good evening ladies gentlemen how are you all doing this evening how was your recess i know too short and mr melville it is just sort of like a study break quite bad this week has been too long too long already only on monday but at least you guys got a recess i didn't have already says i will just continue working so let's wait another minute or two and wait for more of our friends to join us but while we are waiting it seems like i'm going to preach to the choir again because i've been [Music] really receiving some really stupid questions about the moon assignment stuff i have already discussed in class showed examples in class i showed you websites to find data in class i told you guys not to underestimate project i told you guys what to do when you run into problems and still i get stupid questions and when i try and look at class participation because i can see who participates in the online classes i can see who watch the videos on blackboard collaborate downloaded it from blackboard collaborate i can see you watch the videos from google drive who downloaded it from google drive and i can see you watch it on youtube the last lecture lecture 12.1 that's been up for almost two weeks now there's only been four downloads for that lecture across all platforms and there's only been six views for that lecture on all platforms so what is going on everyone are you not participating in my class are you not watching what's going on if that trend continues then i will remove all lecture slides from click up i will remove all lecture videos and the only information you will get is via participating in the online classes and those students that already reached out to me asking for help with their assignments asking good questions about assignments i'm very proud of you guys because i know you guys are actually thinking about the assignment you're paying attention you are working very hard and that makes me happy and well done to you guys well done to the students has actually made appointments to come and see me to for help of the assignment well done but if i'm looking at who's in the class right now i'm actually preaching to the choir because with everyone online in a class you're all doing very well so that means there are people not paying attention we are currently around about 170 students registered for this module and we are 53 online in this class and in the next week probably another 20 is going to watch the recorded videos so there is a problem i will send out an email on this matter as well as well of whatsapp and as well on this board so just to remind you guys tomorrow i will be on campus i will be there about 9 30 i will be on campus until about 1 30 i have another meeting start at off campus that i need to prepare for so um i'll be there from 9 30 to about 1 30 so if you have any problems any questions you're more than welcome to pop in during those times otherwise you're more than welcome to set up a consultation session and that's it that's me finishing my rant before we start off with tonight's lecture do you guys have any questions anything i can assist with no you don't have to make an appointment to pop in tomorrow so when i'm on campus i have an open door policy as long as i'm there you are more welcome to pop in anytime pleasure and if you would like to have more consultation sessions if you'd like me to be more on campus just send me an email i can see the whatsapp as well but i prefer email as communication language which it just makes it easier to start a paper trailer if i need to escalate something to the hid or to the dean so that just makes it a bit easier but if i'm on campus you guys are more than welcome to pop in anytime if you are off campus and you want a video consultation i'm available on all platforms or video consultations you can do zoom you can do skype we can do google meeps we can do microsoft teams we can even do a whatsapp video calls so if you need consultation i'm here to help you make use of it they said you know what caused the heavy rains and flooding and caves again so one contributing factor is the el nino effect we are currently in so the el nino effect means we are now in a period we have abnormal amounts of rainfall and that is one of the contributing factors to the floods and gases in pleasure before we continue if you guys have any interesting science news you can share something interesting happening currently right now in the science world and especially in the astronomy world so yes there are two so the one is making sure the solar first time ever a solar eclipse was observed from a different planet and yes the curiosity rover observed a solar eclipse for mars that was very cool and then yes the alignment of planets so right now in the morning you'll probably still be able to see it this week just after five the morning there's actually a planetary alignment where mercury venus mars jupiter and saturn and the moon is actually now lining up that is also very beautiful to see so some planetary alignments happen quite often especially with the planets closer to the sun but this one is actually very cool to see on our groups i will post photos of it that few of my colleagues has taken and something exciting the lhc law chadron collider turned on from this weekend it had been undergone maintenance for the last three years and now has been turning back on to start doing experiments again and one thing that i can try to look for in this experiments is dark matter and dark energy so that is also very cool and something worthwhile so before recess we ended off right about here where we discussed the structure of galaxies so if we look at it on we can see the central bulge the sun is located at the end this is the disc if you look at side on here we can see global clusters the halo the essential bulge yes everybody is still happy with this everybody's still following everybody okay with where we stopped and i will continue again so the next important question is how do we determine the mass of the galaxy and what is in the mass of the galaxy to find a mass of an object astronomers must observe the motion of another object orbiting it just in a binary star system so humans don't live long enough to see stars moving significantly along their orbits around the galaxy but astronomers can observe the radial velocities proper motions and distances of stars and then calculate the sizes and periods of the orbits the results can reveal the mass of the galaxy stars in the disk of the galaxy follow nearly circular orbits that lie on the plane of the disk current observations indicate that the sun orbits the center of our galaxy at about 225 kilometers per second moving in the direction of sickness the evidence suggests the sun's orbit is nearly circular so given the current based estimate for the distance to the center of our galaxy that is 8.3 kilo per 6 you can find the circumference of the sun's orbit when multiplying by 2. if you divide the circumference of its orbit by its orbital velocity you will discover the sun has an orbital period of about 225 million years the radius of the sun's orbit around the galaxy is eight thousand three hundred pasic and each parsic equals two point one times ten to the power five astronomical units multiplying you find the radius of the sun orbit is 1.7 times 10 to the power 9 astronomical units so the orbital period is 225 years so that means the mass of our mass is one times 10 to the power 11 so 100 billion solar masses the mass of the sun this is only a rough estimate because it does not include any mass laying outside orbit of the sun allowing for that overlooked mass gives a lower limit to the total mass of a galaxy of at least 400 billion solar masses the rotation of a galaxy is actually the orbital motion of each of its stars around the center of mass stars at different distances from the center revolve around the center of the galaxy with different periods so stars starting near e starting new each other will draw apart as time passes a situation called differential rotation the different different rotation of the galaxy means that stars at different distances from the center have different orbital periods this example the star just inside this orbit has a shorter period and pulls air at the sun while the star outside falls behind to fully describe the rotation of our galaxy astronomers graph orbital velocity versus radius to produce a rotation curve the rotation curve of our galaxy is stored here as orbital velocity versus radius data points show measurements made by radio telescopes observations outside orbit of the sun are much more uncertain and data points scattered widely orbital velocities do not decline outside orbit of the sun as you would expect if most of the mass of the galaxy were concentrated toward the center rather the curve is approximately flat at great distances suggesting that a galaxy contains significant mass outside orbit of the sun if all of the mass of the galaxy were concentrated near center you would expect to see orbital velocities decrease as you move away from the center that is what you see in our solar system where nearly all of the mass is concentrated in the sun for that reason it is called a keplerian motion referred to kepler's laws in contrast the best observation of the rotation curve of the milky way showed that orbital velocities in the outer disk are constant or even increasing with distance from the center of the galaxy those high