into the black hole because black holes don't suck you can well they definitely don't suck you can just orbit them like you can orbit any other massive body uh yeah yeah so here is the uh Galactic bar this is the center of the Galaxy inside here somewhere there is this super massive black hole and we are orbiting it I mean we're not orbiting just the black hole we are orbiting essentially the entire galaxy um but the black hole definitely is a very big part it's of the mass of the Galaxy what would be the closest we would ever like actually get to being near the black hole like well so we are about at least 25,000 light years away from this black hole so if let's say you wanted to visit it and let's say that you could travel very close to the speed of light you could visit it but it's going to take you 25,000 years to get there now there may be smaller black holes that are closer to us that we could potentially visit in a less ridiculous amount of time um but we haven't detected any yet do you know what happens to stuff for people that enter the plat things that enter this black hole are going to be torn apart into their individual atoms and cease to exist essentially so you definitely don't want to do that uh even if somehow you manage to stay alive you won't be able to escape the Black Hole uh you're just going to be stuck in the forever so I I do not recommend [Music] it yes so what happens for the Galaxy like well the uh so black holes supposedly emit radiation called Hawken radiation and again we'll learn more about that next term and uh they decrease in Mass gradually because they lose mass by emitting this radiation however this radiation is extremely small so it's going to take trillions of years before this black call gets smaller in any noticeable way okay so let's uh move on now in this image from the hble Space Telescope you can see around 10,000 galaxies so each dot like this dot over here and this tiny Dot and there's this tiny dot you can't even see all of these are individual galaxies and each one of these contains billions or trillions of stars and we discover more galaxies all the time so this image is a much more recent image than a previous one this is an image from last month from the James web Space Telescope and in this image you can also see thousands of galaxies and many of these galaxies are galaxies that we've never seen before last month the reason we're seeing them now is that Hubble the Space Telescope that took the previous image wasn't uh sensitive enough and also didn't look in the correct wavelength um and this new telescope can see fter away than we've ever seen before of course remember seeing farther away also mean seeing F into the fast which is also one of the reasons uh we built this Space Telescope now you should understand that this entire image covers a patch of sky that is the size of a grain of sand held at arms length Okay so you take the grain of sand you hold it like this and and then out of the entire sky around you not just above you but also below you and everywhere around you this is only the size of that grain of sand so there is really a a countless number of galaxies in the universe now there are some small galaxies close to the Milky Way but the nearest large Galaxy is called Andromeda it's 2 and a half million light years away and it also has a few small satellite galaxies the Milky Way Andromeda and at least 80 smaller galaxies form this thing called the local group local group of galaxies so here is the Milky Way here is Andromeda these are the largest galaxies but then there are also uh smaller galaxies all around this collection of galaxies we call it the local group because it is local this is where we are this local group is 10 million Li TI in diameter from here to here it is 10 million Li years the local group is itself part of something bigger called the vgo super cluster which is 110 million Li years in diameter and it contains at least 100 not galaxies but 100 Galaxy groups and clusters so here this is kind of a schematic map here is the local group but there are also all these other groups in different places around us that are millions of light years away and all of these are included in this Virgo super cluster so now uh to answer a question that was asked here does the Milky Way orbit anything then yes the Milky way uh is part of this system of uh Galaxy clusters well first of all it's it's part of this system of galaxies and it it's also part of this system of galaxies and Galaxy clusters and there is kind of a center of mass of this super cluster and the Milky Way or that Center of mass just like the sun orbits the center of the Milky Way and the Earth orbits the Sun and the Moon orbits the earth uh any questions so far yes um do all stars have their own set of planets like orbiting them that's a good question so the question was if all stars have planets orbiting them most stars have planets orbiting them um but it may be that there are some stars that for some reason never developed any planets during the development of the solar system yeah how much um do the other planets affect earth like does it influences at all right so the question is if other planets affect the Earth so yeah there are there is gravity between anything that has mass so the other planets affect the orbit of the Earth because they pull it in different directions uh we'll talk a bit more about that when we learn about orbits yeah are you saying that the mil way are is local grou uh it's not the center so so the question is whether the Milky Way is the center of the local group you can see that um well the Milky Way is at the center here because this is from our point of view but I mean this is just a matter of definition any point here could be the center there is one point that you can calculate if you know the masses and positions of all the galaxies which is called the center of mass and it could call that the center of the local group uh if you want to be more precise of course whenever we look at something it always looks to us like we are at