hello and welcome to General astronomy lecture 9 the origin of the solar system we'll begin this lecture with a pretty good question what did our solar system look like before the planets were fully formed the answer May lie in these remarkable images from the huble Space Telescope the two on the left each image shows an immense disc of gas and dust surrounding a young star astronomers strongly suspect in its infancy our own sun was surrounded by a similar disc from which the planets of our solar system eventually coales as we'll discuss in this lecture care careful study of the major features of our solar system has enabled scientists to put together a detailed theory of how Earth and our solar system was born the theory of solar system formation is important not only because it helps us understand our Cosmic Origins but also because it holds the key to understanding the nature of the planets and again I threw in this image here on the right uh you saw this image in my very first lecture I talked about how interesting this was and how it looks how it is a region in which planets are forming uh so you're going to learn about how that actually happens today in this lecture so the small sizes of the terrestrial planets compared to the Jovian Planets suggests that the same chemical elements are quite common in our solar system While others are quite rare the tremendous masses of the Jovian Planets such as Jupiter which has the mass of all of the other plan PLS combined means that the elements of which they are made primarily hydrogen and helium are very abundant the sun 2 is made almost entirely of hydrogen and helium its average density is 1,400 kg per meter cubed in the same range as the densities of the Jovian Planets hydrogen which is the most abundant element makes up nearly 34 of the combined mass of the Sun and the planets so you can see that in this graphic here on the on the right um helium is the second most abundant element together hydrogen and helium account for roughly 98% of all of the mass in material in our solar system all of the other chemical elements are relatively rare and combined they make up about 2% the dominance of hydrogen helium is not merely a characteristic of our local part of the universe by analyzing the Spectra of stars and galaxies astronomers have found essentially the same pattern of chemical abundances out to the farthest distance attainable by most powerful telescopes hence the vast majority of the atoms in the universe are hydrogen and helium the elements that make up the bulk of earth mostly iron oxygen and silicon are relatively rare in the universe as a whole um as are the elements for which the living organisms are made made out of things like carbon oxygen nitrogen phosphorus and other things a wealth of evidence evidence has led the has led astronomers to conclude that the Universe began roughly 13.7 billion years ago with a violent event called The Big Bang only the lightest elements that is hydrogen and helium with small amounts of lithium and maybe brillium uh emerged from the enormously high temperatures that followed the cosmic event of the Big Bang all the heavier elements were later manufactured by stars either by thermonuclear Fusion reactions deep in their interior or by the violent explosions that Mark the end of massive stars were it not for these processes that take place only in stars there would be no heavy elements in the universe no planets like our Earth and no humans to contemplate the nature of the cosmos because our solar system contains heavy elements it must be that at least some of its material once was inside of other stars but how did this material become available to help build our solar system the answer is that near the ends of their lives Stars cast much of their matter back out into space for most stars this process is comparatively gentle in which a star's outer layers is gradually expelled this ejected material appears as the Cloudy region or nebulosity where nubes comes from the Latin root word cloud that surrounds the star as it is illuminated by it which you can see in the image here on the right the star anies in this image is shutting material from its outer layers forming a thin Cloud around the star we can see the cloud because some of the ejected material has condensed into tiny grains of dust that reflect the starlight's light I'm sorry that that reflect the Stars light um so dust particles in the air around you reflect the light in the same way which is why you can see them within a shaft of sunlight in a darkened room uh anies lies some 600 light away from Earth in the constellation Scorpio just for reference a few Stars eject matter in a much more dramatic way at the very ends of their lives this is a spectacular detonation known as a supernova which will completely blow apart the star uh so we did discuss supernovas briefly in my first lecture uh and here we're just bringing them up again uh to let you know that this is where some of the elements will come from is these explosions they enrich the universe with their heavy element M so these are just three images of supernova remnants so what was left over after a relatively recent Supernova explosion well no matter how it escapes the ejected material contains heavy elements that were drudged up from the Stars interior where they formed this material becomes part of the interstellar medium a tenuous collection of gas and dust that pervades the space between the Stars so as different Stars die they increasingly en en Rich the interstellar medium with heavy elements observations show that new stars form as condensations of the inner of the interstellar medium unlike the previous figure which depicts an old star that is ejecting material into space this image here on the right shows young stars in the constellation Orion that have only recently formed from a cloud of gas and dust the bluish wispy appearance of the clouds is caused by Starlight reflecting