orbital velocities indicate that the larger orbits enclose more mass and imply that our galaxy has much more mass than is contained within the radius of the sun's orbit both observational evidence and mathematical models show that the extra mass lies in an extended halo sometimes called the galactic corona that may reach out to 10 times further than the edge of the visible disc and could contain more than a trillion solar masses most of that mass is invisible neither emitting nor absorbing light so astronomers refer to it as dark matter some of the mass in the galactic corona is made up of low luminosity stars and white dwarfs but much of the mass must be some other form of matter the most striking feature of galaxies like the milky way is that patterns of spiral arms that wind outward through the disc these arms contain swarms of hot blue stars clouds of dust and gas and young star clusters the young object suggests that spiral arms involves formation but as you try to understand the spiral arms you need to consider two problems first how can anyone be sure our galaxy has spiral arms if interstellar dust obstructs lv and secondly why doesn't the differential rotation of the galaxy destroy the arms the solution to both problems involve star formation as you have already learned o and b stars are often found in associations and are very luminous thus they are easy to detect across great distances unfortunately at those great distances the parallaxes are difficult to observe the available data indicate that ob associations near some are not located randomly but lie along several spiral arm segments so here we can see the galactic centers this way this is the sagittarius arm the urine cygnus spur and the perseus arm here's where the sun is located and here we can see where other stars are so we can see nearby young o and b stars in a galaxy fall along bands that appear to be segments of spiral arms and then an image beam many of the galaxies in the sky are disc shaped like our galaxy most of those galaxies have spiral arms marked especially by hot luminescence and emissions nebula that must be very young this might cause you to suspect that the spiral arms are related to star formation the image is we can see images of other galaxies show spiral arms that are especially prominent and well defined those spiral segments have been named for the prominent constellations through which they pass astronomers can now use infrared and radio telescopes to penetrate the interstellar dusk locate more distance ob associations and trace the spiral arms even further objects used to map spiral arms are called spiral traces aside from ob associations spiral traces include young open star clusters clouds of hydrogen ionized by hot stars so this means emission nebula and certain high mass variable stars so notice that all spiral traces are young objects formed recently astronomically speaking those stars for example live for only a few million years the typical orbital velocity is about 200 kilometers per second so they cannot move more than about 500 parsecs in their lifetimes this is less than the width of the spiral because they don't live long enough to move away from spiral arms they must have formed there studies of other galaxies show that its spiral arms are also marked by hot blue stars and other spiral traces the youth of spiral traces is an important clue about spiral arms obviously spiral arms are associated with star formation before you can follow this clue you need to extend your map of spiral arms to show the entire galaxy so this brings us back to this image so here we can see again our spiral arms is where these ob stars play here's everyone's still following awesome so question does the model account for the light from the back of the spiral taking longer to reach us stay in the front yes it does that is something we are going to discuss in a couple of slides radio astronomers use the strong spectral line emissions from carbon monoxide to map the location of giant molecular clouds in a plane of the galaxy the dust that blocks our view at visual wavelengths is transparent to radio waves because radio wavelengths much larger than diameter of dust particles recall from chattanooga and section 2 that dry molecular clouds are sites of active star formation if you point a radio telescope at a section of the milky way you will receive a combination of signals from gas clouds in direction you are looking that lie at various distances across the galaxy from those different clouds can be separated by measuring the doppler shifts of the spectral lines astronomers then use a simple model of orbital velocities at different distances from the same dark galaxy in order to untangle the observations and locate the individual clouds maps constructed from such observations reveal that the giant molecular clouds like omb stars are located along segments of spiral arms so here we can see an image a add a wavelength spectral lines emitted by carbon monoxide skills astronomers find many modern molecular clouds along the milky way but clouds overlap in a confusing jumble by using a model of a rotation of a galaxy to interpret the radial velocities of the clouds radio astronomers can estimate the distances to each cloud and use them to map spiral arms then an image b this map represents the view from 0.2 kiloparsecs directly above the sun the molecular cloud shown here as hemispheres extending above the plane of our galaxy are located along spiral arms so angle labels indicate the galactic longitudes acid in chapter 9 section 1 especially figure 9.3 observing in a wavelength of 21 centimeters allow the depiction of a spectral line of atomic hydrogen that type of gas is generally warmer than a material in molecular clouds just as for the molecular clouds clouds just as for molecular clouds analysis of the atomic cloud radial velocity data requires that astronomers start with estimates of the orbital velocities at a different distances from the center of the galaxy the atomic origin map doesn't trace as clear as spiral pattern as the molecular gas in part because the warmer atomic gas has more random motions due to its temperature as well as turbulence within the clouds that restore the radial velocities so this image we can see the 21 cm radium map of galaxy confirms the concentrations of neutral atomic origin gas are approximately in spiral but a pattern is complex and suggests branches and spurs observations toward the center of the galaxy have an additional problem the orbital motions of gas clouds are perpendicular to line of sight and all of the radial velocities are zero that is why the mapping figure 12 14 is empty in the weight shaped region directly towards the center clearly we live in a spiral galaxy but a spiral pattern appears to be slightly irregular with many branches and gaps the stars you see in orion for example appear to be in a detached segment of a spiral arm referred to as a spur by combining observations of our own galaxy with studies of other galaxies astronomers can make educated guesses about what a milky way galaxy might look like for you from outside so here we can see an estimate of what our own galaxy would look like so this is a two armed model for the multi way and it's based on observations of the spitzer infrared telescope so notice the large central bar and here we can see branches and spurs are common in this model and the sun should be around about here in addition to helping map the arrangement of spiral arms infrared observations indicate that a central bulge of a galaxy is not a sphere but rather elongated bar pointing partially away from earth in the next chapter you will see that such structures are common in other galaxies one important fact revealed by radio maps is that spiral arms are regions of higher gas density spiral traces tell you that arms contain young objects so you suspect they must also have active star formation radio maps confirm your suspicion by telling you that the material needed to make stars abundant and concentrated in spiral arms having mapped the spiral pattern of our galaxy and seen these patterns in other galaxies you might ask just what are spiral arms you can be sure they are not physically connected structures like bands of magnetic fields that hold the gas in place if they were the differential rotation of a galaxy would destroy them within just a few hundred million years winding them up and then tearing them apart like paper streamers scored on a wheel of a spinning car yet spiral arms are common in galaxies so they must last for billions of years astronomers conclude that spiral arms are dynamically stable they retain some appearance even though the gas dust and stars in them are constantly changing to see how this works think of a traffic jam behind a slow moving truck seen from an airplane the traffic jam would be stable moving slowly down the highway but you could watch an individual car approach from behind slow down when it's turned finally reach the front of the jam passes the truck and resume speed the individual cars in a jam are constantly changing but a traffic jam itself is a stable pattern moving at its own speed along the highway in a spiral density wave theory spiral arms are dynamically stable regions of competition that move slowly around the galaxy just as the truck moves slowly down the highway gas clouds moving at orbital velocity around the galaxy overtake the slow moving arms from