the center this is exactly why people thought that the Earth is at the center of the solar system because that's just how it looks like if you're on Earth yeah um in the super cluster you said that milky wave orbits a center of mass is that like an actual object or is it just like different like gravitational holding uh good question so uh the question is does the Milky Way orbit when I say that the Milky Way orbit is the center of mass is it an actual object that is being orbited around so no Center of mass is just an imaginary point which is kind of like the average of all the masses um we'll talk more about that when we learn about gravity but usually there is no actual object at that center it's just something you can calculate um okay so let's move on now the vgo super cluster you can see that this just keeps going so the Virgo super cluster in turn is part of the lania super cluster it contains more than 100,000 galaxies and has diameter of 520 million Li years okay so this is here the local group and in this area is the Virgo super cluster but there are all these other clusters in groups over here and all of these uh are part of this one thing that is called the L super cluster the most distant known Galaxy as of today this might change tomorrow but right now the most distant Galaxy we know of is called gnz11 and is located 32 billion Li away it's obviously very very far away so you can't really get a high resolution photo of it now around 13.8 billion years ago at what we call the big bang which is kind of an unfortunate term uh the universe began to expand from a hot and dense State as it expanded it became colder and less dense and eventually it was cold and less dense enough that stars and planets could be formed and you can see here basically the whole history of the universe as we understand it today with a big bang here and then you can see space expanding and you can see that here there are no stars the fair stars appear only after about 180 million years and this keeps going until we get to now now despite its name the Big Bang wasn't an explosion it was simply the time when the expansion of the universe started so the universe has been expanding ever since that point in time and it might keep expanding forever although of course we are not 100% sure about that uh the universe isn't expanding into anything instead distances become longer so it's kind of hard to imagine but this this photo is actually a bit misleading because it shows as if something is actually expanding inside some ambient space right this it looks like there's this outside space and then the universe just expands within that space but that's not really how it works the way it works is the distances just become longer so like the distance from me to you right now is let's say 20 M but then a million years from now that same distance will be 40 m so we haven't expanded into anything just just the distances became longer this analogy isn't perfect because actually this distance is not going to expand because there is gravitational and electromagnetic pull between the atoms of the STS and that actually prevents the distance from expanding but when you have long stretches of empty space between galaxies there's nothing to stop this expansion uh yes un is always does that mean that there's some point in end so the question is uh if the universe is always expanding does that mean that there is an end to the universe so um that's a bit hard to answer right now the short answer is we don't know the universe might be infinite which means there is no end or it might be finite but if it is finite then it still with have no end it's just that you go in this direction and then after you pass through the entire universe you actually go back to the same place you came from yeah with it continuing to expand and becom older could this potentially go from being a good thing to a bad thing down the line or is that something that there's no worry for that right so uh good question so the question is um Universe keeps expanding and it becomes colder and colder can that turn into a bad thing so yeah definitely uh something that could happen which is called the heat death of the universe is essentially that the Universe becomes so cold that nothing can uh happen in it anymore there's not going to be any stars there's not going to be any life uh there's not going to be any way to really do anything and that however is unimaginably far in the future you have another question the universe's expansion is accelerating uh we'll learn why later although probably only next STM in 1 P2 uh any more question questions yeah if the universe expand from such a small space is there a amount of or exp um well atoms can be created and destroyed so um so the question is is there a finite amount of atoms and the answer is if the universe is finite in size then probably yes but if it's infinite then that means there's an infinite amount of atoms in it yeah could there be a second Universe well there are lots of different hypothesis uh that hypothesize that there are different Universe of different kinds um I also have a hypothesis like that actually uh I can about in my payers but remember what I said last time hypothesis is just an idea that hasn't uh being verified experimentally so right now the only thing we know is that there is one universe and that's all we know yeah if the universe keeps on expanding does that mean Galaxy clust will eventually um yeah so when the universe EXP expans galaxies and Galaxy clusters are always moving away from each other because the distances between them increase so um eventually they're going to be so far away from each other that if you look out you're not going to even see any light coming into where you are because things are just going to be so incredibly far away um yes how are galaxies created then sorry how are galaxies created how galaxies created that's a great question but uh it has a long answer so you'll have to wait until next term when we learn about Galaxy creation like you know how they come together