A series of talks organised by local astronomical societies to help beginners to the hobby of astronomy gain knowledge on a wide range of interesting topics. This is the third session in programme of 6 sessions.
The collaboration between Bath Astronomers, Bristol Astronomical Society and Cardiff Astronomical Society was conducted on Tuesday 27th February 2024 by Zoom for members of local astronomical societies.
Index
00:00 Stephen Price of Bristol Astronomical Society – introduction
02:45 Dr Jane Clark of Cardiff Astronomical Society – the study of the universe as a whole
31:10 Dr John Henn of Bristol Astronomical Society – nebulae, star clusters ang galaxies
well hello everybody my name is Steven price from the Bristol Astronomical Society and it’s my pleasure to be chairing this evening’s session on behalf of the bath Bristol and Cardiff astronomical societies I’d like to welcome you all to this beginner’s course for astronomy and hope that you really enjoyed the topics we’re going to cover this evening this is the third session of a six-part program and this evening’s theme is large scale Cosmos but first of all as ever some quick guidance on attending this evening’s session over Zoom please will everybody remain muted during the session so we can avoid any distractions to the speakers and the audience we can mute attendees if required and we really do appreciate your cooperation in this if you have a question during or after a talk then please type your question in the chat box on Zoom we’ll have a few minutes at the end of the talks to answer some of these questions and we’ll respond to other questions after the session is closed and let you know the answers so that brings us to our two talks for this evening our first talk is entitled cosmology the study of the universe as a whole by Dr Jane Clark from the Cardiff Astronomical Society a few words about Jane Jane is a retired amateur astronomer and has a PhD in physics and an MBA from Mor University Jane became interested in both astronomy and photography as a teenager in the 1970s photography much more seriously although as her career progressed and family commitments increased both interests lapsed she acquired a telescope in 2006 shortly after completing her MBA and quickly became hooked on observing this experience made her realize that astronomy is a lot more fun than Business Administration in 2017 she achieved her ambition of having an observatory in her backyard Jane is a member of Bristol Cardiff and New Town astronomical societies and was a Founder member of the West norol astronomy Society Jane give talks on astronomy to astronomical societies schools and other societies as diverse as the CB Scouts the University of the third age and the church’s wives groups so with no further Ado I’d like to invite Jane to talk to us on cosmology the study of the universe as a whole over to you Jane so we’re out in the entire universe at the moment uh and really um the study of the universe as a whole what exactly what are we studying well that’s changed over the centuries as our view has moved outwards uh and I think you know the people who started cosmology uh in ancient Greece would be very surprised by what we’ve got as a view of the universe now uh and if truth be told I find the whole thing very surprising and you know how do we get here uh anyway how big is the universe well the brief and simple answer is that we don’t know but uh you may have heard of a telescope out in space called the Hub Space Telescope it took a photograph called the ultra Deep Field basically they pointed it at the same part of space a place where it was known that the view wasn’t going to be cluttered by stars from our galaxy and in this tiny region that they photographed they I mean they exposed it for weeks and weeks and weeks and weeks um and they found 10,000 galaxies in this tiny region now if you assume the universe is the same in all directions which is a pretty reasonable assumption then that implies you know we have the capability to observe 130 billion galaxies and each of those galaxies uh like the ones in the picture contain about a 100 billion stars and each of those stars is a million times the size of the Earth so you know we don’t know how big it is but we do know it’s enormous now the solar system this was really discovered by five men uh whose pictures are shown in the Rose Gallery there uh and of those five men it started really with cernus uh and it took a quarter of a millennium to get to New Newton who really completed the disc discovery of the solar system uh and I categorized the people cernus was a theorist he never made any observations um he just didn’t like the Greek theories and thought he could do better um Tao this gentleman here was an observer Galileo was an observer and an experimentalist Kepler who actually worked for Tao at one stage and after Tao died suddenly Kepler took over was a theorist again he did he actually invented a form of a telescope he wasn’t really an observer um he was a mathematician and Newton did experiments but they weren’t on mechanics in the solar system they were mostly on Optics or Alchemy so from this point of view and of course he was one of the greatest mathematicians that ever lived so he was mainly a theorist uh now these people as they put together this picture of the solar system which is pretty much our modern picture of the solar system um they had some idea that the sun was a star like the the Stars they could see at night but they couldn’t measure Stellar distances uh I mean this we’re talking now kind of Newton’s time in the um 1600s early 1700s and a contemporary of Newton called hens estimated that Sirius was about half a light year away it’s actually about eight light years away so um having discovered the solar system they had some awareness that it was not the whole universe and um once telescopes were introduced over time they gradually improved uh so introducing them which was done by people like Galileo was a revolutionary step but since then development of telescopes has largely been evolutionary um and herel in the late 18th century that’s late 1700s worked out the distribution of