behind and slam into the gas already in the arms the sudden compression of the gas can trigger the collapse of the gas clouds and the formation of new stars so here we can see a representation of this so newly formed stars in the remaining gas eventually move on through the arm and emerge from the front of the slow moving arm to resume that travels around the galaxy so here we can see orbiting gas clouds overtake the spiral arm from behind so we can see the gas cloud the galactic center the spiral on the compression of a gas cloud triggers star formations years massive stars and lower mass stars massive stars are luminous make the spiral arm prominent the most the most massive stars die quickly low mass stars live long lives but are not highly luminous so according to the spiral density wave theory star formation occurs as gas clouds are compressed when they pass through these spiral arms what evidence can astronomers have that star formation is caused by spiral density waves spiral traces are the key stars of all masses are forming in spiral arms but as you already learned the omb stars live short lifetimes that die before they can move out of the spiral these massive high luminosity stars along with gas and dust confirm that spiral arms are sites of style formation you can expect that lower mass stars like the sun can't be used as spiral traces they're also forming sterile arms but during their relatively long lives they leave the arms in which they were born the sun probably formed the spot of an association in the style forming region within a spiral arm almost five billion years ago escape from that association and our circle the galaxy more than 20 times passing through many spiral arms plotting the locations of sun-like stars does not reveal the spiral pattern confirming this picture the evidence seems to fit the spiral density away fairly well but a theory has two problems firstly how does the complicated spiral distribution begin and how is it sustained computer models indicate that spiral density waves should slowly die out in about a billion years so something must generate a spiral wave mathematical models show that a disk of a galaxy is naturally strong affected by certain types of disturbances as you learned earlier observations show that the centre of our galaxy is not smeared by the bar the gravitational effects of the bar's rotation could continuously perturb the galaxy's disc and stimulate the formation of spiral density waves a close encounter with a pair pausing galaxy could also stimulate a spiral pattern some galaxies without the visible ball large companion never lays have prominent spiral arms so astronomers continue to make observations and computer models investigating the questions of spiral arm formation the second problem is for the density wave theory involves the spurs and branches observed in the arms of our own and other galaxies some galaxies called grand design galaxies have a symmetric two armed patterns other galaxies have a great many short spiral segments giving them a fluffy appearance but no overall grand design these galaxies has been termed flocculant meaning woolly so our milky way galaxy seems to be intermediate between extremes of flocculant versus grand design what process might make small-scale spiral segments instead of a galaxy-wide spiral pattern the answer may lie in self-sustaining so contagious star formation so here we can see image some galaxies are dominated by two spiral arms but even if those galaxies minus spurs and branches are common spiral density waves can generate a two-armed grand time pattern that cell sustained star formation might be responsible for irregularities then image b many spiral galaxies do not appear to have prominent spiral arms buttered spurs and branches nebulae suggests star formation is proceeding robustly in such galaxies observations indicate that our milky way galaxy spiral pattern is intermediate between these two examples so the answer may lie on self-sustaining circumvention style formation so you learned in chapter 9 section 2 that star formation is one [Music] location can lead by supernova explosions stellar winds and outflows to star formation in neighboring locations look back especially to figure 9.9 the differential rotation of the galaxy would drag the inner edge of star forming cloud aid and laid the outer edge back behind this will result in a region of star formation shaped like a segment of spiral arms so a sphere so here we can see the foreign rotation drags the inner edge of gas cloud aid of its outage a cloud can become elongated by continuing differential rotation star formation in a gas cloud can produce massive stars whose high luminosity and supernova explosions can compress surrounding gas and trigger more star formations if star formation continues long enough a cloud can be elongated in a spiral segment so cells are stating star formation may be able to produce long clouds of young stars that look like segments of spiral arms astronomers suspect that while a spiral density wave can generate beautiful two-armed patterns the self-sustaining star formation process produces the branches and spurs of prominent and some galaxies including our own this discussion of star formation in spiral arms illustrates the importance of natural processes the spiral density wave creates graceful arms but it is the star formation and the arms that makes them stand out so prominently self-sustaining star formation can act on some galaxies to modify the spiral arms and produce branches and spurs in other galaxies it can make the spiral platen flocculant by searching out and understanding the details of such natural processes the astronomers can begin to understand the overall structure and evolution of the universe we live in the most mysterious region of our galaxy is the very center the nucleus at visual wavelengths this region is totally eaten by dust and gas that dim the light it emits by 13 magnitudes so if a trillion photons of light left the center of the galaxy on a journey to earth only one of those photons would make it through the dust and the gas consequently visual wavelengths images reveal nothing about the nucleus observations at infrared and radio wavelengths can see floating interstellar material and those images show a region of tremendously crowded stars orbiting the nucleus at high velocities to understand what is happening in the innermost regions of a galaxy you need to carefully compare observations and hypotheses so one branch of astronomy is called active galactic nuclei that actually studies the nucleus of galaxies one of my colleagues is an expert on this but what would you guys guess is the mechanism in the center of our own galaxy or in most galaxies yes a black old you guys are correct and how many gays does a black hole a why is a black hole important than central galaxies so yes we smell malevo you're right we're also leading up the discussion on quasars so yes gravity it causes the stars in a galaxy to orbit a central point and keep everything together so if you look up at a milky way on a dark night you might notice a slight thickening in the direction of the constellation sagittarius but nothing specifically identifies this as direction of the heart of the galaxy even shapely study of globular clusters identified the location only approximately the first complete infrared map of the central polish made by eric beckland in 1968 showed the location of the intense radiation where the stars are most crowded together the gravitational core of the galaxy so high-resolution radio maps of that status center has revealed a complex collection of radio sources with one sagittarius a abbreviated or a and usually pronounced search a star laying at the exact location of the galactic nucleus radio interferometer observation so that such a star is lacing one astronomical unit in diameter but nevertheless a strong source of radio energy as well as x-ray emissions there are three important points to consider when we talk about this and the first thing is observations at radio infrared wavelengths reveal complex structures near such a star caused by magnetic fields and by rapid star formation supernova remnants show a massive stars has formed there recently and exploded at the end of their lives and secondly the center is very crowded tremendous numbers of stars heat the dust which embeds strong infrared radiation and then lastly there is evidence that such a star is a supermassive black hole in which gas is flowing observations of the motions of stars orbiting the central object indicate its mass is about 4 million solar masses and that is right in the ballpark of a supermassive black hole the supermassive black hole is an exciting idea but scientists must always be aware of the difference between adequacy and necessity the supermassive black hole is adequate to explain the observations but is unnecessary could it be other explanations for example some astronomers have suggested that gas flowing inward could trigger tremendous bursts of star formation causing some of the turmoil observed in the galactic nucleus such hypothesis has been considered and tested against the evidence but none appears to be adequate to explain all the observations so far the only viable hypothesis is that our galaxy nucleus is home to a supermassive black hole meanwhile observations are allowing astronomers to refine their models for instance such a star is not as bright