and then break apart into smaller kind oftom that going of be like forming but like insanely lar um not exactly like I said it's it's a bit complicated so uh if you continue to astronomy 1 P2 we'll talk about it in uh in depth uh okay yeah so um the question is if there's gravitational pool how is the universe expanding so like I said let's say between you and me there is sufficient gravitational pool that it can cancel the expansion of the universe as long as that expansion doesn't exert a stronger pull in the other direction so right now um you only have expansion between galaxies but if the expansion keeps accelerating eventually it could even tell about galaxies themselves make stars uh expand away from each other and then maybe even in the very very very far future start making atoms expand far away from each other all right let's move on so um there are plenty of misconceptions of about the big bang and the expansion of the universe and we are going to discuss them in astronomy 1 P2 so I know you have lots of questions about it and they're all great questions but uh this is going to be like two or three lectures just devoted to that so you know you have to be patient so the oldest thing with we can see in the universe is called The Cosmic microwave background what it is is electromagnetic radiation that was emitted only 380,000 years after the big bang which is not a lot of time on Cosmic scales the reason it was emitted um again it it's going to take a bit of time to explain this in detail but essentially before that time the universe was so dense and so hot that it was completely opaque so light couldn't actually travel from one place to another and then 380,000 years after the big bang the universe started becoming transparent and light could start travel and this is the first light in the Universe essentially now everything I talked about so far in the last two lectures is part of the observable universe observable meaning the part we can see from Earth so what we can observe the observable universe is a sphere with a diameter of 93 billion light light years which means the edge of that sphere is 46 and2 billion light years away from us okay so here uh we are in the Virgo super cluster and we're right in the middle and here we have 46 and a half billion light years until we get to the edge of the sphere now it's important to understand the reason we can't see anything beyond the edge of the observable universe isn't that there is something in the way like an actual barrier or Edge or that our telescopes aren't good enough to see beyond it it's because as I said before light takes time to travel so objects that are outside the observable universe are so far away that the light from them just has not had time to reach us yet in fact it's never going to reach us because the universe keeps expanding so the edge of the observable universe is an imaginary line Beyond which light cannot get to us but there isn't any actual Edge to the Universe imagine aliens living far away in some different galaxy let's say a billion liers away they will see a different observable universe and then the edge of their observable universe is going to be at a different place still at the same radius of 46 and a half billion light but a different place than the edge of our observable universe and again this is because of the travel time of light there are uh galaxies that uh from which light has had time to reach us so they are within our observable universe but they haven't yet had time to reach the aliens in this other Galaxy so they are not in the aliens observable universe and the aliens will also see them at the center of the sphere so the sphere isn't an actual sphere it's just an imaginary sphere that we are at the center because like I said before when you look around it looks to you like you are at the center of things doesn't mean you're actually at the center it's only just your point of view the size of the whole universe including the parts that are not obser observable but don't get confused if they're not observable doesn't mean they don't exist it doesn't mean that we haven't seen the light from them yet um so the size of the whole universe is unknown it could potentially be infinite or like I mentioned before uh it could be of finite size but cyclic so you actually you go that way and then you find yourself back in the same Pond you started from now uh to answer a question that was asked so the universe is 13.8 billion years old and the edge of the observable universe is 46 and a half billion light years away so if light travels at a rate of one lightyear period by definition then light could only have traveled 13.8 billion light years since the Big Bang so how can this be well the reason is that the universe is expanding the galaxies that are currently at the edge of the observable universe or maybe I should say of our observable universe used to be much closer to us but the distance to those galaxies expanded with time so the distance expanding meaning light has traveled more distance to get to us yes that's a good question so the question is is the universe expanding faster than a speed of light and the answer is that essentially the expansion of the universe doesn't have a speed what happens is that things so distances are becoming longer and if you look at something that is far enough away from us it looks like it is moving away from us faster than light in fact that is why it's outside the observable universe because it's moving faster than light from it can reach us uh but this doesn't viol at relativity which says that nothing can travel faster than light um because nothing is actually moving faster than light it's just the distances become larger so it looks like it's moving faster than light okay 10 to the N is a power of 10 so if you have if you see something that looks like 10 and then an exponent n where N is a positive in integer that means very simple one followed by n zeros so a th is 10 to the 3 okay so n equals 3 so it's one followed by three zeros a million 10 the 6 is one