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so today our topic is m52 which is a young open cluster in the constellation of cassiopeia the interesting thing about this cluster is just the huge variety of variable stars that are found within you might ask well what is a variable star and in fact there are a number of ways in which stars can exhibit variable behavior can be nice smoothly periodic variations in luminosity or very sudden catastrophic nova or quick increases in luminosity so if we go back a hundred years or even more variable stars were one of the hottest topics in astrophysics and you only have to read the really great book the glass universe by author davis sobel to understand the whole industry that was studying stars
and particularly variable stars at this time so this book talks about the efforts of the harvard observatory in around the period from about 1890 for the next following decades and describes the incredibly detailed work that the astronomers there did mostly a huge team of female astronomers who were leading this work and studying in great detail the luminosity and the stellar properties and unraveling the astrophysics behind these stars now in particular the study of variable stars was the subject of very early citizen science the american association of variable star observers as far back as 1890 was enlisting people to use their telescopes to make very specific measurements um you know they'd be allocated a list of stars and told
what's it what to do and they would send these measurements in and all of this information was being used and so one of the outcomes of this was the discovery by henrietta swan levitt of the period luminosity relationship for cepheid stars and that really was the key to unlocking the whole distance scale of the universe we realized you could actually link that change in luminosity over time to the intrinsic brightness of the star and if you know the intrinsic brightness you know the distance and that was really quite something to help us figure out that the universe was not just our own milky way but our milky way was one galaxy in a sea of hundreds of thousands of others i want to go back now to m52
and look at just what kind of variable stars are contained within this cluster so remember this is an open cluster it's a young association of stars they're loosely bound together and so the paper that i uncovered studying this object was on a hunt for variable stars so they will have set up their observations so they were taking many pictures of this region of sky over quite a long period of time for each star in the object they would be measuring the brightness in each of those images and then constructing a curve of how the brightness was getting brighter and dimmer over time and in doing this they found that this was quite rich hunting ground for variable stars so you might want to ask what would cause a star to
be variable in the first place and so you might have something like an eclipsing binary where you have two stars and one passes in front of the other alternately dimming and brightening the pair or you could have something to do with the rotation of the star itself for example if it had some star spots that meant the surface of the star itself was variable that would change the observed brightness as the star itself rotates but the types of stars that were mainly found in this object were pulsating stars are not just homogeneous balls of gas they have layers they have structures and in particular stars are always in this really delicate balance between the attractive forces of gravity pulling all that material towards the center and the
intense heat and pressure pushing that material out there are two important factors that influence the different types of pulsations that we see the first is the star's mass and we know that the stellar mass is one of the single most important aspects that drives sort of the properties of the stars that we observe and governs the evolution of its lifetime and that leads into the second point which is the evolutionary phase of the star so stars undergo quite different periods in which they're burning different types of fuel in their core that has an impact on the structure inside and that in turn can influence the types of pulsations so what kind of stars have we found in m52 well one type was slowly pulsating b stars they
actually found 18 of these and these are b stars which are quite massive stars quite hot and luminous stars they also reconfirmed a different type of variable star which was a known star of a type called delta scooty and so these are actually dwarf versions of the very important cepheid type variables that helped us unveil the scale of the universe and these are stars that pulsate in multiple different modes and lie on a particular part of the hertzsprung-russell diagram which is the very important diagnostic diagram that we use in stellar evolution studies and so this is an area called the instability strip in addition there were even more variable stars so there were a
handful of gamma duratis type stars and in contrast to the really hot massive b stars that they found lots of these are variable stars where the star is actually less massive so more what we call a type f or type a star and has a period of about one day in terms of the change in its luminosity and even on top of that they found a handful of eclipsing binaries so these are the kind of stars that are not pulsating they're not varying in their brightness because they're pulsating but they're varying because they're actually blocking each other's light as they orbit around each other so the end result is a whole zoo full of different types of variable stars that only scratches the surface of the many other
different types of variable stars that are out there but means that this one object is really rich picking grounds for further studies for astro seismology and trying to understand the physical processes reaching into the interiors of these stars through observations of their periodicity and trying to understand what makes them tick absolutely let's go and look at the most famous moon rock so this is apollo 15 sample 15415 it's the genesis rock this is the