and x-rays as it should be if it has hot accretion disk with meta constantly flowing into the black hole observations of x-ray and infrared flares lasting only in few hours suggest that a mountain-sized blobs of matter may occasionally fall into the black hole and be ripped apart and heated by tides the black hole seems to be mostly dormant lacking a fully developed pop accretion disk because little matter is flowing into it at a present time such a supermassive black oval could not be the remains of a single dead star it contains too much mass it probably formed when a galaxy first formed 13 billion years ago in later chapters you will see that such supermassive black holes are found at centers of most large galaxies so what is the origin mystery of the milky way galaxy so just as dinosaurs left behind fossilized footprints our galaxy has left behind clues about its youth astronomers have compelling evidence that the stars of the spherical component are all must have formed long ago when the galaxy was very young some of that evidence comes from comparing abundances of chemical elements and stars located in different parts of the galaxy so if you ever had a dark place and you can see this band of the milky way this is in the direction of sagittarius so this is looking towards the center of the galaxy and because it's more crowded you can see more stars in this direction so you can recall what you have learned about stellar structure and stellar evolution in previous chapters you understand the chemical evolution of the galaxy in astronomy jargon the term metals refer to all of the chemical elements heavier than helium in a later chapter you will learn about evidence that when the universe began it contained about 90 oxygen atoms 10 percent indian atoms and little or no metals the first stars that formed early in the universe's history therefore had to be nearly pure hydrogen and helium all the other chemical elements have been produced nuclear synthesis the process of stars that fuses hydrogen and helium to make the aba atoms as you already know medium mass stars like the sun cannot ignite carbon fusion but during helium fusion the heat and density can trigger some nuclear reactions that could the gas to produce small amounts of elements heavier than helium when aging star pushes away its surface layers to produce a planetary nebula some of those elements are spread back into the interstellar medium where they may become part of newly forming stars and planetary systems so lithium is a metal astronomy yes that's exactly true so the most massive stars fuses elements up to iron and spontaneously cooked the gas and they caused small amounts of many different atoms including sulfur and calcium window stars die in a supernova explosion traces of those elements are spread back into the interstellar medium along with rare atoms such as gold platinum and uranium produced in the supernova explosion itself which can also be incorporated eventually into new stars and planets nevertheless metals are actually quite rare in the universe conventionally astronomers graph the abundance of the elements using an exponential scale but if you replot a data using linear scale you can see how ray these atoms actually are so here we can see the abundance of chemical elements in the universe so in mha when elemental abundances are plotted on an exponential scale you see that elements heavier than iron are about a million times less common than iron and at all elements heavier than helium so the metals are quite rare and then an image b here we can see the same data plot on the linear scale provides a more realistic impression of how rare metals are carbon origin oxygen makes small peaks near atomic mass 15 and the peak representing iron is just visible in this graph so yes repeats we can see is hydrogen helium carbon origin oxygen iron elements heavier than iron but product linearly you can see hydrogen you can see helium carbon the little blob yes iron and so on so here you get idea how rare these things actually are question sir how would the bulge of our galaxy initially form or would a black hole be the connection point but to my knowledge there are things older than the black hole in the center of our galaxy if it were to be a black hole that resides there so that is a very very very good question so we will now in a couple of slides discuss how the bulge of the galaxy is formed and we will also touch on a few objects that's older than a black hole but the black hole is key to a few of these but we will discuss that momentarily same 1940s astronomers realize that there are two families of stars in the galaxy they form and evolved in similar ways but they differ especially in the abundances of methods population one stars are metal reach containing one to three percent noodles whereas population two stars on metal pool 0.1 metals the difference is clearly evident in the spectra so here is another question for the testing exams i can ask you what is the population one stone what is the population 2 star what are the differences so this is just a tip to keep in mind so here we can see an image a difference between spectra population 1 stars and population 2 stars is dramatic examine the upper spectra here and notice the hundreds of faint spectral lines the lower spectrum has fewer and weaker lines than image b a graph of such spectra reveals overlapping absorption lines of metal completely blanketing the population one spectrum the lowest spectrum is that of extremely metal poor star with only a few weak metal lines of iron and nickel this population two stars contain about ten thousand times less metals than sun do you reckon that a universe is currently in a black hole i don't think so so hr diagrams of star clusters reveal the population one stars are relatively young and population 2 stars are old population 1 stars belong to this component of the galaxy and are sometimes called disc population stars they have nearly circular orbits in the plane of the galaxy and formed within the last few billion years the sun is population 1 star as are the type 1 chapelaid variables discussed in this chapter and in population 2 stars belong to the spherical component of the galaxy and are sometimes called halo population stars these stars have randomly tipped orbits ranging from circular to highly elliptical they are old stars that formed when the galaxy was young the mental poor globular clusters are part of the highland population as are the are earlier and the type 2 ship aids further observations show that there is a gradation between populations extreme population one stars are found only in the spiral arms slightly less middleweight population one stars called the immediate population of one stars are located throughout the disk the sun is an intermediate population one star stars that are even less metal rich as their stars in the central bulge belong to the intermediate population too the most middle poor stars are those in the halo including those in globular clusters those are referred to as extreme population two stars the two populations of stars in a galaxy are clearly different you can see that in table 12.1 those differences are clues to the history of galaxy so when you observe population 2 stars such as those inhaler you are looking at the survivors of the early generations of stars in our galaxy the first stars formed from gas that was metal poor and only survivors of these early generations are low mass long-lived stars a spectra which indicate the composition of their atmospheres and now their cause still shows their composition of tomato pool gas from which they formed population 1 stars such as the sun formed more recently after interstellar medium had been enriched in metals and a spectra shows stronger mid lines stars forming now have even higher metal abundances so what is the age of the galaxy so you are the same age as the universe because the mata can then be created on sorry that you guys even thought about that yeah so you can just say age is really just a number but how do you guys think we can determine the age of the galaxy because astronomers know how to find the age of star clusters they can estimate the age of the oldest stars in the galaxy giving a lower limit to the age of the entire galaxy the process sounds straightforward but uncertainties makes the easy answer hard to interpret the oldest open clusters are 9 to 10 billion years old these stages come from the turn-off points in the hr diagrams but finding the age of an old cluster is difficult because old classes change so slowly also the exact location of the turn off point depends on the chemical composition which differs slightly among clusters finally open clusters are not strongly bound by gravity so all the open clusters may have dissipated as the stars wandered away as it is reasonable to suppose that a galactic disk is somewhat older than the oldest remaining open clusters which suggests the disc is at least 10 billion years old globular clusters have faint turn-off points in the hr diagrams and are clearly old but finding these ages is difficult again slight differences in chemical composition make noticeable differences when calculating the stellar models from which ages are determined also define the age of the clusters astronomers must know its distance parallaxes from the hippocause satellite may allow astronomers to improve the calibration of the shipbait and rr neura variable