off of the Interstellar dust grains within the cloud the grains are made of heavy elements produced by earlier generations of stars thus these new stars have an adequate supply of heavy elements from which to develop a star system of planets satellites comets and asteroids our own solar system must have formed from enriched material in just this way thus our solar system contains recycled material that was produced long ago inside a dead star this recycled material includes all of the carbon in your body all of the oxygen that you breathe and all of the iron and silicon in the soil beneath your feet so that is why I have this quote here the cosmos is within us we are made of star stuff and we are way for the universe to know itself we are that recycled material we are literally made out of material from dead stars uh so it's really a beautiful thing and you'll hear me bring this up a lot uh because it it's a prevalent theme throughout the course all right well Stars create different he heavy elements in different amounts for example oxygen as well as carbon silicon and iron is readily produced in the Interiors of massive stars however gold as well as Silver Platinum and uranium is created only under special circumstances consequently gold is rare in our solar system and in the universe as a whole while oxygen is relatively abundant although still much less abundant than hydrogen and helium so this graph is is just showing you that um so you can see the relative abundances so this shows you how much there is in the universe so it gives you an idea um in fact we mentioned specifically that gold is very rare uh I just recently saw a a quote I don't know how accurate is but I do believe it um I was surprised though that all of the gold that's ever been excavated from the earth uh is only enough to fill up three Olympic siiz swimming pools uh so that gives you an idea of how rare it is and that you know why why it's a pretty expensive item it's because there's only three full swimming pools of it out there uh anyway um so the small Cosmic abundances of elements other than hydrogen helium help to explain why the terrestrial planets are so small because the heavier elements required to make these terrestrial planets are rare only relatively small planets can form out of them by contrast however hydrogen and helium are so abundant that it was possible for these elements to form the larger Jovian Planets so from the abundances so you can reference the previous slide do you expect that water which is H2O might be a common substance in the galaxy take a minute to reference the image pause your video and return When ready well the answer in this case is a resounding yes it turns out that hydrogen which is one of the main components of water it is the first most abundant element and oxygen the other component of water is the third most abundant element so with that train of thought if it's the first and third most common element in the universe well it's not unreasonable to think that water would be quite common um so it actually is water is common throughout the Universe it's just often Frozen because it is so cold the heavy elements can tell us even more about the solar system they help us determine its age the particular heavy elements that provide us with this information are radioactive a radioactive nucleus therefore ejects particles until it becomes stable in doing so a nucleus may change from one element to another physicists refer to this as radioactive decay all right so we're going to get into some slightly technical stuff I'll skip over all of the really tougher stuff and just give you kind of a general idea if you'd like you can look this up a bit more for more detail um but a lot of it is external to to this introductory course so just know that there are some heavy elements that we consider to be radioactive and that means that um they will eject particles from their nucleus until they become stable and turn into something else so that is the process in very quick vague terms of radioactive decay so for example a radioactive form of the element rubidium which is the has the atomic number 37 on the periodic table it decays into the element strontium when one of it the neutrons it is decayed into a proton and an electron so again these might be terms that you're not too familiar with if you haven't taken many science courses yet um but the idea is that one element may turn into another because of this Decay the first step in understanding how we learned the age of our solar system is to understand exactly what we mean by the age of a rock a rock is a collection of a great many atoms held together in solid form the atoms must be older than Earth having been forged in the Big Bang or in stars that lived long ago right so the elements that make up our rocks are older than the our planet itself because they had to come together and then form our planet we cannot determine the ages of the individual atoms because old atoms are indistinguishable from new ones however some atoms undergo changes with time that allow us to determine how long they have been held in place within a rock's solid structure in other words the age of a rock is the time since its atoms became locked together in their present Arrangement which in most cases means the time since the rock last solidified experiments show that each type of radioactive nucleus decays at its own characteristic rate which can be measured in the laboratory furthermore the older a solid rock is the less of its original radioactive nucleus remains this behavior is the key to a technique called radioactive dating which is used to determine how many years ago a rock cooled and solidified or simply to determine the ages of rocks for example if a rock contained a certain amount of radioactive rubidium uh when it first solidified over time more and more of the atoms of Rubidium within the rock will Decay into strontium the ratio of the number of