stars so he uh got the first idea of the Milky Way shape and hersel in fact collaborated with his sister uh and it was the Hershel siblings who did a lot of their work uh and Caroline herel the sister uh during her lifetime held the world record for the number of comets discovered so she was a substantial scientist in her own right uh I was really the first female scientist celebrity she was you know she was much sought after in upper class Social Circles um now the first person who accurately measured the distance to a star and got pretty much the answer we know now was this guy Bessel uh those of you there may be one or two of you who’ve done science or engineering or math degrees I may have heard of Bessel in the context of math mathematic because he was also a substantial mathematician um but uh his contribution to astronomy was that he was the first person to succeed in measuring the distance to a star he started out as an accountant by the way and like me he found out that business administration wasn’t where it’s at um now we all pretty much know what a telescope is I think um but there’s another key tool which we’ll come back to time and time again in the study of astronomy called spectroscopy this was a revolutionary change uh it started with Newton who who did do a lot of Optics experiments and he discovered in 1672 that light splits into colors um herel uh that was a gentleman we mentioned earlier as having the first suggestion as to the general shape of our galaxy he uh put thermometers next to the Spectrum and what he found was that yeah I mean where the light fell it the thermometers showed a higher temperature but where you couldn’t see any light there uh the thermometer still went up so he discovered that not all light is visible um and uh walliston Henry walliston discovered in 1802 that the solar Spectrum which you know goes from red yellow yes all the way down to Violet but in fact there are discontinuities in it these dark lines appear um so there was an interesting thing that meant there was more to Spectrum to a spectrum that meets the eyes um and from having discovered it a guy called frown Hofer um developed a lot of knowledge of the solar Spectrum in the early 1800s uh and one of the things about Spectra is that they’re chemical tools you could tell what what uh atoms are present and molecules um and then there is a hydrogen Spectrum with lines missing so that’s like the sun Spectrum uh but you also get what’s called an emission spectrum uh and you’ll notice that these lines line up like that so if you spot that pattern of lines whether it’s in an emission spectrum or an absorption Spectrum you can identify hydrogen so that’s chemical information uh and there’s the Spectrum by hydrogen helium neon sodium and Mercury um and I remembered we had to do when I was a physics undergraduate we had to look at a sodium light and I think I discovered about 18 lines in that were visible you can’t see them all here but they were there and some of them were pretty faint um now it took a 100 years for 100 more 120 years for people to understand why this happened uh well FR H first started the discovery of it um and Heisenberg you know he of the uncertainty principal actually worked in a spectroscopy Department in his university and he was the one who uh figured out why uh these spectral lines do what they do and that was in 1925 now another trick you could do with Spectra I mean this is really you know you’re beginning to get the idea I hope the Spectra are pretty important is you can use them as thermal tools and the victorians gradually discovered this and it was finally solved by two people in Germany plank in 1900 and Einstein in 1905 um and stars are classified by color and temperature and the observational work to do this was mostly LED from Harvard University they had telescopes both in the northern hemisphere uh we talking the early 20th century now uh and in the southern hemisphere they did not let women use these telescopes but they did let women analyze the photo s that were taken uh and in fact the there were some very substantial discoveries made by these women um but uh the spectrum that you get you know for instance I mean if you’re not used to looking at graphs uh that’s the intensity in some units or other uh but for at 7,000 Kelvin which is effecting 7,000 degre Centigrade give or take a little bit um you get a peak there in the Violet and at this wavelength you get some emission say at Red you get some and so on down there now if you’re at 3,000 Kelvin which you think of it as 3,000 degrees Centigrade um you’ll get see it’s predominant it’s brightest in the red and so at 3,000 Kelvin if you see a star whose surface is at 3,000 Kelvin or 4,000 it will be redish you see a star whose surface temperature is 6,000 Kelvin like the sun it it looks like that um and the sun looks kind of yellow to the eye uh and at 7,000 kin it will look blue so hot stars are Bluer cool stars are redder that’s the deal now um and in fact helium was you know the stuff they put in balloons to make him float was first noticed in the sun Spectrum before it was discovered on Earth uh and it was discovered um that the sun is mostly hydrogen I mean you may have heard this notorious story um that Cecilia pay karkin disc was a research student at Harvard University discovered this uh and her external examiner for her PhD was this guy Russell who said no that’s not true you’ve got to put some words in there to say that you it looks like this but it’s not actually true so she did and then a few years later Russell clayy discovered it himself uh so it’s a bit of a blot on uh Russell’s character but anyway um this woman did she went on to become the first female professor at Harvard University so she didn’t do so badly for itself uh now Russell and this guy herong in Europe discovered that if you plot brightness that way against temperature that way Stars don’t randomly fill that graph there is a sequence there like that the main sequence which is what they first discovered um and so and then there’s a sequence there for Giants a sequence there for super Giants beetle juu is a super giant and a sequence