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we have messier 55 otherwise known as ngc 6809 there it is it's a globular cluster it's got the defining feature of all globular clusters which is it's a cluster and it's globular in other words it's round it's actually one of the most southern of the messier objects it's a declination of minus 30 degrees so it's actually you know above a latitude of -30 which means it's quite hard to see from the northern hemisphere messier struggled really hard to see this object he was told it had existed from observations made from cape town and struggled for a long time to actually make his own observations of it but did eventually find it himself from paris and so was able to add it to his catalogue the reason why globular
clusters appear around is because they're intrinsically pretty spherical things and the reason why they're spherical is largely because all the motions in them are completely random there is no preferred direction in them all the stars are all orbiting around in random directions kind of like a swarm of bees and there's this sort of trade-off in gravitating systems like this that the more random the motions are the rounder it will appear and as soon as you start having ordered motions then actually things start appearing flatter and so you can think about the extreme case of this as a system like the disc of the milky way where all everything's moving around on nice neat circular orbits and
that makes for a very thin disc so this is trade-off the more the orbits are ordered rather than random the flatter the system becomes now in this particular case the object is appears completely round but what we don't know is whether it's intrinsically spherical so really is round in three dimensions whether it's actually somewhat flattened but we just happen to be viewing it exactly face on and then you know even a disk-like thing viewed exactly face on will appear around and so we can't tell just from a picture like that whether it really is rotating at all and hence whether it's flattened or not and from the traditional way that astronomers measure rotations of things we're completely
stuck because you rely on the doppler shift which only measures the line of sight motions of things which means if a thing is completely face-on the stars are all just orbiting in the plane of the sky which means that you don't see any motion towards you or away from you so you don't see any doppler shift so we can't tell but that's what's now changed and it's changed because this wonderful satellite called gaia which has been up there for a few years now measuring very precisely the positions of lots and lots of stars and then making repeated measurements of them and from that it can do various things it can measure the distances to them but the main thing that it can do that's useful to us is
that it just tracks them over time very accurately so you can actually see the individual stars move so for the first time ever we're not just restricted to the motions of stars away from us or towards us we can actually see the movement of the stars on the plane of the sky and so the later release number two from gaia actually has enough data in it that we can actually monitor about 8 000 stars in this globular cluster there's some of the stars that the guy is following and we can see how they're moving and so you can actually tell for the first time ever whether this thing's actually rotating there it is messier 55 otherwise known as ngc 6809 and this is as you go away from the center of the galaxy the movement of the
stars and you can see that actually so this is this movement on the plane of the sky and it's measured in these not very helpful units of milliarc seconds per year but if you translate that into velocity speeds it works out that's about one kilometer per second so you can see that as you work your way upwards indeed the stars on this thing are actually all rotating around and as you go out it stays more or less constant at around one kilometer per second as you work your way out with so indeed it is rotating on the plane of the sky from an astronomer's point of view one kilometer per second is incredibly slow so for example the sun is going around the milky way at about 200 kilometers per second so from that
perspective it's a very low speed but these globular clusters they're not very massive which means the movements of stars are typically only a few kilometers per second so this is actually you know a normal kind of speed that you see things moving in a globular cluster and it means some significant fraction of the motions of the stars in the scopus cluster are actually rotation on the plane of the sky and this is the first time we've ever actually been able to see this effect in systems like globular clusters that we can actually tell indeed this system is rotating which means it probably is somewhat flattened but we can't see the flattening because all the flattening is that way rather than that way
so when you're measuring the motion of stars what are you using as your reference point because everything's moving it's really tough because as you say there are no fixed points very distant stars are obviously moving much less things are moving appear to be moving more when they're closer to you very distant stars move much less so in principle you can use that partially as a reference frame but then at some point you actually want to tie things down to a real absolute reference frame and one of the things the gaia satellite does is it measures the positions of very distant quasars so these very bright very distant star-like objects they really aren't moving on these kinds of time scales at all so they are the ultimate thing that kind of ties down
the reference frame and allows you to take the gaia observations and sort of put them in the context of any other observation so that's how the reference frame is ultimately tied down how come we don't see really flat globular clusters then like you know the occasional one that is the right angle for us they all seem pretty round because actually although that rotational motion is about one kilometer per second the random motions are probably about three or four kilometers per second and the thing that really dictates the flattening is that ratio so about a factor of four or they're about fact it depends on that ratio squared so actually it's about a factor of 16. so actually really that even though that rotation is significant it's still a
small fraction of the total motion of the stars the motion is still dominated by random motions which means it's still roughly round so if i could like shimmy around the corner and look at a different angle of m55 it would still look pretty round it would still look pretty round it's actually one of the faster rotating globular clusters in terms of this balance between the ordered motions and the random motions so it is a significant factor but it's still basically going to be more or less round when you see it on the sky yeah i'm not looking at a pancake very much not no just slightly flattened so once they have formed there's a couple of things that can happen you get star interactions in a global
cluster so you have like flybys of stars it's not intended as a detailed reproduction of what's out there it's a kind of cartoon view to kind of aid thinking and he's trying to figure out what i think why it is you see these stars in certain directions on the sky