stars and careful studies with the newest large telescopes have refined the chemical compositions and beta defined the hr diagrams of global clusters the base analysis of all the data suggests that average goblet clusters are about 11 billion years old though some globular clusters are younger than that and some are older studies of the oldest global clusters suggest that a halo of our galaxy has at least 13 billion years old observations of stellar populations and classes show that a disc is younger than the halo so you can combine these ages with the process of nucleus synthesis to tell the story of our galaxy so here we can see clusters of recently formed hot stars in our galaxy the ring of dust here we can see very distant young galaxies with a chaotic structure here we can see our dwarf companion galaxies and that is the large and small magellanic clouds here we can see remote galaxy seen through the ngc 4911 here we can see faint stars in the milky way globular clusters so if you put everything together we know we can start putting everything together so the lack of metals in a spherical component of the galaxy tells you it is very old a fossil left behind by the galaxy when it was young and drastically different from its sprays and disc shape the study of element building and stellar populations leads to the fundamental question how our galaxy formed so if you look here here we can see the galaxy from the top you can see our sun but if you look at sideways you can see the edge view of the milky way here we can see the galactic center so here we can see is young stars lies in this plane and all the stars in this planet so how did the galaxy form so in the 1950s astronomers began to develop a hypothesis sometimes called the monolithic labs hypothesis also known as a top-down hypothesis to explain the formation of a galaxy so late observations forced through elevation of the hypothesis so this is another good question for you testing them so yeah you can see the top a spherical cloud of turbulent gas gives birth to the first stars and star clusters the rotation of the cloud cloud of gas begins to contract towards its equatorial plane stars and clusters are left behind in the halo as the gas cloud flattens new generations of stars have flattered distributions here we can see the disk of the galaxy is now very thin so the monolithic collapse or top-down model for the origin of the galaxy begins with a spherical gas cloud that flattens into a disk so the monolithic collapse hypothesis says that a galaxy formed from a single large cloud of gas over 13 billion years ago that cloud contracted to form our galaxy as gravity pulled the gas inward the cloud began to fragment into smaller clouds and because the gas was turbulent the smaller clouds had random velocities that caused the stars to start clusters that formed from these fragments to have orbits with a wide range of shapes a few were circular most were eccentric and some were extremely eccentric the orbits were also inclined at different angles resulting in spherical cloud of stars the spherical component of the galaxy of course these first stars were metal poor because no stars has existed earlier to enrich the gas with metals the second stage is the in this hypothesis accounts for the formation of the disc component as the gas clouds in originally spherical cloud collided turbulent motions in the gas cancelled out as do it is in recently stirred coffee and a cloud was left with a uniform rotation a rotating low density cloud of gas cannot remain spherical a star is spherical because its high internal pressure balances its gravity but in low density cloud the internal pressure is much too low to support weight like a blob of pizza does spun an air the cloud must flatten into a disk eventually producing the galaxy's disk this contraction from a sphere into a disc would have taken billions of years and while that happened the metal abundance gradually increased as generations of stars were born from the gradually flattened gas cloud the stars and global clusters of the halo were left behind what a cloud as it flattened the subsequent generation of stars formed in flatter distributions the gas distribution in the galaxy now is so flat that the youngest stars are confined to this only about 100 parsecs thick these stars are made to rich and have nearly certain orbits the monolithic collapse hypothesis accounts for many of the milky way's properties advances in technology however have improved astronomical observations and beginning in the 1980s contradictions between theory and observations arose for example not all global clusters have the same age but surprisingly some of the youngest clusters seem to be in the outer halo in contrast the monolithic labs hypothesis says that a halo formed first and predicts that the clusters within it should either have a uniform h or the most distance one should be somewhat older another problem is that all the stars are observed to be metal poor but not completely metal free there must have been at least a few massive stars to create these metals during each generation for the formation of the all the stars now seen in halo can the region hypothesis be modified to explain these observations astronomers suspect that galaxy began as a large gas cloud containing almost no metals models of metal-free stars show that they are very massive so that first generations of stars evolved rapidly and exploded as supernovae which enriched the gas cloud with traces of metals none of those massive first generation stars survive but the metals they created are detectable in the oldest population two stars it seems likely that a central bulge and halo formed from such a gas cloud or from the accumulation of a number of gas clouds a thick but density disc of stars may have formed at this early stage this would explain the age of the central bulge and the metals in both the older stars inhaler and those scattered above and below the disc the thin part of the disc could have formed later as more gas fell into the galaxy and settled into the gas clouds of the thin disc where stars are forming today perhaps entire galaxies were captured by the growing milky way astronomers have found streams and rings of scattered stars surrounding our galaxy and hypothesize that they were produced when smaller galaxies were captured pulled apart and absorbed by a home galaxy you will see dramatic evidence in the next chapter that such galaxy mergers do occur so i can only imagine if i only imagined these astronomers had to go through trying to figure out how things that are light years out of reach work yes and that if you remember when we first started this module back in february in the opening chapter we had a discussion of scientific models what is the scientific models why do we need scientific models and that is exactly why we need to build up a model to try to explain what we can see and detect so uh questions sir does the fury not assume that a galaxy begins as a sphere where would the sphere come from so remember that's actually a very good question so if you don't have gravity pulling on something towards something it will end up here as a sphere so why a raindrop is a severe if you um it's a raindrop falling it's here if you put a blob of something in space or in a vacuum it will end up as a sphere it all depends what forces are working on it but in nature everything usually ends up as a sphere if no external force is acting on it so if their young milky way galaxy effect was partly a symbol from stellar units absorbing several small but partially evolved galaxies in some of the global clusters astronomers seen a halo may be hitchhikers astronomers now have evidence that our galaxy formed through merger of number of smaller pre-galaxy clouds of gas and stars plus later editions of infinite gas and more small galaxies so sir what are so many things in space thing to turn into disks so when it turns into disks is usually it depends on the rotation so if you have a rotation of something you have your if it is spherical that means while it's spinning its equator will become bigger and bigger and where the morph itself always that will become squished together so thinking about it an experiment you can do if you take a piece of dough so usually um you can buy though at spa or pick and pay or any of those shops and that's usually around but if you take that piece of dough and you start spinning it you will see it become flat like a pizza desk and that is exactly what happens so it is due to your rotational forces on it and a bit of gravity so yes you're correct that is described as a scepterian motion so someone had a hand raised so what forces of gravity act on galaxies so what forces act is literally the nucleus or the center black hole because it is so if you remember with take for example our own solar system as uh example we have the sun everything orbits the sun everything is rotating around the sun and the sun is what's keeping everything in place with the galaxy that's the nucleus of the galaxy and in most cases it's a black hole so here we can see the bottom up galaxy formation so you can see mixed clouds of normal and dark matter are drawn together the first stars are massive evolve fast and begin making metals and dust as a large proto-galaxy accumulates more clouds of dark matter plus gas dust and stars fall in and begin building a halo and a thick disk small galaxies are ripped apart