followed by six zeros a billion 10 the 9 one followed by 9 zeros and so on so all these numbers we learned about last time I'll uh just one with some number of zeros after it now if you multiply two powers of 10 let's say you have 10 to the n and you multiply that by 10 to the m the result is 10 to the sum of N + m for example 10 to the 3 * 10 the 9 3 + 9 is 12 so the answer is 10 12 now if you translate it to Worlds then 10 3 is a th 10 N is a billion and 10 12 is a trillion and indeed a thousand times a billion is equal to a trillion and why are we learning about this because we want to learn scientific notation and scientific notation is the notation we use to write very large numbers of course as we've seen in astronomy we deal with very large numbers in terms of distances and time and so on so we need to have some Compact and well- defined way to write these large numbers so a scientific notation is just a number times a power of 10 for example 2.3 * 10 6 is scientific notation what does it stand for well 10 the 6 is one with six zeros 2.3 * one6 zeros which is 1 million is 2,300,000 you can do this on your calculator you don't have to do this in your head okay another example 4.7 time 10 the 12 so again it's 4.7 times one with 12 zeros after it which is a trillion and that comes down to 4 trillion 700 billion again you can do this easily in your calculator but you just need to understand what this this notation stands for it stands for this just a short way to write this number now if I want to multiply two numbers in scientific notation essentially it goes this way so I have one number times the second number the first number is a * 10 to the N second number is B * 10 to the m and the product is a * B * 10 ^ of n + m so you multiply the two coefficients and you add the two exponents and here is a simple example 1 and2 * 10 3 * 3 * 10 6 so 1 and2 * 3 is 4 and A2 3 + 6 is 9 so the answer is 4 and 1/2 * 10 to the nine again you don't need to do this in your head you can do this in a calculator uh but you should understand really just what these numbers stand for so a quiz what is 2 * 10 9 * 4 * 10 6 okay who who thinks it's a who thinks it's B raise your hand okay who thinks it's C okay so okay so the people who said B are correct because to yeah great great job so two * 4 is 8 9 + 6 is 15 so it's a * 10 15 okay so let's uh finish here and we'll continue on Wednesday so now I want to calculate uh the size of the observable universe remember the observable universe is the part of the universe we can see from Earth and stuff outside the observable universe we just can't see because the light can't get to us due to the expansion of the universe essentially it's expanding faster than light is traveling so um I told you last time the size of the observable universe is 93 billion light years which is 93 * 10 to the 9 light ears ly means light ears so a billion is one followed by nine zeros so that's 10 to the N um also last time we calculated that one light here is 9 and a half trillion kilometers and again in scientific notation 9 and 12 * 10 12 kilomet because trillion is one followed by 12 zeros and that is 10 the 12 okay so the quiz now is who can calculate the size of the observable universe in kilometers yes it's 8835 10 21 K okay good so um we multiply these two numbers so 93 * 9 and2 is this 10 9 * 10 12 that's just 9 + 12 that's 21 now all this is just an approximation right I mean we don't know the size down to the centimeters so uh we might as well round it up so this is 883 and a half we can just round it up to a th000 which is 10 the 3 so we actually have 10 the 3 3 * 10 21 again multiplying 2 power of 10 I just add the powers so 3 + 21 is 24 the size of the observable universe is approximately 10^ the 24 kilom which is 1 0000000000000000000000000000000000 00 0 kilometers which is big so now I'm going to show you a video that you can also find on YouTube that shows you all of these scales uh that we talked about last time starting from Human scales and all the way up to the whole universe at 10 to the 24 kilomet trying to comprehend how big the universe is it's one of those questions that astrophysicists grapple with all the time trying to explain it in a way that's easy to understand well that's a whole other challenge in 1977 Charles and Ray e hugely influential American designers released one of the most elegant and creative pieces of science communication of modern times powers of ten it took the viewers on a journey from a picnic blanket near Lake Michigan to the edge of the known universe and back again over 40 years later as a humble amage to this groundbreaking film we're going to take a similar Journey Through Time and Space and see how our understanding has changed along the way as in 1977 we'll start with a picnic though this time we're on the island of Sicily in Italy rather than Lake Michigan we'll start with a scene 1 M wide viewed from 1 meter away and every 10 seconds we're going to move out to 10 times further away so the seene will be 10 times wider this square is 10 m wide and in 10 seconds the next Square will be 10 times that the movement may seem linear but we're actually accelerating exponentially into the distance this square is 100 m wide the distance someone can running 10 seconds well if they're running at 43 km an hour just under the speed of your cat 1 kilometer though our picnickers are indistinguishable now we can still clearly see the impact of human activity on the world 10,000 M or 10 4 M this is the distance a supersonic plane travels in 10 seconds and we're now reaching the highest altitude flown by such a plane 10 5 m this is the distance the International Space Station travels in 10 seconds there it goes from here on human activity will be lost to sight we're at the scale of countries 1 million M or 10 6 M we've long left Earth's atmosphere and soon we'll see the whole planet what a jewel 10 million M the invisible magnetosphere Shields us from the dangerous ionizing radiation of space 10 to 8 m this line extends at the speed of light this is the time