M21 - 8-million-year-old stars - Deep Sky Videos

yeah Messier 21m 21 unfortunately this is m20 the trifid nebula which we've already done but it has a nearby friend down here which is well the less inspiring looking called Messier 21 and in fact they were both discovered by Charles Messier on the same day so I suspect probably he just spotted them at the same point in 1764 when he discovered the trifid nebula at the same day you so found this well they're less exciting little object down there so it's one of those open cluster things actually quite a young cluster of stars it's probably no coincidence it is somewhere near the trifid nebula because the trophy nebula also has a cluster of young cluster stars that's still in the process of forming Messier 21 is just a little bit further along the line I think that's about 70 old bright stars but so probably a few hundred stars in Tokyo okay so it turns out you can do some science with this object so here's the paper from a little while ago when was it published 1993 this was published the age spread an initial mass function of the open cluster NGC 6 5 3 1 which is Messier 20 ones other name so in fact I'm just going to talk about the the eye each bred and save the initial mass function for another day but the age spread is kind of the interesting one so I'm gonna have to I'm afraid so if we delve into this paper it's a very short paper it's one of these it's actually from a conference so it's actually quite kind of a short paper I have to talk about these things again the color magnitude diagram or host from rustle diagrams which we talked out about a lot of times for clusters of stars so basically you measure the color of a star with blue to red on the bottom and then the luminosity of the star from fainter bright on the axis up here and you find that stars are not entirely randomly spread in this space they kind of tend to cluster in particular places so this is the kind of the raw data when you actually look at all the stars essentially this gives a quite a good indication quite how many stars there are in the cluster and you can see there's a bit of a pattern there but there's an awful lot of mess you know a lot of spread around at least in part that's just down to the fact that there's actually still dust in this cluster it's sufficiently young that this is still obscuring material in the cluster fortunately if you measure lots of different colors so you measure kind of the light in lots of different bands blue green red infrared and so on you can actually thick separate out what the kind of the intrinsic color of the star is from the obvious raishin effects so you can kind of correct the color and that gives us this plot over here which you can see is everything's kind of much neater and tidier that huge scatter has kind of gone away but it's kind of a couple of things I wanted to talk about about this kind of cleaned up version of the color magnitude diagram the line drawing here is the thing called the zero age main sequence now that's where a store where a star ought to be if it's just a boring star like the Sun which is just burning hydrogen to helium in its core that's what this thing called the main sequences and the zero age part implies that it's just that's where it is when it starts doing that and you can see you know to some extent the stars do indeed lie along this 0h main sequence but there are a several kind of systemic effects okay in fact it's probably three that maybe you can pick out you'll see that the things at the top here all lie systematically to the right of the line you can see down here is a big gap where's nothing at all and you can see over here the things lie systemically to the right of the line as well now it turns out those things are not unconnected to each other let's start with the ones up here the ones up here are the most massive stars so that the bright brightest stars which tend to be the most massive stars massive stars have very short lifetimes just because even though they're kind of they're massive so they've got more fuel to burn they burn it incredibly quickly so they live fast and die young so these are the massive stars and these ones are actually already approaching the end of their life and so they've already started to evolve off this main sequence you can actually figure out how long ago they were on that main sequence how long ago the cluster actually formed from how far they've moved and the answer is about eight million years ago so this tells you this cluster is about eight million years old which in astronomical terms is it's very mean it's very young cluster of stars okay so you can get the age from that so that's their end of things then the other end of things actually that both these effects are sort of connected to one another the reason why these stars are all over here is because they haven't actually even made it to the main sequence yet they're still kind of collapsing and forming and turning into normal hydrogen burning stars one of the things that happens as a star collapses to the main sequence is its heats up gets hotter and hotter before it reaches the point where the hydrogen in the core starts to burn the helium-3 one of the isotopes of helium that's in there actually burns at a lower temperature there's not very much of it so it actually doesn't sort of do anything very much but it does actually sort of generate some luminosity for last and heat for a while and so what happens as the stars kind of collapsing it kind of gets hung up at that point where it's just turning helium three into helium four for a little while and actually that's why there is this gap here that actually all these guys are kind of hung up and eventually they all have exhausted all their helium three and then they'll join onto the main sequence quite quickly but you end up with this kind of gap appearing because everything's just kind of hung up at that sort of pre main sequence stage and again you can actually figure out okay so how long just from where these gap is you know basically which stars are currently at this sort of hung up stage and you can actually use that to figure out how long these things have been forming for and it works out that these things have been forming for about eight million years it's taking them about eight million years to get this far it's gonna take them a little bit longer which was the same number that we