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which one do you want to do first how about messier 39 all right i haven't done any homework professor i don't even know what messier 39 is an open cluster in the constellation of cygnus we're doing it mainly because i thought it'd be nice to actually get a picture of my own or one of these objects to illustrate our chat and the night i went out to do this the clouds were rolling in it was a really grotty night and i had to pick a bit of the sky where there was still a little bit of clear sky that i could actually look at and also i wanted something that was kind of unchallenging to look at because i knew i only had a couple of minutes before the clouds rolled in i'll use your picture professor but you won't
be offended if i also use some professional ones please do although i think you'll be hard pressed to find an exciting picture of m39 i mean i'm sure everyone by now knows what an open cluster is yeah so there's these two types of cluster that's the globular clusters which are these beautiful round things generally very old and then there are the open clusters which are generally somewhat messier collections of stars generally a bit younger and they tend to be in the plane of the milky way whereas the globular clusters are kind of in a big halo around the milky way there is a gravitationally bound component of an open cluster ultimately they will all diffuse away and so yeah messier 39 is
just one of those in its relatively early stages is a few hundred million years old so these are sort of like stellar childhood friends having their youth together before they go and have their lives out in the milky way absolutely and they have lots of things in common which it turns out to be quite useful they have similar stellar properties but also they're kind of moving together because there is bound cluster of stars part of the reason why this cluster doesn't look very exciting in the picture that i took and actually in lots of other pictures is because it's enormous it's actually about half a degree across so it's about the same size as the full moon there's probably only about 40 or 50 stars associated
with the cluster and in fact even if you want to see this a cluster there at all what i ended up having to do is kind of move the telescope off to show that there are fewer stars a little bit to the left and a fewer stars a little bit to the right but a few more when you're actually pointing at the cluster itself it's actually very nearby it's only about a thousand light years away so in the grand scheme of things it's one of our closer neighbors of these open clusters which means it ends up looking pretty big on the sky the interesting result that came out i'll show you the paper here there's a paper entitled extended stellar systems in the solar neighborhood five discovery of corona of
nearby star clusters discovery of corona yeah so the new resource we have to play with are the results from the gaia satellite which has been measuring the positions of literally billions of stars with incredible accuracy and so what they were able to do is survey the gaia stars in the region of messier 39 actually they did a few other clusters as well and they can pick out those stars which seem to be moving the same way so remember gaia not only does it tell you where the stars are now and actually how far away they are from this technique called parallax but it actually also measures this thing called their proper motion which is how they're moving on the sky so they were able to
survey this region and find all the stars that are moving in the same direction as m39 but it turns out even that because it's such a big bit of sky you're looking at that's not a trivial thing to do because for example if you imagine supposing you were watching a train passing so you're standing next to the train tracks and you're watching a train passing actually the bits of the train that are closest to you move faster at least in terms of the angle as they move across than the ends of the train because the ends of the train are further away so even though the ends in the middle are all moving at the same actual speed the apparent speed of the bit in the middle is higher than the
bits at the end so they have exactly the same problem here that they actually have to be able to separate take out that effect to actually say okay so you know we need to we actually expect the bits closest to the middle to be moving a bit faster than the bits towards the end and so on so they've used some rather clever techniques to take all that out to try and figure out which stars seem to be moving along with m39 then they use the fact that you have this parallax information which actually gives you the distance to those stars and so that distance then says okay and these are the ones that are actually likely to be in this cluster because that they're at the right distance away
by doing all that they were actually able to identify hundreds of stars remember they're about as i say 30 40 stars maybe that we can actually see in the cluster they detected 350 stars that were originally part of this cluster that are all moving together and it's now this enormous structure right remember i said the cluster itself is half a degree across that's about the size of the full moon that's already very big in astronomical terms in terms of coverage of the sky the whole thing is like 20 degrees across ah so it's a huge bit of the sky that this thing is covering it's really exciting to sort of capture this happening it is the way you kind of say whether something's in the
cluster or outside is whether it's beyond the tidal radius which is basically saying is it still bound to the cluster has it now kind of escaped and it's part of the milky way most of these stars are beyond that tidal radius so they've actually already escaped and are really starting to mix into the milky way but they just remember that they were members of the cluster so they were all still traveling in vaguely the same direction the amazing thing is now they're looking at the stars that are a long way away from where m39 is now you might say well how many of them are likely to be you know contamination or other can they really tell if they're still clustered members from the test
they've done they can get down to a point where only one in a thousand of the stars they're looking at is still a cluster member and they can still say it's that one what's the corona part of the title of the paper then what's that referring to i mean in terms of when you're thinking about the sun for example we talk about the sun and then it has this corona of very diffused gas around it and it's just really by analogy with that they're talking about the cluster and then there's the corona of the cluster which is this diffuse little bits and pieces of the cluster around it professor you know one of my holy grails and i ask you about it all the time is the cluster that the sun was
born in this makes me think about that this makes me think we're starting to develop the research methods that may one day lead to us pinpointing where the sun came from i think the problem with the sun is i mean so these things are already spread out over you know tens of degrees across the sky the problem is the sun went through this process so long ago that all its friends now are kind of spread throughout the milky way so we really aren't going to be able to reconstruct where they all came from this kind of method i'm never going to give up they did this for about 10 clusters they've looked at and actually all of them show the same kind of thing right there's this sort of central cluster but
these diffuse corona and again they've all been stretched out in the same way by these tidal effects and they produce this beautiful 3d toy that you can play around with you can actually look at the data and visualize it in 3d move it around so you can actually really see these three-dimensional structures that these clusters of stars are getting stretched out into and actually one last thing to say is that there's a neat trick that they can do to just convince themselves they're not being fooled by this which is once they've detected which things they think are the members of the cluster then they can construct one of these things called a colour magnitude diagram for them which we've
talked about quite a lot of times before the important thing about them is for any given cluster of stars the stars only fall in very particular sequences in these colour magnitude diagrams whereas if you're just picking random stars they tend to be scattered around all over the place if they then construct one of these colour magnitude diagrams for all the stars they've extracted they lie beautifully on the sequences you'd expect for the m39 cluster so that they know they really have found the stars that originated in m39 professor if we rewound the clock and had a look at this cluster a long time ago when it was a lot newer and all these stars were closer to each other would it still have the appearance of an
open cluster i mean it's not going to be a globular cluster is it what's it going to look like if we went right back to you know near the beginning if you go right back to near the beginning you'll actually probably just see the cloud of gas from which it formed because the stars would all be embedded within that cloud so you'd have one of these things called a giant molecular cloud and because there's a lot of dust associated with that gas you probably might not even be able to see the stars at all it's only when that gas kind of finally diffuses away and takes the dust with it that the star cluster will magically appear within the residuals of that gas cloud but presumably when the gas fight
when the shroud finally lifts it's quite dense with stars it's not the it's not the diffuse cluster we see now i mean so there are younger clusters like the pleiades where we're just seeing them starting to emerge and you can actually still see some of that gas in that you see the reflected light from the stars and indeed they are that bit more concentrated but they're still not the same sort of spherical beautiful structures that you see for the globular clusters they're always a bit messy in appearance nice you can see there are two horizons and i don't know if i can visualize it i can try so you can see i'm looking for two there are two horizons in there's one when the throat