down there for a special type of style called a white dwarf there’s a companion around Sirus which is a white dwarf uh so this was was really the key piece of observational evidence that people used to work out the theory of Stellar Evolution uh and uh well the first thing they had to discover was because nobody could understand why stay Stars would stay so hot for so long before Edington pointed out in the 1920s that it was nuclear fuel bear in mind the atomic nucleus had not been discovered till 1910 by Rutherford um or thereabouts um still a structure that first person to work on it was emden but in the 1920s Edington is the name most associated with it uh he was at Cambridge University uh meanwhile back at the nebulas in the sky nebula is a term you’ll hear in astronomy nebula is Latin for clown and telescopes were finding nebul objects so there’s a 19th century telescope owned by a guy called lord Ross who was a sort of of um you know Victorian industrialist who made a pile of money um and people were some of these nebuli that they were finding with telescopes gradually over the years people began to identify spiral nebul as a subtype of these uh and I mean the later the these early ones are definitely drawings or paintings these later ones are photographs and photograph photography greatly accelerated this work and this is an early photograph from 1888 of the great nebula in Andromeda so where we now people by about 1900 telescopes could resolve far away stars as well as nearby ones of course you know you can just look up and see nearby ones henrya lit in 1912 found a way to measure the distance to a small southern hemisphere nebula um which we now know to be a Galaxy called the small melanic cloud and she used a type of variable star which people were beginning to realize existed called Sate variables um and if you plotted the magnitude see the nor magnitude is bright 25th magnitude is unbelievably dim um and for the LA sorry it says LMC it should say small uh that was leaveit law for the SW maganic Cloud but the period over which it varied in days was directly related to the the brightness Hubble later recognized some saf variables in the Andromeda nebula um and he’s his plot of those I mean he was this was well within what people could observe then so it’s a fairly accurate looking straight line there Hubble was absolutely at the limit of what people could observe in his day and he was using what was then the biggest teles whoops the biggest telescope in the world um so his data were kind of you know less obviously linear but they were still pretty obviously linear and the brightness of those of these there five magnitudes less bright than that so if you do the maths on that it turns out that means that the Andromeda nebula must be 10 times as far away as the SW melanic cloud and it therefore was clearly outside our galaxy so the idea that there are more than our galaxy is just one of many galaxies was born and now the universe is seen to be made up of galaxies not just planets and stars now spectroscopy next trick it had up its seve was that movement away from us stretches waves see those if you’ve got an object moving that way there these waves get stretched and these waves get compressed and red wavelength has a longer wavelength than red light there a longer wavelength than blue light so if your spectral lines are shifted to the red that implies you’re moving away if you don’t understand that don’t worry just accept it the red shift means moving away but blue shifted means moving toward WS so in the direction in which you are looking not sideways but in the direction in which you are looking uh spectroscopy gives you speed information now the galaxies it turns out are red shifted and there was a guy called vesto Slifer in the 1910s who noticed that but just noticed it as a fact um and the further the galaxies are away because Hubble had a method of measuring the distance to these Galax IES now the further away the galaxies are the greater the red ship so this was the plot that Hubble came out with showing that the further away a galaxy is the faster it’s moving away from us in other words the universe is expanding so not only have we now in the had this time of very great change in the 1920s where it was discovered that the Universe consisted of galaxies but we also found that these galaxies are moving away from one another so it’s an expanding universe of galaxies and this is expansion has since been confirmed by many many observations uh and the argument runs well if they’re expanding like that red shift is proxy for how far away they are um then what happens if you wind the clock back you wind back the movie all the galaxies must have been in the same place about 14 billion years ago billion is a thousand billion you’re not sure so it seems that there was an event of some kind in which they all flew apart but actually it’s not the galaxies that are moving what’s happening is that space is expanding and space was created in this event when it was all together uh and is still being created as these galaxies move apart or appear to receive from one another uh that you know in other words the big didn’t explode into anything space exploded and took the big bang with it which is kind of a difficult concept to wrap your head round I know I’ve struggled with that one for a long time um so the evidence for the Big Bang the galaxies are getting further apart the redling is seen in Spectra we’ve done that um and light is stretched as space expands so that’s what we’ve just discussed also there’s an echo in Cosmic microwave radiation the reason is complicated there was the alternative to the Big Bang was a steady state Theory which was proposed in the 1940s and that doesn’t predict the cosmic microwave background radiation so while that theory was taken seriously before the microwave background radiation was found uh and the microwave background radiation was predicted about the same time um by these guys um but in the 1960s these two people they were working for the American Bell Telephone Company uh and of course they were interested at that time in radio communications to extend our