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I have Abel 1795 which is this collection of little faint fuzzy things in the middle here so the center of it is that relatively big bright blob and but then actually most of these little orange slightly fuzzy dots you can see are what make up a 1795 it is a cluster of galaxies so each of those little fuzzy dots is a whole galaxy and it's an enormous distance away it's about a billion light years away amazing isn't it just you know this little faint smattering doesn't look like anything very much but that's thousands of galaxies you're looking at there all clustered around this one big bright galaxy in the middle there's got to be stuff living in there you would certainly think so wouldn't you that many galaxies each with that many stars
we don't live in a rich cluster we live in a poor group so most galaxies don't live in complete isolation few do but actually most live in groups so maybe four five 10 20 galaxies the rare things are where you get thousands of galaxies all in one of these clusters the AEL C was put together in the 1950s by George Abel there was a survey of the sky had been put together I think called the Palomar Sky survey and he literally went through all the photographic plates by eye identifying objects like this I think in his original catalog that's getting on for 3,000 of these clusters that he found all across the sky or the northern sky because the survey he had actually only covered the northern
sky so they're all Galaxy clusters all clusters of galaxies there was an extension called the Abel Corwin uh catalog which did the southern hemisphere as well so we now actually have these clusters all across the sky so if you want a really long set of deep sky videos you know there's like two or 3 thousand of these we can do and then you could do each Galaxy within the cluster absolutely yeah all right so here's a nice paper called mmt FH Alpha and HST fuv Imaging of the filamentary complex in Abel 1795 this is kind of a zoom in of the picture I showed you before so actually the orangey stuff you can see here is exactly the same as the orangy stuff we were looking at before but kind of zoomed in on that Big Bright
galaxy in the middle and then the pinky red stuff that's this H Al for emission and then the blue stuff here is far ultraviolet emission both the H for emission and the far ultraviolet emission are the signature of young Bright Stars so young Stars Bright Stars heat up the gas which make it Glow to produce the H Alpha and then the Stars themselves are actually so hot that they emit in the far fire ultraviolet so this is actually the signature of star formation going on within this cluster but you can see that the star formation is actually nothing to do with the Galaxy right it's you know I mean some of it's in the Galaxy but there's this whole strand of star formation going on
out here I mean that's on a galactic scale that filament and Beyond right it's it's bigger than the Galaxy so how is it that there's star formation going on quite intense star formation going on that's nothing at all to do with the galaxies somehow there has to be a whole L gas that's being brought together to make stars outside the Galaxy but let's complete this multi-wavelength view of this cluster by looking in the X-ray part of the spectrum so I have another paper here called a very deep Chandra observation of a 1795 chander is an x-ray satellite still currently doing its thing up there in space and they pointed at it a 1795 for a very long time so here it is in X-rays and it's a
very bright x-ray source so there masses of x-rays coming out and actually superimposed on top of it the blue Contours here are the H Alpha observations I was showing you before so this is where that star formation is going on the existence of the X-ray emission tells you there's a whole load of gas there because the process by which x-rays are emitted is a thing called Brimstone which is breaking radiation which is if you have a plasma so you got if you just think about hydrogen gas to keep life simple you got a whole load of once you've ionized it you've got a whole load of protons and a electrons the electrons are no longer bound to the protons because it's all been ionized it's all been ripped up
into this plasma but once in a while one of these electrons will go somewhere near one of the protons and in the process it may be decelerated so it'll scatter past it and it'll lose energy and as with a lot of processes in physics when that when energy changes way you kind of make sure that energy is globally conserved is that a photon gets emitted now these are such energetic processes that this breaking process ends up producing x-ray photons and the fact we see so many x-ray photons tells us there's a lot of hot gas there so you can actually do a calculation of saying okay we got all this x-ray emitting gas sitting there how long would we expect it to stay there for how long does it
take to cool down and that's a reasonably straightforward calculation to do because we can figure out how much energy is being lost through this brim stoling process how much energy is going away in photons and that if you take that energy away that means the gas kind of cools down over time and if you make the gas dense enough then the cooling time gets quite short at least in astronomical terms um so actually the gas that hot x-ray emitting gas stops being hot and x-ray emitting and cools down to a point where it turns back into normal hydrogen gas and that's what we're seeing here in this tail so here they've kind of they've taken this original x-ray image and kind of remov
the smooth distribution of x-rays to see what's left and if you remove that smooth distribution of hot gas you find there is still x-ray emitting stuff on the scale of this Hal Alpha filament and actually lying in kind of the same place when you see kind of an excess of X-ray emission uh like they found when they kind of subtracted off this smooth stuff that's telling you that's a place where the gas is particularly dense and the fact that it's emitting more x-rays um actually means it's going to cool down faster so this feature is cooling down suff ly quickly that some of it's just turned into normal hydrogen gas and some of that normal hydrogen gas is now turned into Stars this is a thing
called a cooling wake it's gone all the way from being this hot glowing diffuse x-ray emitting gas all the way down to cooling down into clumps that then turn into Stars it must be something to do with the fact that this galaxy is moving through the core of the cluster so you've got this hot x-ray emitting gas that permeates the cluster and then you've got this galaxy moving a bit relative to it which means that actually it's kind of dragging a wake behind it creating this wake behind it as it's moving of that cooling gas that physics of figuring out the time scale on which gas cools down you can apply it to any cluster of galaxies and what you find is that the time scale for the central
parts of clusters to cool down is relatively short remember I said that the cooling time how quickly it cools down depends on the density squared so actually if you go to the center of the cluster where there's a whole load of gas that means that its cooling time becomes quite short because the these emission processes become strong and in fact they become much shorter than the lifetime of the universe so this effect that we see in this weird wake in AEL 1795 by a simple argument you should see the same phenomenon in every cluster of galaxies going on in the middle because the cooling time is sufficiently short that they should be turning into stars and to some extent that's happening but not anywhere near the extent we expected
it to happen and what that's telling us is that there's some process that's actually heating the gas back up again stopping it from cooling down so that we end up with loads and loads of stars forming and it's thought that the process the main process that's heating the gas back up again is that at the center of that massive galaxy in the middle there's a super massive black hole that super massive black hole once in a while becomes an active and it starts throwing out very energetic Jets of material and so on and that those Jets of material smashing in to the gas that surrounds it Heats it all back up again so it's thought that the reason why you don't see phenomena like the one
one in a 1795 all over the place is that the clusters of galaxies themselves have their own heating mechanism to stop it from happening too much if you look at clusters of galaxies you actually find that as well as that sort of diffuse x-ray Mission you actually find that there's Optical light on large scales as well there's this thing called intracluster light and it's thought that is just stars that are permeating the cluster rather than living in individual galaxies now for the most part most of those stars are things that got ripped out of galaxies so they probably just formed in normally in a galaxy but that gal interacted with another galaxy and as part of that process some of the Stars got kicked out into the
cluster space so most of that intercluster light is probably stars that started their lives in galaxies and then got kicked out but at least some of it would have formed through processes like this so those are stars that were never in a galaxy there's something so romantic about stars not in a galaxy I think like I love the idea of those yeah it's a it'd be a sort of it' be a sad and lonely life wouldn't it really and look I know this probably isn't going to win any Astro photography awards but here's Abel 1795 photographed by Professor marfield himself in his backyard in Nottingham with his beloved EV scope and while I have your attention can I take this opportunity to thank
everyone who supports deep sky videos on patreon you can see them now their names floating through space thanks everyone you can join them by going to patreon.com deepskyvideos I'll put a link in the video description