by tides and absorbed adding to the 11 disk then gas and dice settle into a thin disk where star formation continues so the botnet hypothesis for the formation of the molecular galaxy proposes that smaller star systems accumulated to form larger ones to see how the scooter have built our galaxy start with the top frame depicting the situation only a few hundred million years after the beginning of the universe a small clouds of matter began accumulating and stars begin forming in them in the second frame the central object has grown larger and in a third frame the galactic alum disks are forming by today represented by the bottom frame the disk of the galaxy has become very thin so there is no external gravity acting on galaxies no there's no external gravity acting galaxies to any way these external gravity that can act on galaxies if his two galaxies approach one another then the two galaxies would play tug of war between them with the gravitational field scores in havoc until one of them is basically engulfed by the other one this bottom-up hypothesis for the formation of the galaxy would explain puzzles such as the observed ranges of globular clusters ages and metal facilities the metal abundances and ages of stars and star clusters in our galaxy seem to be important clues but mental abundance and age do not tell the whole story astronomer bernard pedro was thinking of this when he said cats and dogs may have the same age and metallicity but there are still cats and dogs so here we are now so our planet's hunting neighborhoods most of the planets found to date lie within about 300 light years from our own sun so if you put everything in perspective this is where we are in compared to our own galaxy so from the end of this lecture slides with this quote to boldly go where no man has gone before do you guys know who said this way this came from yes you are correct this is from star trek one of my favorite shows ever so ladies and gentlemen this was this lecture site so let's take a five minute break before we continue with the next set of slides so it's now 6 40 we will restart the class at 6 45. so question how do we know what our galaxy looks like if we are in it so that is a very good question so as we discussed in just before recess we know how other galaxies look like we can observe mechanisms from other galaxies so by observing other galaxies we have an idea of how we would look like so say for example stupid example say for example we don't didn't have mirrors on earth we didn't have cameras we didn't have any material that can show our reflection how would you know you would look like you would look at the other people around you and you will have more or less an idea of how you would look like and that is exactly what we did with our own galaxy so now quiz it's basically an educated yes yes it's basically an educated guess so if a photon is its own anti-particle then why aren't they always annihilating each other so that is a very good question so i'm not a particle physicist so i don't know much about this but i will find out and report back tomorrow evening on that question so that's so we will sort of something we i'm going to announce the end of this class but we still can have our next quiz next week but i will announce everything on click up emails in all our groups in question 3 do you think the gravidon is a dog that is anti-particle uh yeah for me it makes sense so one other thing i want to talk to you guys so there's one thing i want to talk to you guys also about before we start our next set of lecture slides in your third year you doesn't matter what you are studying you can be computer science can be engineering so engineering will be your fourth year physics you will do it doesn't matter what you are studying in your final year you are going to do a small research project called a mini dissertation so for your research projects you need to find a project and a supervisor for that project and you will work the entire year on that project so when it comes time at the end of your second year you will need to pick a topic and a supervisor but remember you're going to work the entire year on that project and work inside your supervisor so make sure number one it's a topic that you will enjoy and secondly also a supervisor you can actually work with because say you have a great great topic and project but union supervisor are bumping it then it's going to be a miserable project and a measurable year but you can have a great supervisor but horrible topic and you will hate every moment of it so i would suggest in you have time go to these yourself to your different lecturers find out what their research interests are and when it comes time to do a project then you know with who you would like to do a project and what project you would like of course remember usually at the end of each year or all lecturers needs to submit projects for students to do the following year so if you are going to so usually beginning of your third year or end of the second year you can just pick a project from a list and you ever submitted that project you get that as your supervisor or beforehand you are welcome to [Applause] create your own topic as long as you have a supervisor that can approve it so that's something just to think about in the future so i'm not sure what you're studying but usually you will do that for any course you are doing [Music] so usually that form sport of your final year so that's just something to keep in mind so why do electrons act differently when observed as that signal consciousness no that doesn't signal consciousness so for the consciousness so that comes down to uh shows is true you remember the experiment of charging the scat yeah that's the uncertainty principle so if you observed subatomic particles if you know where they are you don't know where they are moving what speed they are moving but if you know what speed i'm moving the weight i'm moving you don't know where they are so that's the uncertainty principle so sir are you one of these supervisors so yes i am will be one of the supervisors so it all depends on what you want to do so if so i would say if you're not bothered by anything you just want to get it i will pick a topic from a list and that's your supervisor and topic but usually about six months before your third year before you need to do the project you're more than welcome to reach out to your lecturers find out what their research are what your interest in and you can propose your own topic with a specific supervisor that you want but that's just something to keep in mind that's still two years away for you guys so sir i remember once hearing that our galaxy will combine with the andromeda are we combining with it was a tugging at us so that is actually a very cool question so when you do the math and plot so if you think about the universe the entire galaxy is also moving so we have the sun and say for example earth earth is moving around the sun but the sun is orbiting the center of our own galaxy a galaxy is also moving through the universe so if you play the trajectories of the different galaxies the andromeda galaxy and our own galaxy the milky way is moving towards each other so right now is still too far away that we can feel any interactions but we will discuss this in a later chapter as well as the two becomes closer to one another both will tug on each other and then we will be coming closer and closer and closer the gravitational fields will cause havoc on one of one another until it eventually creates one bigger galaxy please just so in the previous sets of lecture slides we discussed our what a galaxy is our galaxy is formed but within this there are still a few different key differences so with this lecture we are now going to look at normal galaxies and active galaxies but for us to understand so uh sir can ask questions you're more than welcome to ask a question so with this is to understand our galaxies we need to understand a family of galaxies just as we did the family of stars how do we classify these different galaxies screen we need to measure the properties of galaxies speak about the evolution of galaxies then discuss the active galactic nuclei and quasars and then lastly disks jets eruptions and galaxy evolution so it isn't related to work but what is the purpose of an honorary degree okay so that is um let me ask for you guys do you guys really want a discussion on that what is the difference between a different degrees and why it's necessary not a second major what is the difference between a bachelor's honors master's phd okay so one thing this so one thing you get is your normal [Music] it was something that was quite regularly in the past but usually you did an associate's degree that's a two-year degree but the associate's degree was replaced by so that is you get a broad knowledge to be able on a certain topic or field to do it but then you get a bachelor's degree so for bachelor's degree you have a specific focused area and you learn general knowledge so for example if you do a bac that stands for bachelor of science it's like physics for example so if you're a bachelor of science in physics that means you have a broad background of physics meaning you can do basically anything in any branch oh yeah you have a basic understanding of anything in branch of physics so that means you have basic understanding of nuclear physics particle physics astronomy quantum mech