telecommunication range the satellite Communications and stuff so from an engineering point of view they had to work out what all the noise was in the radio signal get rid of it and quite by accident they found this background radiation and got a Nobel Prize for um because they spoke about it to the astronomers at the local University and they saidwell ah have you read this paper by galov and Company now another prediction that the Big Bang makes again don’t worry if you don’t understand why it’s all complicated stuff um but it predicts that there’s about about 24% of the universe the universe is vast majority of it is hydrogen and helium it predicts 24% is helium we observe 25% so that was also a big nail in the coffing of the steady state Theory um because the steady State Theory required a universe hundreds and hundreds of billion years old to make the helium in stars but we just don’t see any evidence of this the universe has aged and matured quite detectably in the last 14 billion years um an example of that is a phenomenal called quazars don’t worry if you don’t know what they are uh but they were found there are no quazars near us they’re all a very very long way away and they’re all very very bright so uh you know since when we look back because light has a speed we look back in time that’s an example of evidence that the Universe has matured um and the galaxies that we saw see far away earlier times were noticeably different from today’s galaxies so by now the evidence for this big bank is just overwhelming uh but we don’t know everything I mean we are still on a journey through cosmology to discover what’s going on in the universe and we are by no means there yet first puzzle is a puzzle of missing Mass galaxies are known to Cluster together and you saw that a little bit if you had your eyes open in that sequence of galaxies um with that rather lovely music at the beginning um and we can measure how it and what you can do is when you’ve got a cluster of galaxies in in the radial direction to and from us you can measure how fast they’re collapsing we can’t really measure the sideways collapse speed but we can measure the front back collapse speed and if there’s a big cluster of galaxies that’s enough to tell how fast they’re collapsing under Gravity um and these galaxies the you you use I mean the the most up to-date theory of gravity we’ve got is Einstein’s theory um and we use that and you know according to that we need more mass to make these galaxies collapse that fast so there are two possibili ities one is there is some dark matter that we can’t see and the second possibility is that on a scale of cluster of galaxies our gravity theory is wrong um because the gravity Theory well first Newton developed it for the solar system Einstein um developed it a bit further uh but on scales much greater than the solar system Einstein’s theory has been shown to be more accurate than Newton but it might be that actually we still haven’t found the ultimate theory of gravity yet and I would say probably 90% maybe more of professional cosmologists um work on dark matter and about the rest of them which is a much smaller number work on modifying gravity and there’s been a huge amount of effort uh because the the presence of a rival Theory means that the Dark Matter people have had to work a bit harder to prove their right as it were or to show they’re right um so these guys have acted as Devil’s Advocate but the thing is that there has been no observation yet which knocks one or other of these theories out so we still don’t know and we have no clue what dark matter is you know I mean the particle physicists particle physicists are a bit like cosmologists um you know they’re often wrong but they’re never modest um and they think they know everything well they told us we were going to discover it with this large Dr cider in at CERN in Switzerland well they haven’t so um the and you know as the properties of Dark Matter people are beginning to have to make it look more and more magical to make the theory of Dark Matter hold up so there’s a big question mark there we don’t know one minute two slides Okay one minute to go thank you thank you um the second puzzle is star formation we know that galaxies make stars and we know that a whole galaxy is needed to make a star forming nebula of gas um you can’t just form a nebul in the middle of nowhere but we don’t yet have a theory of that and there are hundreds of professional astronomers through the world working on this uh and the third puzzle is the expanding Universe the expansion is getting faster we don’t know why uh is it repulsive gravity at long distances and when Einstein put his theory of gravity together uh he did show that you could have a repulsive term that solved his equations um but he was very comy com about whether that was physically real uh and the the allegation is that dark energy is the repulsive term in Einstein’s theory now this theory is out by a factor of 10 to the^ of 120 that’s what 10^ the 120 is it’s one with 120 zeros after it so is a huge number uh and if your theory needs to to be multiplied by that huge number to get it to work it’s wrong very wrong the most wrong theory in history and with that I’ll leave you well thank you very much Jan for an excellent introduction to cosmology um really was life the universe and everything in that sort so it was really good and uh please feel free to give virtual Applause to Jane for an excellent talk thank you Jane there will be a bit of time at the end of the session for questions or any questions for Jane but we’ll move on first of all uh to our second talk for this evening please and the title of that talk is nebuli star clusters and galaxies and that’s going to be given by Dr John Hen from the Bristol Astronomical Society now a few words about John when John was a boy he always wanted to build flying sources and launched them from Peru and he was inspired by the radio series journey into space in 1971 as a member of the Hawker cly Dynamics Team John watched Europa 2 being launched from kuu and French Guyana the North coast of South America the first stage was Blue Street which