got from the massive stars at the top which I guess at some level might not be a surprise because it's telling you basically well this means the whole thing's about eight million years old what's interesting about that is it tells you something fundamental about the star formation because it tells you that basically these are the low mass stars and these are the high mass stars this is saying that they all form at the same time that when you form a cluster of stars it's not like you formed the low mass things long before the high mass things or the high mass things long before the low mass things because if that were the case we'd have ended up with different ages for the low mass stars and high mass stars the fact that they all come out with the same age is kind of a rather nice piece of evidence that actually when you form a bunch of stars the little ones and the big ones all format more or less the same time because of the way this temperature scale is arranged so this is blue these are hot things this is red these are cool things and this is bright and this is faint so when something before a star is born it starts out as cold and faint but it's just basically not a star yet so cold and faint is down here somewhere okay and the things at the bottom kind of evolved across to here and then these guys kind of they went up and then across so they get brighter and then they get hotter and they'll end up here so the things follow different tracks when they're coming across actually the cont the guys down here probably went up and then came back down again but basically things follow different tracks everything kind of comes up and then the the massive stars track straight across the low mass stars come up and then they come back down again to here so the different masses of stars follow different tracks in this in this diagram as they approach the main sequence and we're just catching them just to that point before they've all arrived or just after they've arrived main sequence so is that if the main sequence the finish line it's where a star spends most of its life but then obviously when it runs out so that's what it's where it sits when it's turning hydrogen into helium in its core which is where a star spends you know 90% of its life but after that it will then turn helium into carbon and carbon into heavier things and so on depending on how massive the star is and so the stars will evolve off turn into red giants which live up here somewhere and actually these massive stars tend to shuttle backwards and forward several times before they then blow up as a supernova so little guys because their lifetimes once they arrive on the main sequence this they're so pathetically faint that actually they use their fuel incredibly slowly so they'll stay on that main sequence for 10 billion years so I've seen from the kind of the lifetime of the universe eventually when the universe gets old enough they'll also evolve off and probably turn into red giant stars as well but the really low mass things they take forever before they reach that point there's there's hardly anything on the main sequence it feels like the main sequence should be the place where most things are but they're in an older cluster or an older object would we see more actually we would see so on an older cluster these guys would all be sitting on the main sequence so this end of the main sequence will be very well populated there'll be a whole bunch of stars down here these guys would have gone through their entire life and disappeared entirely as supernovae so this end of the main sequence will be entirely unpopulated so when you look at a color magnitude diagram you can pretty much tell the age of that object straight away by looking at some of those things like like a naked top and a well-populated bottom means yes so yeah really I mean you can really you can read these diagrams very quickly at least of that kind of crude level of saying this is an old cluster this is very young cluster just really because because the stars are Pele have such short lifetimes if you see any stars up here at all it tells you it's a very young cluster you see no stars up here it tells you it's a kind of middle-aged or old cluster under what circumstances would we see stars to the left of the main sequence or a large number of stuff over here yeah never I think is this you answer so yeah this is really where there's a few so you'll find some white dwarf stars are these very very hot very small objects so they're hot which means they're very blue and they're small which means they're quite faint because there's not a lot of surface area to them so they live way down here somewhere so they're kind of certainly to the left of where the main sequence stars are but there's not a whole lot over here you sometimes find stars a little below the main sequence if they have very little by way of heavy elements in them so very you know lots of hydrogen and helium but nothing much heavier than that and that's really because the heavy elements tend to absorb blue light very effectively in the atmosphere of the star and so if you don't have those heavy elements in there more the blue light gets out which kind of shifts the main sequence a little bit but only a little bit you don't end up with things way over here in the diagram this cluster it shouldn't really be moving very fast there are sort of typical random motions within this cluster is only about a kilometer per second or so so if the Sun was going to escape from there typically it would probably come out at a few kilometers per second

Channel: DeepSkyVideos

Views: 71,836

Rating: 4.9707265 out of 5

Keywords: astronomy, m21, NGC 6531, age spread

Id: UMaFfszJAw4

Length: 9min 38sec (578 seconds)

Published: Mon Sep 12 2016

Reddit Comments

[u/[deleted]]

Jesus f Christ on a stick! A new Deepsky video! :D I miss those so much! <3

[u/[deleted]]

Hey brady I know that you get a ton of suggestions but PLEASE do a vidoe on dox otherwise called SDSSJ1240+6710

It is a white dwarf that is predicted to have an almost pure oxygen atmosphere which is extremely strange.

Thanks for the great video also

[u/BarrenStory]

Yet another great video /u/JeffDujon

I have noticed that you don't mic yourself in your videos when you act as the interviewer. I was wondering, since I know this must be a conscious decision on your behalf, what your reasons are for doing so?