classical mechanics you can do a bit of everything but with honest degree is you start to specialize into a specific field so for example if you want to specialize in for example astronomy you do a bachelor of science honors degree in astronomy meaning all the modules you take and classes you have will just be on astronomy modules and then when you do a master's degree it's meaning you say example you're doing a masters in astronomy and meaning you are becoming a specialized in astronomy meaning you are taking one little piece one aspect of astronomy and you are focusing on that and when you do a phd you even choose a smaller focus area of astronomy and then you become the world expert on that that is why you have a phd a phd is for unique work so if you pick something astronomy into a phd on it then you are the world expert in the topic you've chosen in astronomy so if you are working in industry then usually for most places a normal bachelor's degree is fine then you have the basic background and you can do your work so take for example if you want to do software development or a software engineer then a normal degree is fine you have your normal background and you can continue but if you want to be in a specific industry where you need a certain skill set then you need a honest degree to build up that specific skill set but if you even need a more specialized skill set you will need to do a master's so in the past if you do a bachelor's degree then you find but these days industry is looking at honest degrees and if you are an industry and you have two people with the exact same experience applying for the exact same position then the master's degree will be better than honest degree but if you want to work in academia because academia is research based then you need a phd so question you say in your honors here you will only be doing modules related to your course so yes so for example in your honors degree if you want to do a honors in astronomy then you're just going to do autonomy modules if you want to do a for example a honors degree in nuclear physics then you are just going to do nuclear physics modules and that's how it works and then when it comes to academia you get certain levels as well so you just don't become a professor so if you want to become a professor you need a phd and then you need to do a certain amount of research so if you have done a certain amount of research and a certain amount of publications behind your name then you've supervised a certain amount of students then you can get promoted to a associate professor and then if you have done even more research to even more students and even get more publications and research behind your name then you can be promoted to a full professor so i hope that that answers a few of your questions it's a pleasure yes i'm on course my phd and i'm sure a few of you will do it as well so it's difficult to be considered for a master's degree now it's not difficult to consider that all depends on where you're going to study and what are the minimum requirements are so for um all the pain so big difference is if you're undergraduate doesn't matter where in the world you are studying say example you do a bachelor of science in physics in undergraduate it doesn't matter at which university you are where in the world the work will be the same work but you all have to do the same modules at undergraduate level so you're all going to do newtonian physics in first year in in physics it doesn't matter which university you are so if you want to do a bachelor's degree the university doesn't really matter but soon as you are looking at those graduate qualifications daily different university plays a big role because certain universities have certain research groups and research fields where the employers or lecturers are specializing in so say for example let's say up for up as an example say for example you want to work in micro electronics then ub is extremely good because ups a well and founded microelectronics research group but if you want to do for example renewable energy then you go to uj because uj has a very good group in renewable energy for example if you want to do photonics research then uj has a very very good photonics research lab but if you wanted to semiconductor research and particle research then ub has a very good research groups that so depending where you want to do your your masters in phd that is very important and honest also so yes you're correct and resources so ub is radio specialist so we have optical astronomers at ub as well but if you want to do more optical astronomy work then you need to go to vets and uct but yet up we look more at radio astronomy research as well so cosmology yes we do cosmetic at up as well and we do cosmology a bit so it all depends what you want to do so that's way to specialize where you need to go to do your masters and phd programs and it all depends on department to department so usually were in some degrees if you want to do a master's degree then you just need an honest if you have honest then you get entry into a master's program but if you want for some different departments if you have honors you need to have a minimum average of 65 percent to get entrance into the masters program for example but it all depends on faculty per faculty and boardman department so how astrophysics so yes ub um office courses in astrophysics we have a good astrophysics resource group at ub we also have a um it's a good astrophysics research group at fitz at uj and uct as well so then you need to look at astrophysics what part of astrophysics do you actually enjoy and there you will need to find the appropriate supervisor at the appropriate university so it's impossible to do more than one honest degree yes it's possible to do more than one honors degree so you can do honest degree for example in astrophysics you can do an honors degree in nuclear physics so it all depends on what you want to do and yes you can do more than one honest degree so right now recently we have started a new research group at ub it's collaboration between um department of electrical electronic engineering and the physics department where we are going to look at more at radio astronomy where we are going to look at instrumentation development for uh astronomy so we are going to look more definitely at astronomy seeking processing what so what i'm doing and we are also going to look at correlation and one thing we are also going to look at is white band receivers so um white band receivers is something that is quite a new hot topic in the astronomy world because you don't want a receiver that only looks at a narrow band of the electromagnetic spectrum you want one receiver that can look at a wider range of electromagnetic spectrum and that is what you are what we are going to we have received now um the mission to start a new research group and we've also received funding for the group so that is actually now happening so do other countries have honest degrees in other countries only have masters and phd only but not honest so yes that is something and depending on the system we are following so if you look at the commonwealth country so that means all the countries that form part of the british empire have honours degrees so basically a honest degree is your fourth year so am i right you have to do your honors performances yes you have to do your honors performance your honors is um yes you have to have honors before the master says some degree say for example uh llb laws you don't hear people do honours in law because the law degree is fourth as a four-year program so that means you don't get your degree on a bachelor's level you receive it on a honest level so that is why from an llb you can directly do an llm for example inside for example the engineering department so ub is the only university that offers a separate honors degree for engineering but other universities you get your degree on honest levels so you do a big inch it is a get an honest level you can directly do a masters so usually in countries such as the form of commonwealth countries commonwealth countries you do your bachelor's your honors your masters your phd but in other countries such as the u.s they don't use the honor system so they have the four-year degree so you do your four-year degree that you get on honors levels basically your honors degree their masters did phd and you get paid for phd masters or paid myself so it's a degree you enroll for so you have to pay for it but usually on master's level about 99 of time you apply for a master's or phd bursary adversary will pay for your research and that personally will pay you a you call it salary each month that you live from okay cool so far from visitors we have gone through all 20 lectures so for our no not all 20 lectures i will break it down in the scope for you guys so talking about this truth is stressful no it's not stressful you're gonna have it so what are we writing so it's gonna be an online test so you guys probably recognize this image would it be open book notebook won't be open book it will be a closed book yes the exam will also be online pleasure so do you guys recognize this image yes you are correct this is taken by the hubble space telescope and this is the hubble deep sky image if you just google double deep sky image you will um get this is just a photo taken of one tiny dark spot yes that is so this is in the night sky one of the most