strontium atoms that The Rock contains to the number of Rubidium atoms uh gives a measure of how old the rock actually is and this image here this graph shows you another example of that when The Rock first formed it might have had it might have been entirely potassium and no argon but over time pottassium will Decay into argon so over time we can see the relative amount of each isotope so knowing the exact amount of the Rock by taking some samples you can compare it to what we know and you can figure out the exact age of that rock so it's a very useful technique and this is how we date things on the earth this is how we know the age of our Earth and of our solar system speaking of that this radiometric or radioactive dating tells us how long it has been since a rock solidified which is not the same as the age of a planet as a whole for example we find rocks of many different ages on Earth some rocks are quite young because they formed recently from molten lava and others are much older the oldest Earth rocks are about 4 billion years old and some small mineral mineral grains date to almost 4.4 billion years ago but even these are not as old as the Earth itself because Earth's entire surface had been reshaped throughout time moon rocks brought back by the Apollo Astronauts have also been dated and many are older than the oldest Earth rocks demonstrating that parts of the Moon surface have not changed since the very beginnings of its history the oldest Moon Rocks come back to be about 4 1.2 billion years old to go all the way back to the origin of the solar system we must find rocks that have not melted or vaporized since they first condensed out of the interstellar medium meteorites that have fallen to Earth are source of such rocks many meteorites appear to have remained unchanged since they condensed and accreted in the early solar system careful analysis of radioactive isotopes in these meteorites show that the oldest ones formed about 4.54 billion years ago so this must Mark the beginning of our solar system we have seen how processes in the Big Bang and within ancient Stars produced The Raw ingredients of the solar system but given these ingredients how did they combine to make the sun and the planets so that's what we're left at now that is our question astronomers have developed a variety of models for the origin of the solar system the test of these models is whether they explain the properties of present day of of the present day system of sun and planets the central idea of this model dates back to the 1700s when the German philosopher Emmanuel Kant and the French scientist Pier Simon De laas turned their attention to the manner in which the planets orbited the sun both concluded that the arrangement of the orbits all in the same direction and a nearly the same plane could not be a mere coincidence the nebular theory begins with the idea that our solar system was born from the gravitational collapse of an Interstellar cloud of gas called the solar nebula that collapsed under its own Gravity the solar nebula probably began as a large roughly spherical Cloud uh excuse me I excuse me I lost my train of thought uh a roughly spherical cloud of very cold and very low density gas initially this gas was probably so spread out perhaps over a region of a few Lighty years in diameter that gravity alone may not have been strong enough to pull together and pull it together and start its collapse instead the collapse may have been triggered by a cataclysmic event such as the impact of a shock wave from the explosion of a nearby star so we start out with this big collection of gas and dust and we think that it would collapse when something triggers it um we don't think gravity alone would do it something must have triggered it that's the current theory once the collapse started gravity enabled it to continue remember remember that the strength of gravity follows an inverse Square law with distance the mass of the cloud remained the same as it shrank so the strength of gravity increased as the diameter of the cloud decreased because gravity pulls inward in all directions you might at first guess that the solar nebula would have remain spherical as it shrank indeed the idea that gravity pulls in all directions explains why the sun and the planets are spherical however we must also consider other physical laws that apply to a collapsing gas cloud in order to understand how orderly motions arose in our solar system so at this point we know we had some collection of gas and dust out there called the solar nebula each part of it had some gravitational attraction to the next part but it wasn't enough to drive collapse of the entire Cloud so a shock wave is what we believe would trigger that um and once the shock wave triggers some collapse then gravity allows that to continue uh and then from there we start to see it rotating and flattening into a dis all right so as it contracted the greatest concentration of matter occurred at the center of the nebula forming a relatively dense region called the Proto Sun as its name suggests this part of the solar nebula eventually developed into the sun the planets formed from the much sparer material in the outer regions of the solar nebula indeed the mass of all the planets together is only. 1% of the Sun's mass when you drop a ball the gravitational attraction of earth makes the ball fall faster as it falls so it's that same idea here once it starts collapsing toward the protos Sun it starts moving faster and faster so in the same way material falling inward toward the protocon would have gained speed as it approached the center as this fast-moving material ran into the Proto Sun the energy of the Collision was converted into thermal energy causing temperatures uh deep inside the solar nebula to rise this process in which the gravitational energy of a Contracting cloud of gas is converted into thermal energy is what we call Kelvin helmholtz contraction after the 