worked perfectly however the Italians were proud that their satellite was still sending signals when it hit the ocean John has a degree in physics from Mor and a PhD in magnetic suspension from Sussex he’s worked at philon on various space and defense projects and later produced online learning software John says being a dreamer I like to try the impossible so I got involved in 1984 with trying to restore the 70m long very derit welchire and barkshire Canal and he’s still very much involved in that today John was brought back to astronomy by evening classes with Dr Rodney hiler and joined the Bristol Astronomical Society a few years ago becoming a commit member and in John’s own words has been causing problems ever since well I wouldn’t agree with that John but I would agree to listening to your talk now please so John nebula star clusters and galaxies and over to you please so that’s so that’s the Milky Way scene over Stonehenge the same scene would have been seen for thousands of years so when our ancestors and ourselves look into the sky you’re not only looking at a pretty scene you’re looking at at a history the the reason for that is that light is traveling from those stars at uh 180,000 miles per second so it’s a finite number or 300,000 kilomet per second why is that not okay right um so what we’re going to do is look at how we measure distances the um distance between the Earth and the Sun varies between 152 million kilm and 147 million kilm so the average distance is of 150 million kilometers or 93 million miles and for those who know a little bit of astronomy that this distance between the Earth the average distance between the Earth and the sun is one astronomical unit now light takes about 8 minutes to cover the distance between the Earth and the Sun thus eight light minutes is equivalent to a distance of 150 kilm or 93 million miles so as uh Jane was saying space is big enormous so we don’t use kilometers or miles to measure the distance we use Li ear and a lier is something like 9 trillion kilometers or 6 trillion miles so you can see that that’s a very important uh way to to measure distances so now we’re going to look at nebula we’re going then look at star clusters and galaxies so we start off with something we can see as faint patches in the sky and nebula so space is large and very oh it’s almost like a vacuum but it’s not a vacuum it consists of low density gas which is mainly hydrogen and helium some dust and radiation so that’s in space nebula are slightly more dense clouds uh so a cloud you know a nebul cloud the size of Earth would still only weigh a few kilograms so this is huge so you can see there’s there’s not much uh material in in space even in nebula so how come you know where why do nebula for form that is because you get clumping of the material gravity of a a small Clump attracts material to a bigger pump Etc or you could have a shock wave from a dying star from an explosion of a Dying star That Could That Could Bunch the material up now many nebula are strong strongly related to the life cycles of stars so this is the Crab Nebula and it’s associated with the death of a large star so a large star here in I think it was observed by the Chinese in the mid 11th century um and it’s a it’s a a massive star 15 20 T times plus the size of our sun and the result of this explosion is a tiny neutron star in the middle of that and in this case this neutron star is rotating very fast and it becomes a pulser so this is the death of a large star death of a smaller star like our sun um it end will end up as a as a white dwarf and in this case sometimes these Stars they puff out a lot of material during the sort of dying stages so 90 I think it’s something like 90% of stars in the galaxy will end up as a eventually as a white dwarf so those were that’s the death stars or the star death of stars for the nebula now we’re looking at the birth of stars and if you recognize this this is the Orion uh constellation on its side and in the Orion constellation there is a molecular cloud a huge cloud of of gas and dust um obviously again it’s very diffuse but within this within this Cloud you’ve got various nebula the Orion Nebula here and the horad nebula here and the flame nebula so they’re all within the cloud so the Orion Nebula m42 is about 1300 Li years from Earth it’s 24 lities across and has a mass of 2,000 Suns it’s an emission um nebula because there is light from Stars within the nebula um keeping it bright and if you look very closely there there’s four very bright stars known as the trapezium and that’s powering the light in in the nebula we have the next is the horse head nebula which is an absorption nebula that means there’s dust in there which blocks the light from behind it so this nebula is about 14 to 1500 Li years from Earth still part of that huge molecular cloud and what you can also see here is another red emission nebula the flame nebula and then the blue reflection nebula so reflection nebula sort of reflect light from nearby Stars so there’s quite just that’s a very brief introduction to different types of nebula um and you know it’s a it’s a great big Topic in its own right so from nebula we we we started off with the death of stars and the the nebula we just seen are the bir of stars and these Stars they’re not they they are part of a nebula and then they form from the same nebula they form Stars so they they form open clusters of stars which contain dozens or hundreds of stars and they’re often rich in gas between the Stars they they are there we know there are about a thousand irregular clusters in our galaxy and it well that we can view and probably probably there are a lot more because they’re the other side of the uh of the nucleus of the Galaxy so this is an open cluster the pades are seven sisters and we can see that with the naked eye it’s about 400 light years away 86 Li years in diameter contains a thousand stars dominated by a number of hot blue and extremely luminous stars so that’s a quite a quite a good constellation to look at at the night sky so we’ve had the open clusters which are sort of new stars being formed and then we’ve got the globalia