darkest spots uh i can't remember where it was but it was just somewhere beside the moon in some way but with the naked eye and through smaller telescopes it seems like there's nothing there but then hubble took a picture and everything you see in this image is a galaxy so look at all these different and beautiful galaxies so instead of these two dots are two stars in the foreground but the rest are all galaxies and from this we can see galaxies comes in different colors and different shapes and sizes so from this we need to build up our family of galaxies the same way we did as our family yes this thing they exposed by i know it is extreme so milky way galaxy is only one of the many billions of galaxies visible in the sky this chapter will expand new horizon to discuss difficulties different kinds of galaxies complex histories and violent interruptions so i'm just looking here for almost any other comments i yes miss uh could see her you were more welcome to come to a consultation session to talk about different ideas and you're more welcome to pop in any time or just send me an email and we can set up a consultation session so just bring back to consultation i will be on campus tomorrow from about 9 30 to 1 30. i just need to leave campus on 1 30 because i have a different appointment off campus at 3 o'clock and so you're more welcome to pop in during that time and then if you want me on campus for other consultation sessions just send me an email um set up basically a time and date uh eat some of it in my mason tuesdays as well so here you can expect answers to five important questions so one what are the different types of galaxies two how do astronomers determine distances sizes luminosities and masses of galaxies thirdly do other galaxies contain single supermassive black holes and dark matter as does our own galaxy four how do collisions interactions affect the evolution of galaxies and then lastly how did supermassive black holes in the galaxy nuclei form evolve and affect those galaxies do these questions look familiar to you guys these are almost the exact same questions we asked about stars so remember we asked what are the different types of stars now supporting different types of galaxies so every time i get a supermassive black hole there's some news plays in my that is a song i've heard a long time ago and if i'm not mistaking that song always it or came out more or less the same time as the twilight series so when we talk about our different galaxies we have a few different galaxies so these are all classifications i could ask for at least an exam so we have our bed spiral galaxies our eerie galaxies our spiral galaxies killer galaxies elliptical galaxies and lenticular galaxies now that is not a twilight series so the twilight series is supposed to be the same as black mirror but twilight movies with the werewolves and vampires so it's a peculiar yes it is peculiar that's why i've actually received the name so something you would realize is astronomers aren't very good at naming things so usually it's named what i see so it's peculiar let's call it peculiar galaxy that is something we will discuss in this chapter as well so science fiction euros effortlessly between the stars travel between the stars but almost none travels between the galaxies as you leave your own galaxy the milky way behind you will voyage out into the depths of the universe out among galaxies into space so deep this unexplored even in fiction so why do you hear some sci-fi movies they don't leave the galaxy or don't explore different galaxies not as far as scary is very empty but if you want to leave the earth we need an escape velocity to scan the gravitational pull of the earth if we want to leave our solar system we will need at the escape velocity of our solar system so if you want me to leave our galaxy we need to escape velocity to our galaxy as well that's the one thing and the second thing is the galaxy is so big we don't have the technology to even leave it so even in star trek in star trek voyager when voyager got flung to the other side of the galaxy it took them almost 60 years to get back so that is why that is why it is a unique topic as well to talk about so in sci-fi movies we'll jump into millennium falcon go to hyperspace and explore our own galaxy but to explore different galaxies are a different story on its own so astronomy books often include pictures of spiral galaxies like movie stars spiral galaxies get a lot of attention because they are beautiful but many galaxies are nearly featureless clouds of stars and other are distorted models of gas and dust you must begin by sorting out this jungle of galaxies so here we can see a century ago photos of galaxies look like spiral clouds of haze modern images of these relatively nearby galaxies reveal dramatically beautiful objects filled with stars plus clouds of gas and dust so here we can see young blue stars illuminate the spiral arms here we can see clouds of gas and dust in a small foreground galaxy visible insulated against the background galaxy here we can see dusty disk of the galaxy warped by the interaction with another galaxy and muse starts forming in the dusk clouds and here we can see dust clouds glow red in this infrared image so astronomers classify galaxies according to their shapes in photographs at visual wavelengths using a system by edwin hubble in the 1920s such system of classification are fundamental technique in science so firstly many galaxies have no disc no spiral arms have almost no gas and dust these elliptical galaxies so class e range from huge shrines to small dwarfs insecurity disc shaped galaxies usually have spiral arms that contain gas and dust some of these spiral glasses galaxies classified is have a central region safe like an elongated bar and are so called barred spiral galaxies so abbreviated is b so you learn in chapter 12 that the milky way galaxy is a broad spiral a few disc galaxies contain relatively little gas and dust these are called lenticular galaxies glass is zero finally notice that the irregular galaxies class i which are reason are generally shapeless and tend to be rich in gas and dust it is surprisingly difficult to figure out what proportions of galaxies are elliptical spiral or irregular in catalogs of galaxies about 70 are spiral but that is misleading because spiral galaxies contain hot luminous stars and clouds of ionized cast make them easy to notice most ellipticals are fainter and harder to notice small galaxies such as dwarf ellipticals and dwarf irregulars are actually very common but are hard to detect from careful statistical studies astronomers estimate that ellipticals are actually more common in spirals than that irregulars make up about 25 of all galaxies so how many galaxies are there long exposure images of small areas of the sky are called deep fields because they detect most distant galaxies deep in space so here we can see in this image and apparently in this spot on the sky only 1 13th diameter of the full moon contains more than 1 500 galaxies in this extremely long exposure known as the hubble deep field north only four stars are visible in this image they are sharp points of light with the diffraction spikes produced by the telescope optics eventually the entire sky is similarly filled with galaxies so what are the properties of galaxies what are their diameters luminosity luminosities and masses just as our study of standard characteristics the first step in the study of galaxies is to find out how how far away they are once you know a galaxy's distance its size luminosity are relatively easy to find later in the section you will see that just as for stars following the masses of galaxies is more difficult so now we're going to start using almost the same principles we need to know how far away it is to luminosities its size then we can turn everything out so ladies german i see we have two minutes of this lecture left so let's call it night we will continue tomorrow night of this lecture and then we will discuss the different properties of galaxies so remember i will be on campus tomorrow for consultation we will have a quiz next week i will upload information on that in the next day otherwise you guys are more than welcome to log off have a great week and i sure everything will go well for the second quarter so i have a great evening everyone and i will see you in tomorrow night's class so which room and building will be in i'm a natural science 1 building 5th floor office 68th well the quest for next week do before the start of next week or will it be next week so we'll be opening next week so most likely open next week monday and close the following sunday it's a question about the project can i edit nasa image to represent my observations unfortunately not you will have to do your own observations um so for tomorrow i will be in my office from 9 30 to about uh 1 30 the afternoon so i mean only the image of the moon because my drawing is bad so you can add it above your drawings you can do your drawing and then you can add that with the drawing pleasure so i'm going to remain active for another minute or two if you guys have more questions so should you read a free body problem probably my favorite sci-fi book series either i will definitely read it awesome so if there's no more questions then i'm also going to log off have a great evening everyone and i will see you all in tomorrow night's class good evening