it takes for light to reach us from the moon's orbit the age of moonlight the Earth is now just a pale blue dot in A Sky Full of Stars even as we accelerate away the stars appear stationary because they're so much further away so much empty space let's illustrate the orbits of the planets in our solar system otherwise this could get a bit dull here comes the orbit of Venus then Mars and now Mercury since 2010 the NASA Solar Dynamics Observatory has been using an extreme ultraviolet filter to monitor the activity of our sun finally we reach the orbit of the outer planets the gas giants but just specs at this distance there's the the orbit of Pluto one of the dwarf planets of the Kyer belt 10 to the 13 M and we're moving out of the solar system in 2012 the Voyager 1 spacecraft became the first human artifact to make this journey followed in 2018 by its twin Voyager 2 both were launched in 1977 the year the EM were working on powers of 10 we're heading into Interstellar space our sun is just one of billions of stars and still at this distance the night sky looks very similar to to what we see at home this square is 10 to the 16 M the distance light travels in one year one light year here's our closest neighbor Alpha centur three stars for the price of one with planets orbiting around them I'd love to know what's going on there thanks to data collected by the Gaia spacecraft we're building a detailed 3D map of the Milky Way There are between 100 and 400 billion stars in our galaxy alone and clouds of dust and gas like these nebula where new Stars Are Born images sent from the Hubble Space Telescope have been blowing our minds for a generation as we move away we begin to see the great flat spiral of our galaxy a few hundred billion stars rotating around a black hole Sagittarius A star 4.2 million times more massive than our sun we now think super massive black holes reside at the center of nearly all galaxies these two dwarf galaxies are the melenic clouds which together with at least 80 others make up what's known as the local group of galaxies 10 to the 22 1 million light years soon we'll pass the super giant elliptical galaxy m87 and if we switch to radio waves we can Glimpse the super massive black hole at its Center switching back to visible light as we Traverse the Virgo supercluster each tiny dot not a star but a Galaxy billions of stars floating in an Ever growing void 10 to the 24 M the limits of our vision in 1977 but over 40 years later we can show a bit more clusters of galaxies arranged along filaments like the Pisces CA supercluster complex at 10 to the 26 M we switch our view to microwave and we can now see the current limit of our vision this light forms a sort of wall all around us the light and dark patches show differences in temperature by fractions of a degree revealing where matter was beginning to Clump together to form the first galaxies shortly after the big bang this light is known as the cosmic microwave background radiation 10 27 m 1 followed by 27 Z Beyond this point the nature of the universe is truly Uncharted and debated this light was emitted around 380,000 years after the big bang before this time the universe was so hot that it was not transparent to light is there simply more Universe out there yet to be revealed or is this region still expanding generating more universe or even other universes with different physical properties to our own how will our understanding of the universe have changed by 2077 how many more powers of 10 are out there from a picnic blanket on Sicily to the very edge of our understanding I salute the EM for the way they told this beautiful story The story of the [Music] [Music] universe right so uh one thing I should mention he was talking about meters and I was talking about kilometers so there is a bit of a discrepancy between the numbers when I say here that the observable universe is 10 to the 24 kilom each kilometer is 1,000 M so that's 10 to the 27 M and that was the highest scale in the movie uh this movie is basically a remake of a movie from 1977 as he said and you can find that original movie on YouTube obviously it's not as high quality and not as up to date uh but it's still a very nice illustration of the scales of the universe we live in so we talked about the biggest things in the universe right we zoomed out basically starting from Human scales and then increasing to planets Stars galaxy galaxy clusters and the cosmic mcro background and the whole observable universe IE now let's instead zoom in and check out the smallest things in the universe so of course to do that we need to do more math we talked about positive powers of 10 there are also negative powers of 10 so those are numbers that look like this 10 the minus n where n is some positive integer and what means is really simple it's just 1 ided by 10 to the N so 1 divided by one followed by n zeros you can also write 10 to the minus n as n Zer followed by a one with a decimal place after the first zero so it's basically kind of like an inverted 10 to the N so instead of one followed by n Zer it's n Zer followed by one here are some examples so [Music] one, is 10 Theus 3 so that's 1 over 10 3 or one over 1,000 and you can also write it as 0.01 so notice there's three zeros followed by a one and a decimal point after the first zero 1 millionth is 10us 6 so that is 1 over 10 6 which is 1 followed by six zeros which is a million so it's 1 over a million we can also write it as a decimal so now it's six zeros followed by a one and 1 billionth is 10 Theus 99 so that's 1 over 10 the 9 so one over one with 9 zeros after it or one over a billion or you can write it as 9 zeros followed by a one with a decimal place over here and these are just three examples obviously you can do it with any number uh on the exponential so as you can see these numbers as I increase the exponential the numbers become smaller and smaller we need more and more digit after the decimal point to specify that number so most things we can see or detect like stars planets and humans are made of atoms every