19th century physicists who first described it as the newly created protos Sun continues to contr contract and become more dense its temperature continued to climb as well after about 100,000 years the protocon surface temperature stabilized at about 6,000 de Kelvin but the temperature in its interior kept increasing to ever higher values as the central regions of the protos Sun became more and more dense eventually after perhaps 10 million years had passed since the solar nubula first begin to began to contract the gas at the center of the Proto Sun reached a density of about uh 10 to 5 K kg per met cubed which is about 13 times more dense than typical iron and a temperature of roughly a few million degre Kelvin under these extreme conditions nuclear reactions that convert hydrogen into helium began in the protos sun's interior these nuclear reactions released energy that significantly increased the pressure in the protos Sun's core when pressure built up enough it stopped that contraction of the Proto sun and thus a star was born in fact the onset of nuclear reactions defines the end of the protostar stage and the beginning of a star nuclear reactions continue uh to the present day in the interior of the Sun and are the source of all of the energy that the sun radiates into space so in very brief terms basically what happens is as this ball of gas contracts it starts to heat up and it heats up enough for Fusion to occur and that's how we get our sun at the center if the solar nebula had been rotating at all everything would have fallen directly I'm sorry if it had not been rotating at all everything would have fallen directly onto the Proto Sun leaving nothing behind to form the planets instead the solar nebula must have had an overall slight rotation which caused its Evolution to follow a different path as the slowly rotating nebula collapsed inward it would naturally have tended to rotate faster we're getting into a little bit of physics here but um this relationship between the size of an object and its rotation speed is an example of the general principle called conservation of angular momentum figure skaters uh make use of this conservation of angular momentum when a spinning skater pulls her arms and legs in close to her body like you can see on the right the rate at which she spins automatically increases in the same way the solar nebula spun more and more rapidly as its material contracted toward the center astronomers see young stars that may be forming planets today in the same way that are our solar system did billions of years ago as the solar nebula began to rotate more rapidly it also tends to tended to flatten out but why so again this is a little bit of physics here but from the perspective of a particle rotating along with the nebula it felt as though there was a force pushing the particle away from the nebula's axis of rotation this is kind of like how passengers on a merry ground uh or spinning carnival ride seem to feel a force pushing them outward and away from the center of the ride this apparent force was directed opposite to the inward pole of gravity so gravity is pulling in and they feel this apparent Force outward and so it tended to slow down the contraction of material toward the nebula Center but there was no such effect opposing contraction in a direction parallel to the rotational axis so basically what this is saying is that after 100,000 years um of its first formation uh when it first started to contract it had developed a structure with a rotating and flattened disc surrounding what would have become the protocon so you've got gravity pulling in but you also have a force pulling some material out so it takes that material and just stretches it and basically flattens this ball into a disc so this disc is what we call a protoplanetary disc since planets form from its material this model explains why their orbits all F in essentially the same plane and why they all orbit in the same direction just think of a big spinning disc all parts of the disc spin in the same direction and in the same plane so um here is a visual overview of the entire process starting from the top left and working down to the right um so quick summary you start out with a giant ball of gas and dust um a shock wave probably triggers its collapse and as that happens gravity can help continue that collapse um as it collapses it has some general overall spin so as it starts to condense that spinning speeds up it becomes more rapid um because of that conservation of angular momentum just like a figure skater but as it starts to spin a lot it starts to flatten out because of that stretching effect uh that we mentioned so you start to get a disc and then from that disc we begin to form some planets now um again I show you that image here that I showed you in the first lecture because you can see it similarity now to what we've been talking about so this was Theory and this is new observational proof notice how similar they are well we actually have more direct visual proof of these uh objects so the Arion nebula which you see on the left is a star forming region located about 1500 light years from Earth the smaller bluish nebula above it um the I'm sorry the smaller bluish nebula is the object shown on the right the four insets are false color close-ups of four Proto protoplanetary discs that lie within the nebula a young recently formed star is at the center of each disc the disc at the upper right is seen Edge on and the one in the lower left shows it um in comparison to our own solar system so these are visual proofs of protoplanetary discs so we know these do occur this is direct of evidence um so what we'll do what we've well what we've done so far is discuss how we go from from just random material in space to a star system but we haven't really talked about how this material now forms the planets so that will be our next lecture lecture 10 thanks for watching and have a great day