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where in space are you taking us today so today we're talking about messier 91. this is a beautiful bad spiral galaxy it's actually also known as the missing galaxy because in 1781 when charles messier discovered this galaxy he wrote down the wrong coordinates and for almost 200 years no one could figure out where this galaxy was or if it actually existed he wrote down the coordinates with respect to messier 58 when he should have written it down with respect to messier 89. people thought maybe he doubly recorded m58 he logged it twice because it's actually quite close in the sky as well so it wasn't until almost 200 years later in 1969 that an amateur astronomer noticed that the coordinates would work out properly
if it was respect to m 89 and not m 58. so he was kind of giving the wrong start point like you know turn left then right and all that but he gave you the wrong start yeah exactly so this galaxy actually was discovered by herschel three years after messier so it was already a logged galaxy then as well like i said it's this bad spiral it's the faintest one in the messier catalogue actually and you can see from the image it's kind of like washed out like there's not much contrast between the spiral arms and the disc this is because this galaxy is an anemic galaxy it's got very low star formation and it's got very little gas content to form stars in the first place anemic galaxies
tend to live in galaxy clusters they're not found isolated in the field and it turns out that this galaxy does belong within a galaxy cluster it belongs in the virgo cluster of galaxies and actually for a long time people thought that it didn't belong within virgo because it's so much dimmer than all the other galaxies in the cluster and also its recessional velocity so you know the universe is expanding right so all the galaxies are moving away from us you expect galaxies in the same cluster to move away from us at the same rate but this one is moving away much slower than all the other galaxies within that cluster so the galaxies in virgo have an average recessional velocity of one 1 100 kilometers per second whereas this
galaxy has just a recessional velocity of about 500 kilometers per second so less than half but they managed to confirm it was part of the galaxy cluster by measuring its distance and they did that using cepheid variables cepheid variables are stars that pulsate and they have this very defined pulsation rate against luminosity so the brighter they are the faster they pulsate so if you know how like fast they're pulsating the period of the pulsation you can calculate its luminosity you can compare that to how bright it looks its apparent magnitude and that gives you how far away the galaxy is and based on that they know it's um 63 million light years away and that's consistent with being within the galaxy
cluster i'm imagining the reason that you rarely find anemic galaxies outside of clusters is the reason it's anemic is all the other galaxies stole all the gas so that's one of the main reasons that you would want to study a galaxy like this one galaxies and galaxy clusters are ideal places to study the interactions between galaxies themselves the interactions between the galaxies and the galaxy cluster and the interactions between the galaxy and the inter-cluster medium so the gas that lives in between the galaxies within this galaxy cluster one of the explanations for this galaxy being anemic is that because it's in a galaxy cluster it's moving around in this dense into cluster medium environment and that strips away all the gas
and that prevents it from forming stars essentially but actually that's not what is happening here and a hint of this is like typically when you see ram pressure stripped galaxies you get these jellyfish galaxies right where you can see like the tendrils of stars and dust coming off the galaxy we don't see that for this galaxy actually what's happening is if you look at the metallicity of this galaxy the star population is actually not so different from the milky way but what is different is the ratio of iron to hydrogen so this indicates like how much star formation has happened because stars like they begin with hydrogen and helium but as you form more and more stars from the kind of stellar wind outputs and the
reprocessed gas then you get more and more iron in these regions so you get more metallicity basically even though the stellar population of this galaxy is similar to the milky way it's iron to hydrogen ratio is four times higher this tells us that this galaxy all the stars have been like having rapid amounts of star formation happening and that must have used up all of the gas it didn't pace itself it went on an early feeding frenzy essentially so it's really interesting to study like galaxies within cluster environments because then you can look at how different their properties are from other galaxies um for example in the field or those that have been in clusters for a long time in comparison to those that are falling in you can
look at how their morphology changes how their luminosity changes etc if there's one thing i've learned from making these videos with you guys about the messi objects it's that no two galaxies are the same yeah it's surprising isn't it famously there's a problem with understanding how this structure is there or at least how it stays there in that if you look at how fast the galaxy's rotating