Clusters um and their spherical collections of hundreds or hundreds of thousands of stars they’re nearly free from gas and dust and contain some of the oldest stars in the galaxy and this is a picture of the Omega centuri cluster located about 16,000 light years away most massive cluster Global globally a cluster in our galaxy containing several million stars now it is believed and this is from the telescope guia that this was actually the the nucleus of a dwarf Galaxy which is been subsumed into the Milky Way and at the center of that is probably a massive or reasonably massive black hole so now so those that’s those clusters of stars now we go and look at the Milky Way as a whole so the Milky Way galaxy consists of a disc of about 100,000 liers in diameter uh Galactic bulge a Halo of stars and you’ll notice within um within the disc there open clusters and you’ll also notice that the sun is approximately 26,000 lighters from the center so the number of stars in the mil Milky Way somewhere between 100 and 400 billion and the mass is about 800 to 1500 billion times the mass of our sun so it’s quite big so the disc as I said is a diameter 100,000 Li years and it’s got a thickness of about 1,300 Li years so the thickness of the the actual disc is about 1300 to 1600 light years what’s interesting it contains most of the Galaxy’s Interstellar star forming material so so again if you look at it again this star forming material is mainly concentrated on the plane of the disc so the disc rotates and half the in interstellar material is within 80 liers of the central plane so you’ve got 80 light tiers there very small and you got 13,000 for the Bulge and um 1,300 for the disc so it’s concentrated in quite a relatively small area or volume rather I should say so again star formation does not occur randomly but in spiral arms so this is the rotation of the Galaxy going around there so there’s due to rotation there’s an outward force and in the center there’s an inward force and obviously because the stars are sort of uh kept in more or less the same position then the force is balance and our galaxy takes approximately 220 million years to do one rotation so that’s the dis now we go to the Halo which is a thinly which is a thin uh distribution of stars and glob GL globular clusters it’s spherical in shape about the galactic center or nucleus and the radius is about 65,000 light is so it’s it’s it’s big so at the center of the Halo is the Bulge so it’s got its greatest uh concentration of stars are in the Bulge and a bulge is centered around the nucleus so the the Bulge is about 20,000 light liers long and approximately 12 12 13,000 lighest thick now what you will will’ll be showing you at the moment but this is effectively a bar in the center of our galaxy and the length to the width of the bar is approximately 3 to one and we’ll come across different types of spiral galaxies here in a minute so that’s the Bulge and then at the center of the Bulge is the nucleus right in the middle and if if you look look at the distances we’ve got thousands of Li years but the nucleus is only about 400 Li years from the center so it’s a very small part of the Milky Way so it’s the D now this is this is I thought this is amazing fact densest distribution of stars so if you look at the uh concentration of stars where we are in the in the dis we have if you make a if if you make a a c a sphere of about 3.3 liers in diameter so as you can see there one star so when we look at uh our location the nearest star proximus interior is about four Li years away so in that in that space that will between as and and proximus and Tory if we take that as a whole volume we’ve got one star in the nucleus there are 50,000 stars in the same volume which is I think is an incredible thing um obviously if you’ve got a planet there’d be no night it’s a region of violent and energetic activity um you’ve got you’ve got the Stars giving off radiation you then possibly got you get uh collisions of stars and possibly the the odd Supernova so life I don’t think if we Earth was stuck in the middle of the of our galaxy I don’t think life would have existed or would exist on that so that’s uh that’s the core and the stars and we’ve been looking at the center of our galaxy for quite a few years and this is an infrared uh sort of uh view uh and we’ve been looking at these s s stars um over the last 10 to 15 years and S2 uh is moves in an orbit of about one light week in a period of 16 years and it’s the closest approach to this point here is 17 light hours or 120 astronomical units so 20 astronomical units is four times the distance of so PL Pluto is approximately 30 astronomical units from our sun so it’s four times that distance the closest approach and radiation regions plus orbital speed details of this star and other stars very close stars is this there’s stars that come even closer to the sarius A so the calculations reckon that Sagitarius a has a solar mass of about four has a mass of 4.3 million solar masses so it’s it’s it is good to assume that Sagittarius A is a massive black hole at the center of our nucleus sorry sorry center of our galaxy at the nucleus or core so that’s the Milky Way galaxy now Jane mentioned the Andromeda nebula and what’s amazing that a 100 years ago people didn’t know whether these nebula were part of the Milky Way or not um um are they were they other separate Island universes of star and of course Mr Hubble using the C variables measure the distance and said well the Andromeda galaxy is far beyond the Milky Way galaxy so on his CV he you could say he discovered the universe so that’s quite a good CV point to put on your CV right so we so we’re now looking at galaxies our so beyond the Milky Way so got spiral galaxies which are the most common they’re yellow in the middle because of the Bulge and blue towards the outer edges and the categorization depends on the the spirals how close the spirals are and how many they are the um B spiral I said the Milky Way is one um and again it’s the same sort of categorization and apparently 2/3 of the these spiral galaxies have bars so this is just a picture of the pin wheel