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all right professor what are we talking about today what where in space are we i'm going to talk about the messier 81 so in space we're in ursa major this is one i've been saving up because messier 81 is a beautiful grand design spiral galaxy professor is there a threshold for a spiral galaxy to go from being a bog standard spiral galaxy to a grand design there really isn't and it's you know there are degrees so the two extremes we describe them as ground design where you get this beautiful sweeping spiral structure over much of the face of the galaxy and the flocking on spiral galaxies where you get lots of little bits and pieces but actually there's lots of galaxies that are somewhere in between that they seem to have two spiral arms but other
little bits and pieces as well so it's not a clearly defined thing but in the case of m81 it really does have two very clear very strong spiral arms in it so it's also known as bodes galaxy that's because in the 18th century johan bode was the first person to discover it was then subsequently independently discovered by maestro who was messier's collaborator in the messier catalogue it is pretty it's glorious and it's sufficiently big and bright if you go somewhere really dark you can just about see it with the naked eye it's what's called circumpolar which means it's far enough north that actually it never sets so it's actually visible for most of the year as one of the objects i took a picture of from my back garden as well
i sent you the picture so you can include or not you're getting really into your backyard garden pictures at the moment no they really are keeping me a little bit saner by taking some pictures and actually i really you know i guess the other thing to say about them is i really believe now in astronomy right that actually now i've actually taken pictures from myself from my back garden it's much somehow even as a professional astronomer you know it's now much less of an abstract exercise because actually i've been out there and taken the pictures myself all right well just to show you up professor right now side by side on the screen we will have your picture from the backyard and a public space telescope picture
there are definitely things you can do with a hubble space telescope picture you can't do with mine and in particular this galaxy is sufficiently close it's about 12 million light years away it's in one of the closest sort of groups of galaxy to the milky way but it's sufficiently close that with the hubble space telescope not only can you get a glorious picture of the galaxy but actually you can resolve it down to seeing individual stars as well and that means that some of the things that we've previously talked about mostly in the context of star clusters within the milky way we can actually do with this galaxy normally we can construct these things called color magnitude diagrams
where you just basically plot how red or blue the star is on the x-axis and how bright or faint it is on the y-axis stars of different ages populate different bits of that diagram so we can even reconstruct the sort of star formation history of different bits of this galaxy by looking at these color magnitude diagrams the other thing that's interesting about it is it's a sort of useful test bed because it does have these grand design spiral structure and we have these ideas about what ground design spiral structure is how it originates and there are two aspects to this one is we think it's usually something to do with interactions so for example the other classic grand design spiral galaxies another the messier objects m51
the whirlpool galaxy where you can see the companion that's kind of stirring up that beautiful structure to it but the other part of the story is that we think it's possible that this ground design spiral structure can have quite a long life it could just be a wave that's kind of propagating around the galaxy and if it's just a wave then it can just keep traveling around and around the galaxy pretty much indefinitely and therefore actually it might not have anything to do with interactions the nice thing about that story is that the long live spiral structure this wave picture of it makes a very specific prediction which is that the way stars form so stars form in the spiral arms because that's where gas is being compressed right if it's a
wave then that's actually gas sort of squeezing together and that's where stars will form but the stars are actually traveling faster than the wave so that means that as the wave travels around the material that's going to turn into stars is traveling around too and it overtakes the spiral arm so what you expect is you have a gas cloud comes along it travels into the spiral arm stars form and then it travels out the front and the stars start getting older that means there should be a very characteristic picture that as you look in front of a spiral arm you should see very young stars just formed right at the front of the spiral arm and then as you go further and further forward from it you should be seeing stars that formed
in the previous generations of material that pass through that arm so they get older and older but of course with these colour magnitude diagrams and the resolve stars from the hubble space telescope uh data we can actually do that so it's a paper entitled testing density wave theory with resolved stellar populations around spiral arms in m81 effectively they did exactly that they analyzed the hubble space telescope data they looked at these color magnitude diagrams for different bits of the galaxy moving forward from the spiral arms they looked for this signature of them getting older as you go further away from the spiral arm and they found absolutely nothing there is no gradient in m81 right you don't see this effect that they were looking for
at all and what this is saying is that at least in this case it looks like this is something which is not a long live density wave it's some sort of short-lived phenomenon that's been stirred up and interestingly if you look at the gas around m81 you find that actually it's very disturbed and it's sort of clearly interacting with m82 and actually with one of the other galaxies in this group as well so it looks like there have been in the recent past sort of fairly close interactions between these galaxies which are the things that have stirred up this short-lived spiral structure so it looks like we just happen to have caught it at the moment where it has this beautiful spiral structure it isn't one of these long-lived density waves
that would have that nice signature to it do those long live density waves that theory of uh spiral galaxies is that like legit or is that in question that's certainly sort of been the conventional view at the very least what this analysis seems to show is that at least in this particular case that's not the answer it could be that you know there are multiple ways that you can actually make spiral structure right that some of them could be these long-lived density waves other ones could have been stirred up much more recently it could be a more complicated story it's not very satisfactory because you know you in science you're always looking for this thing called occam's razor the simplest answer
and the simplest answer would be you know there's one mechanism that makes spiral structure but maybe the universe doesn't work that way maybe there are some of these sort of long-lived density waves and others which are much more rapidly stirred up or could it be that there's been more churn in the stars ahead of the wave than we thought and they've been stirred up a bit more and you haven't got that nice gradient it could be and you know again that sort of fits in with this picture of it being a somewhat disturbed galaxy which maybe has had interactions with other things it could be that several density waves have been excited which are kind of creating a more confusing picture if you look in the
atomic gas for this galaxy you find that actually although that grand design spiral structure is there it looks like there are multiple spiral structures there right there are broken bits there are bifurcating so arms which split which again fits in with the idea that there have been a series of recent interactions each one of which generated its own set of spiral structure but then the sum of those is going to kind of complicate this picture as to what's going to happen with the stars professor we obviously reside in the milky way which has arms do we understand the geography of the milky way well enough yet to be able to point our telescopes towards the leading edges of our arms where the stars are a lot closer and easier to resolve and do
that same analysis but in our own arms we're getting there in the milky way you can pick up these whole clusters and again you can figure out what the ages of the clusters is again from the same kind of analysis looking at these color magnitude diagrams and people have looked at if we look at clusters in the spiral arms and moving away from them and there is some evidence that picture is going on in the milky way but the milky way is a really hard place to do that because we're actually in the milky way and you know there's lots of obscuration from dust plus there's an ambiguity that it's hard to tell exactly how far away something is because we're seeing it through the galaxy it could be very nearby or
further away whereas if you look at a galaxy like m81 basically the whole galaxy's at the same distance away from us so we don't have ambiguities as to what the distances to things are the galaxy around the outside you've got that central bulge there but you've also got this bar running through the middle then at the ends of that bar you'll see there's sort of two concentrations there and if you actually look at a picture of a galaxy here's a nice picture of a galaxy this is a very famous galaxy the whirlpool galaxy m51 and if you look at pictures of galaxies what makes them beautiful is this beautiful spiral structure that you often see in them