atom is composed of a nucleus surrounded by a cloud of electrons so here you can see this is the nucleus here in the middle it's very small and here there this fuzzy cloud of electrons now the nucleus itself is made of protons and neutrons Each of which is around 100,000 times smaller than an atom and here you can see we zoomed into the nucleus and we see here these protons and neutrons so this is a helium atom helium atom has two protons and two neutrons so you can see here uh the blue are the protons and the red are the neutrons or the other way around I guess it doesn't matter um you can also see here the scales so the scale of the entire atom is 10 the- 10 m so that's 1 / 10 10 or 0.000000001 MERS and then the scale of the nucleus is 100,000 times smaller uh 100,000 is 10 five so we get from 10us 10 down to the 10us 15 so um let's talk about Atomic scales so like I said the size of an atom is 10 Theus 10 m so it is a very very small number 0.01 M the size of a nucleus is 10 Theus 15 M which which is even smaller so now it's 15 zeros and a one in the end the size of a proton or a neutron these are the particles that make up the nucleus just a bit smaller than the nucleus now this illustration of an atom you may have seen before is actually wrong because here what you see is that you have uh well the nucleus with protons and neutrons inside it and then there are electrons that orbit in these well defined orbits like planets orbit a star but that's not how it actually works this is how people thought it actually worked 100 years ago but nowadays we know the electrons don't really orbit the nucleus as this illustration suggests uh they'll just probability clouds like I showed you uh in the slide before is just kind of like a fuzzy cloud of electrons and I'll explain what that means when we learned about quantum mechanics in a few lectures why why are we taught like that it's like that then like that it goes around in like an orbit well you talk a lot of incorrect things so my job is to correct them you know this is kind of like a nice picture nice illustration that doesn't require you to understand quantum mechanics so when you teach it in uh High School uh and you don't understand quantum mechanics at that point then I guess this is the best they can do but I'm going to teach you some more advanced picture of an atom which is the correct picture as far as we know uh another question here um how recent how recent how much like recently did we learn that this was not the case like how recent did we say oh okay it's not orbits like it's a probability Cloud so uh quantum mechanics was developed in the 1920s and in the 1920s and 30s that's when we start started realizing this picture of the atom that we had before uh was not correct so schools are just still keeping with that then yeah yeah I mean like I said quantum mechanics isn't a trivial thing to learn I guess they don't teach it in high schools maybe they will in the future but they start I learned a little bit about it okay so you know some people have learned about it so now there are 118 different types of atoms that we know of which are also called chemical elements of course I said that we know of maybe later we'll discover more but this is what we know right now so all Atomic matter in the universe is made of different combinations of these 118 elements and notice as I said Atomic matter there is also nonatomic matter we talk about that later the number of protons so remember the nucleus has protons and neutrons the number of protons is also known as the atomic number and it's determines the type of the chemical element so for example hydrogen has one proton helium has two protons and so on anything that has one proton and any number of neutrons is an hydrogen atom and of course all of you are familiar with the periodic table of elements um you don't need to know any of this this is just to illustrate that these are all the different types of atoms that we uh know of and you can in fact organize them in a well- defined way according to some properties that I'm not going to get into but you can see here for example hydrogen is the first one with one proton so its atomic number is one and then here we have helium with two protons and lithium with three protons berium with four protons and so on increasing until we get to the last one 118 protons now what's interesting is the tyren that has this one proton is the most common element and it makes up 74% of all Atomic matter and then helium which has two protons makes up 24% of atomic matter now if you add this up you'll get 98% is either hydrogen or helium so the other 116 elements make up the remaining 2% so basically the entire universe is mostly either hydrogen or helium and it makes sense because these have the smallest number of protons then in some sense they are the easiest to make now of course some matter is made of molecules which are groups of two or more atoms bonded together and here we have a water molec and you can see it has one oxygen atom connected to two hydrogen atom and together they make up one water molecule and water is made out of these molecules not individual atoms now we get to elementary particles so the protons and neutrons in the nucleus are not Elementary they are made of even smaller particles that are called up and down quarks so in a proton we have two up quarks and one down Quark so you can see here up up and down and you can see how they're all kind of connected together kind of like atoms are connected in a molecule except it's it's a very different type of connection governed by a different type of force and here we have a neutron which is one up quar and two down qual uh you can see here one up and then down and down and again they are all connected together now you may ask why they're called up and down it's just Just some name someone invented it doesn't have any actual meaning so let's summarize all Atomic matter in the universe is actually made of just three kinds of particles electrons up quarks and down quars an atom we said is made of a nucleus