Galaxy with not without a bar the diameter 170,000 Li years and 27 million Li years from Earth quite close really and then you got this B spiral the great B spiral people so that’s go with 200,000 light years across and it’s 56 million light years from Earth so it’s when you look up in the sky there are amazing sites so we now go from spiral galaxies to elliptical galaxies they are yellow rather featureless they normally are in G Galactic clusters or smaller compact groups contain older stars some are low mass some are high mass the lack star making gas but they have dust clouds so here is famous m87 53 million Li years ago it’s in the cluster Virgo uh home to several trillion Stars so it’s big it’s a big galaxy has a super massive black hole which if you remember the Event Horizon telescopes actually photo got a an image of it it’s got 15,000 glob clusters so the Milky Way is 150 forgot to mention the andromeda’s got about 400 and it’s a diameter of about 120,000 liers it’s a big Galaxy this [Music] one so you’ve got the spiral and elliptical now you got Irregulars which don’t have a spiral elliptical shape this is a an image of a dwarf irregular galaxy so some of these odd-shaped galaxies these irregular galaxies result from the interaction of of Galaxy so in this case two spiral galaxies are collided and they formed an IR regular galaxy known as the mice because they’re the long tails coming off the main Gala Galaxy bodies so that’s irregular uh galaxies they can be large or small the most abundant G type of galaxy in the universe are dwarf galaxies but they hard to detect the low Luminosity low mass and small size most are elliptical or irregular but there are some dwarf spirals so diameter 300 light years to 30,000 light years you know the Milky Ways 100,000 light years 10 million to 10 billion solar masses is um these dwarf galaxies most commonly found in clusters often as accompanies to larger galaxies so this is Andromeda with some some dwarf galaxies around it so galaxies the universe got hierarchical structure so stars are not clumped random randomly throughout the Universe but are clumped together in galaxies galaxies are are clumped together into clusters and clusters are clumped together into Super clusters brilliant isn’t it so our local cluster the local group contains three large galaxies and more than 20 dwarf galaxies they haven’t found them all yet this is a this is a photograph of the Mel magelan lamp big genic clouds um large and small which are irregular dwarf galaxies and can be viewed by from the with the naked eye from the southern hemisphere so they’re part of the of the local group so the local group has got these dwarf galaxies and then three major ones the Milky Way the Andromeda and the triangulum one and triangulum Galaxy and you’ll notice the triangulum Galaxy is significantly smaller than the other two our other two galaxies so those are the main galaxies in the local group so we got quite a small cluster of galaxies but if you look at the Virgo cluster which includes the m87 it’s approximately 50 million Li years away and 10 million Li years in diameter and it contains over 2,000 galaxies then you go to the con cluster which is 300 million light years away 16 million Li years in diameter and it contains about 10,000 galaxies so you’re getting a bigger cluster there super clusters and uh the biggest superclusters are 320 million Li years or more long or in diameter and they contain loads of sun the the masses huge amount more than the the than uh the mass of the Sun so our local group and the Virgo Custer are part of the lania supercluster which is about 160 million ligh is in diameter and that little red dot is the Milky Way so it’s so it’s amazing it’s the way the the universe is structured so we now talked about nebula star clusters galaxies Galaxy clusters and now we go to dark matter and I’m just going to just introduce this very quickly so 80 years ago Fritz swiky observed galaxies in the coma cluster were moving so quickly they should have been flung away into space so this is this is a a video of it oh an animation the stars in the cluster provided only 1% of the mass needed to keep the galaxies from escaping the Clusters gravitational pull only 1% of the the matter you could see well the dark matter you know there’s only so only 1% of the mass needed to keep the Clusters together and Vera uh in the 70s ver Rubin discovered the same type of problem in the individual spiral galaxies the Stars at the outer rim of the Galaxy should have been flung out into space but they they weren’t so there was something there that that we couldn’t see but added gravity uh two minutes please John right so we cannot see so how we cannot see it doesn’t interact with atomic particles Etc so how do we know it exists because of gravity so um Einstein um sort of put the space time fabric sort of bent bends um and if you look at galaxies they bend the SpaceTime around it so light passing close to it is bent around the Galaxy producing an intensified multiple images of background sources strong gravity ational lensing and it turns the galaxies into a huge natural telescopes again visible matter is not enough to to uh to to create the observed warping effects so dark matter provides Surplus gravity and this is a picture of the eel 370 Galaxy so there’s mysterious blue streaks which are actually distorted images of remote galaxies it also has various um multiple images and in there that is a Galaxy which has been spread so this is the lensing effect of um dark matter of matter and most of it dark matter so dark energy 69% so this is the composition of the universe dark energ 69% dark matter 26% ordinary matter 4.5% of which only .5% is in the former Stars thank you for watching so we’ve gone through nebula star clusters and galaxies there we go thank you very much well that’s great John thank you for excellent talk on an introduction to nebula star clusters and galaxies and some really fantastic images there you’ve shown in your slide so uh appreciate that very much um so we’re just up to the hour hour now and before we go into any Q Q&A some virtual Applause for John for that