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so this paper came out uh mid last year and as soon as i saw it i was like i have to do a deep sky videos on this it's called the sausage globular clusters the sausage globular clusters there's about eight of them that are classed as sausage globally clusters four of them are messier objects and there was one that we hadn't covered yet which is m75 it's a globular cluster there's 400 000 stars in there so it's pretty big it's actually the densest globular cluster we know because the majority of stars are actually in that central nucleus it's about 67 000 light years away or so and it's actually the other side of the center of the galaxy from us making it sort of the globular clusters in the
messier list that are in the milky way it's the furthest from us but i bet you're still dying to find out why it's a sausage globular cluster there was a paper that came out in april of last year that was looking at the positions and velocities of stars in the milky way and they found something that they dubbed the gaia sausage gaia is a mission that is basically trying to look at the positions velocities and distances of about a billion stars in the milky way it's gonna look at some other stuff as well but its main mission is stars and milky way because what it wants to do is actually build sort of like a 3d map of the milky way what gaia will do as well is actually observe all these stars at like 70 times each and
then what you'll be able to do is track their progress around the milky way and like model what the parameters are that describe their orbit around the centre of the milky way and so you then can get this like 3d map of like a billion stars in the milky way like moving there's a lot of people that are excited about gaia data and they had their first release last year this paper that discovered the gaia sausage was last year it was by bella karov and collaborators and so one of the plots that they made was this one which is kind of messy what they've plotted here is two different velocities of the stars okay they've plotted the radial velocity and what we call the azimuthal velocity so if we have a normal sort of
coordinate system that we're used to you know and we see graphs in x position and y position say this is like the center of the milky way here zero and you have a star over here now in terms of the center of the milky way doesn't really make a lot of sense to use those that coordinate system instead what we use is spherical polar coordinates so that star is at a radius r away from the center of the galaxy and it's at an angle theta on its way around the galaxy so say this zero is maybe like the line from the sun to the center of the milky way then you can measure that around from the sun and so what they've measured in this paper is the speed at which the star is going around the center of the galaxy so that would be
the velocity in the theta coordinate that's your azimuthal velocity and they've also measured the speed at which it's changing in r direction as that would be your velocity in your r dimension so basically you can think of it as like how fast you're going around something but then how fast your radius is changing too so what they've plotted in this paper is that v r against v of theta but they've plotted it in lots of different panels because what they've done is bin their data in metallicity which is how much like percentage you have of the sort of heavier elements as opposed to like your hydrogen elements which kind of gives you an age of the population as well because think
about it the very first stars that will form or form from like pure hydrogen and then they'll sort of run out of fuel and they'll have runaway fusion as they die and then go supernova and throw those elements out and so the next generation of stars will form with a higher percentage of like oxygen nitrogen like all the way up to iron and so your higher metallicity then the older your population tends to be and then also they've plotted it in bins of the height above the plane of the galaxy so we've got in the plane of the galaxy here and as you go down this way you get higher above that plane of the galaxy and then you've got less metals on this side and then the more you go across you get to the more metal
rich stars so older stars basically so the thing what they found the sausage in is the plots on this most right-hand side in the higher metallicity stars so you can see in this top panel you've got two very distinct populations okay one with a lot more stars in so that's why it's a lot darker in the plot and one with a lot less stars in this one is the sort of normal galaxy disc right because on your x-axis there you've got the velocity in the r direction which if you think about stars going around the center of a galaxy you don't expect that to change that much you expect them to stay at the same radius and just go around the center of the galaxy with a speed that is like a big speed in your
azimuthal direction your theta velocity then you've got the sausage this is the you see this sort of sausage shape here and that's where you've actually got barely any velocity really in the azimuthal direction but then you've got a massive difference in sort of minus 400 to plus 400 in your radius direction so these stars in the sausage they're not orbiting like normal stars like we'd expect like just around the center of the galaxy in fact they're they'd barely have any velocity around the galaxy but they have a huge velocity in your r direction so they're sort of doing this the fact that they weren't orbiting like normal suggests that those stars didn't actually form in the galaxy itself
there's something we call in-situ star formation sort of in the galaxy an ex-situ star formation where stars are formed outside of the galaxy and have come in some sort of merger with the milky way so like another smaller galaxy has come in and all of its stars have been sort of flung at the center and the gravitational forces in that have set them on these crazy orbits where they're just changing their radius all the time which is kind of cool right what you really want to know though is like when that merger happened and like what the kind of stars you know how old the stars were and how metal rich what kind of stars came into the galaxy when that happened right and the best way to do that
is to look for globular clusters so hence why they went on a search for sausage globular clusters the reason that globular clusters are so good at like this probe of like the age and sort of origin of the stars in a galaxy is because you can really easily tell how old the whole population of stars is because they all formed at the same time so if they all formed at the same time your hottest and younger stars will die off first so your biggest stars will die off first and you'll be left with the much longer lived smaller stars so if you look for the most massive thing in the cluster then you'll be able to age your cluster because basically everything that was bigger than that will have already died off and we know
the lifetimes of those stars quite well until we can put an age on the cluster to say it formed this long ago so that's what they did in the sausage glob of the cluster paper which was like my favorite at the minute they looked at 91 milky way globular clusters basically what they were doing is looking at the properties of these globular clusters in that velocity space again and seeing if they were good candidates for being a part of the sausage basically in the gaia data so here's all 91 and this time they're also plotting in another panel the velocity in the phi direction this height above the plane and so they've plotted it and they've shown the area of the sausage that they found in that
previous paper in all sort of three plots and basically what they've said is that these eight goblet clusters that they've drawn circles around the red ones with the circles around these are all the ones that lie within the sausage in all three of those plots and so they're really good candidates to be globular clusters as part of the sausage so then with these globular clusters they looked at the age and methodicity to see whether they had different properties to the ones that we see in the milky way all were similar to other clusters that we'd seen that we know have been accreted um in merger events into the milky way so here's the plot of age against metalistic for these globular clusters here is age of the universe and you see
that's in giga years which is billions of years which is pretty cool and then we have metalistic here this is your f e over h so this is your ion to hydrogen ratio and basically you have low metallicity down here and high metallicity up here right and then this is sort of young goblet clusters and this is old global clusters basically so most milky way globular clusters lie on this sort of track here quite old all different metallicities because in the milky way you kind of got mixing of lots of gas and stars so you don't tend to end up with regions of very distinct metallicity because you're sort of always mixing stuff around so you get global clusters forming all sorts of
different metalistic all sorts of different ages but in a smaller galaxy like a dwarf galaxy we call them you don't get as much mixing because there's not as much rotation like ordered rotation set up like in the milky way so we know from a previous merger that's happened in the milky way that there is a group of globular clusters that run along this track here which is from a merger with the sagittarius dwarf galaxy and you'll see that the globular clusters that they found in the sausage again red and circled they also follow that similar track but they're slightly offset according to these authors so you can tell that they've come from a different source like a different dwarf
galaxy that's merged with the milky way so without these globular clusters like we wouldn't have been able to do this so the global clusters basically give away the sausages origin and what does it look like inside so you get the date here and you might do more than one sweep in an evening usually three or four at least and you can see here he begins i saw venus with a new 10-foot reflector so he's changed telescopes at this point variations in density that means you're getting variations in pressure that is sound so there is stuff moving yes yeah