surrounded by a cloud of electrons and the nucleus itself is made of protons and neutrons and the protons and neutrons are made of quarks up and down quarks to be specific so that if we go all the way down to the most Elementary particles we know of there are only three of them electrons up quarks and down quarks make up all the atoms as far as we know electrons and quarks are not made of any smaller particles which is why we call them Elementary particles again I keep saying in this course if I don't say as far as we know then just imagine I said as far as we know because everything is just as far as we know 50 years ago uh we didn't know about quarks but now we know about them there is another common elementary particle called the photon that is the particle of light and electromagnetic radiation and we will learn more about it later there are other more exotic Elementary particles like uh higs bosons and gluons and W and Zed bosons and uh top and bottom quarks and all kinds of other exotic particles that we're not going to learn about right now but they also exist so now let's talk about scales because that's really why we're here so the sizes of the elementary particles they're actually unknown and how to define and the reason for that is quantum mechanics because in quantum mechanics small things are fuzzy so you can't exactly Define where they start and where they end so you can't really Define the size of these very small Elementary particles in fact mathematically when physicists talk about these particles and do calculations with them and write theories that involve them we treat these particles as just points that have no size but experimentally speaking we have some upper bounds on the sizes of quarks and electrons so we know that quarks have to be less than 10 Theus 19 M which is this number which is very very small and we know that electrons have to be smaller than 10 Theus 22 M which is again 22 zos followed by one this does mean that this is the size of an electron it could be much much smaller than this or like I said it could be just of size zero with no actual size it also depends how you actually Define that size but I'm not going to get into that so now we know the smallest and largest scales in the universe the smallest thing that we learn about at least is an electron Which is less than 10us 22 M the largest thing that we know of is the observable universe which is 10^ 24 kilomet so okay here I used meters and here I used kilometers just because there's kind of this convention that in astronomical distances I used kilometers and then for subatomic distances use meters so I can convert this 10 to the 24 km to 10 to 27 M so what is the difference between the smallest thing and the largest thing it is 49 orders of magnitude right so starting from 10 to Theus 22 with the size of a single electron I have to keep multiplying by 10 49 times essentially adding 49 zeros to the size until get to the observable universe so in other words an electron is at least this much 10 to the 49 times smaller than the observable universe and this is a good point to remind you what I said last time which is that the observable universe isn't the entire universe it's just the fault that we can see but just because we can't see something doesn't mean it doesn't exist so you know aliens in some other Galaxy are going to see a different region of the universe as their observable universe so what is the entire universe including the non-observable parts uh it could be infinite and if it's infinite then obviously it is infinitely larger than a single elect or anything else for that matter so you can trade this as just the lower bound this is just the smallest ratio between electron and the universe it may be much larger than this or even infinite okay so where do we fit into all of these scales so humans are around the middle right so we have 10 to the minus 22 at the bottom and then 10 to the 27 at the top so 10 to the zero is kind of like somewhere in the middle it's 27 orders of magnitude smaller than the observable universe so we're basically completely negligible compared to the whole universe and uh 20 two orders of magnitude larger than electron so of course an electron is negligible compared to us the farthest from Earth that humans have been is the Moon the Moon is around 380,000 kilom or 3.8 * 10 to the8 M away now if you do the math this is like moving one atom away if the observable universe was the size of the Earth okay so imagine I shrink the whole observable universe to the size of the Earth and then where have humans been able to travel from this atom to this atom just an infinitesimal distance within the entire universe so we have a long way to go but where can we go so we have sent probes to other places in the solar system and I'm going to talk about these probes in more details later so uh we sent probes to other planets moons asteroids uh but we haven't sent humans to any of these places the only place we sent humans to is the moon and these missions to the Moon they took on average three or four days we want to send humans to Mars This is everyone's big dream right now uh because mes is kind of the most like Earth among the planets um such a journey depending on how fast your rocket can can move you will take several months which is still doable now what about the closest stall Proxima centor is 4.2 light years or 4 * 10 to 13 kilom away so that's uh 40 trillion kilom the fastest human-made spaceship right now is NASA's pal solar probe which is expected to reach a maximum speed of 690,000 kilometers per hour this seems like a lot certainly it's a lot faster than any car or even plane can reach but it's still just 0.06% of the speed of light so it's completely negligible compared to the speed of light which is the maximum speed things can move so traveling to Proxima Sor at this speed the fastest speed that humans have been able to reach so far will take 6,500 years now maybe this is possible I mean we could maybe travel find a way to travel faster than this speed although not