excellent talk thank you John um we’re just going to go on for just a small number of minutes now to look at a couple of questions that have been asked if you need to go now feel free to do so but this will only take a few minutes we’ve had a couple of questions come in I think for you Jane if that’s okay um the first one is from Allison Lana how do we know what shape the Milky Way is well that’s a very good question actually uh what I didn’t mention was that there’s been a tremendous amount of effort it started um in about the year 1900 um there was a Dutch School of astronomers who were beginning to figure out methods of of work people were learning how to meure distances to Stars I think that was the key thing and so so people began to get an idea of the size and shape of it from that it’s still ongoing the real problem is um that we can’t see the center of our galaxy because of all the dust uh let me try and find you a picture uh give me a minute uh and I’ll show you right uh file right we go I’m just gonna put the picture here I want yeah here it is right uh I’ll just right find the zoom window again uh actually what I’ll do is I’ll slideshow current slide and then uh settings swap present slideshow sorry this always takes a minute uh and there find the zoom window again and hit share screen button uh that’s the one I want sh so you should now be able to see a picture of three galaxies can can you see that yeah yeah right that one there is Edge on so you can’t see the middle uh and if I go back a slide that’s right yeah there are I’ve blown it up in this picture here you can’t see the middle of that because of dust we have exactly the same problem with the Milky Way because we are in that plane we can’t see the middle uh and we can’t see you see the D dust extends over most of the Galaxy so we we can’t see through them we are beginning to learn how to see through it in particular by using spaceborn um telescopes and spectrometers to look in wavelengths longer than visible light infrared in particular um because uh visible light is not transparent to dust infrared is transparent so people have are able to do things like measure distances to stars and measure angles the stars and so on so it the picture is still being put together today but that’s roughly the answer right thanks very much Jay thanks for those images too now John’s talk prompted so much deep thought that it took for your talk to finish before the questions came in John but I think you can have a sign of relief I’m not asking you what is dark matter so you’re okay so the F the first question is from Joe why do most spiral galaxies only have two arms I think that’s I think I’m not sure I think most you’ve got a great variation of spiral galaxies I mean I showed you the picture of the pin wheel has got quite a few arms so I’m not quite sure what causes do you know Jana what causes the difference in arms and number of arms but youve got the whole range you don’t yes it’s um the uh each Galaxy tends to be surround he big Galaxy tends to be surrounded by several dwarf galaxies as John mentioned and per bations from those dwarf galaxies tend to be what enhances the spirals the spirals the spirals themselves are not consisting of stars that move like that they’re waves that run through the stars and I think the most common pattern that forms from these disturbances is a pair of spirals you do get galaxies with more than two spirals uh but they are as the question quite rightly asks they are rare great okay that okay thank you for that we’ve have one final question come in that we haven’t answered yet and that is also for John what decides if a star should become a white dwarf or a black hole and that’s from Le so it’s it’s just the mass of the star so mass of stars8 to 10 times the mass of the of our sun likely to go through a supernova explosion and at the end of that Supernova explosion is either a neutron star or a black hole stars that are like our sun perhaps up to a couple of times the mass of our sun gently fade away well they don’t quite gently fade away they have a different different end of life and they don’t have a don’t have a super anov explosion they just sort of puff out bits of gas and end up as a white dwarf so it’s the S it’s just basically the mass of the star there have in fact been a few Stars very few that are much bigger than that like 30 to 50 times and one or two have now been observed in our galaxy to collapse directly into black H black holes yeah without an explosion yeah because the EXP the biggest star that we know of uh is in the melan cloud and it’s it started off at something like 320 times the mass of our sun but but the these massive stars don’t last very long um and it’s only a couple of million years old so it’s life cycle they live fast to Die Young and that one probably is because the problem is the Stars Chuck stuff out you know this solar wind yeah comes out from ours you get these coronal mests and stuff but it comes a point where it Chucks out Mass faster than gravity can pull it in that’s what limits the size of stars and this I mean this 300 solar mass one that John mentioned was a kind of rude surprise to the proponents of that theory because it according to that theory that it shouldn’t be able to collapse that much uh and so the suggestion is that it’s probably because it’s in a dense cluster of stars that some stars have merged to formate yeah it’s it so the thing about astronomy the more you go and investigate the more more more you want to find out that is very true and thank you very much both of you for answering that question we’re about out of time now so I’d like to thank you all for listening and for the excellent questions that have been asked this evening thanks very much and of course thank you for our two excellent talks tonight from Jane Clark and John Hen so uh just want to look ahead now to next week our fourth session will be held on Tuesday the 5th of March again over Zoom again at 715 for 7:30 start and the theme next week will be space exploration and an invitation will be send that very soon to you all so look forward to seeing you all then and goodbye for now bye-bye