faster than speed of light or maybe we could send humans inside you know like cryo Chambers or find some other way to make humans survive this journey you can just make humans Immortal if you want in the far future and then maybe this journey is going to be possible so now let's uh talk about the nearest galaxy the Andromeda galaxy is located two and a half million light years away at the same speed of 690,000 kilm per hour again the fastest speed that human have been able to accelerate a spaceship too it would take 4 billion years to reach the Andromeda galaxy even if we could travel close to the speed of light which is physically possible but extremely hard to do and definitely impossible to do with any technology we can imagine right now um it will still take at least 2 and a half million years to travel to Andromeda because the distance is 2 and a half million light years even if you can travel almost as fast as light and I should mention that you can never travel as fast as light but you can travel very very close like 99.99 99% of the speed of light that is physically possible to achieve although very hard uh but even if you can achieve that speed because the distance is 2.5 million lighter away it takes light 2.5 million years to get there and you cannot travel faster than light it's going to take you at least 2 and a half million years to get there there's just no way around it you can never get there in any time less than 2 and a half million years so the conclusion is that as far as we know right now and again everything I say might change later uh humans will never be able to travel to Andromeda with any conceivable technology basically the only way they could travel to Andromeda is if they are just made Immortal and they could live billions of years or you can somehow uh put them in some kind of stasis for two and a half million years while still having enough energy to sustain the trip for two and a half million years so you know it's it's kind of like science fiction territory and Andromeda is the closest galaxy to us so we're talking about just getting to the closest galaxy the edge of the observable universe is 46 and a half billion light years away so even if you could somehow imagine traveling for 2 and a half million years to Andromeda if it will take you 4 to6 and a half billion years which is much larger than the age of the universe to get to the edge of the observable universe so as far as we know right now based on what we know about technology and human lifespans we are stuck forever in the Milky Way galaxy it's actually interesting that most science fiction only happens within the Milky Way galaxy there is very very few science fiction stories where people actually go to other galaxies because it's just so far away that even with science fiction is just really hard to imagine ever getting to even the nearest galaxy not to mention a farter Way galaxy okay so we talked about distance scales so so far um let's now talk about time scales so the universe as I said several times is 13.8 billion years old as far as we know and modern humans evolved around 300,000 years ago so it's really hard to Define when humans actually started because kind of a very gradual process but let's just put put it at 300,000 years ago so this is around even though it looks very long and definitely it's extremely long in human time scales right it's literally the entire time that the entire modern human species has existed is still 20,000 times short than the age of the universe now the recorded history of humanity only began around 5,000 years ago we don't really have any records from earlier than that so uh the recorded history of humanity is roughly 3 million times shorter than the age of the universe okay now we calculated last time there are 31.6 million seconds in a year now imagine that the Universe only existed for one year so I took this 13.8 billion years and I just compress them all into one year then humans have existed for the last 25 minutes of that year and all recorded history has only existed for the last 10 seconds of that year in other words if the Big Bang took place at midnight on January 1st then Humanity only appeared on December 31st the last day of the year at 11:35 p.m. and recorded history only started at 1159 p.m. and 50 seconds so 10 seconds before the year is over that is when we started learning how to write so in conclusion I hope this lecture made you interesting in learning about astronomy and the Universe I definitely it sounded like people had um very interesting questions and I tried to give you an idea of just how immense and astonishing the universe is in terms of distance and time scills in terms of reading there is this textbook called open STS astronomy you can find a link on the Call's website um if you want you can read chapter one and appendices a to d and that will uh give you some more insight into what I talked about however I also talked about a lot of stuff that is not on the textbook and also omitted some stuff that is in the textbook so uh let me stress that the lectures are the main source of material for this course the uh exams are going to be based on the lectures and a textbook is for those who want to read more and uh enhance their understanding for fter exploration see the end of chapter one and you'll see there is this fter exploration chapter there uh um that recommends books websites and videos and I recommend if you really want to learn more about all the stuff I just talked about over the last three lectures uh you should definitely read some of these extra materials in terms of exercises I am going to post practice questions on teams there's a question over there would things in the further exploration be in the exams or is that just for our own interest so like I said the exams are based on the lectures if you want to learn more about all this stuff because there's a limited number of things I can talk about during a one and a half hour lecture uh there is the textbook and if you want to know even more there is this falter exploration