 Hi everyone, I'm Ration, a PhD student at the Australian National University, and welcome to another Mount Stromlo Astronomy Talk. Today I'll be talking about star clusters and what's the big deal about them. But before I start, I must mention why I decided to do this talk in the first place. So this was in 2017, a group of friends and I had a camp or retreat at the Green Hills Conference Centre, which is actually relatively close to Mount Stromlo. It was a very charming and relaxing place with an amazing access to nature. So during one of the nights, some of us decided to go out to stargaze, and as that place has pretty dark skies with not much light pollution. I decided to let them observe some interesting astronomical objects by using a green laser pointer to guide them to the object while they looked at it through a pair of binoculars. One of the objects I showed them was a star cluster such as this one. To many of us who have observed star clusters before, it's a spectacular sight seeing these distinct beautiful balls of stars through a telescope or a binoculars. Unfortunately, not all my friends were appreciative of the beauty of star clusters, and the most memorable quote of the night came from one of them who said in a very unimpressed tone, but it's just a cluster of stars. Very disappointing. So that is the motivation for today's talk to tell you what's the big deal or what's interesting about star clusters. Here's the talk outline. I'll be covering what star clusters are, including the early history of studying and observing this object. I will also talk about the two main types of star clusters, plus their usage in astronomical research. And finally, I'll go through some common star clusters observed in the southern sky with the hope that you may find some time to go out during the night to observe them using a telescope or even a small pair of binoculars. Just a reminder, as I'm talking, feel free to leave questions down below in the comments. Sorry. So what are star clusters? The most basic definition is that they are groups of stars that are gravitationally bound. This means that stars in a cluster all orbit around the common center of mass, similar to our solar system. Since virtually all clusters have been discovered either by visual examination of the sky with a telescope or from inspection of photographic or electronic images, an operational definition of star clusters is suggested over here, which is an obvious concentration of several stars or more above the surrounding stellar background and identifiable on the visual or infrared images. Now, since star clusters are just groups of stars that are bound to each other, are double stars or triple stars considered star clusters then? The answer is no. So what is the minimum number of stars to be considered a star cluster? This is still quite unclear, but some astronomers have defined it in terms of its total mass, which is 10 solar masses. That means if all the stars are approximated to be of similar mass to the sun, the minimum number of stars in a star cluster is 10. The history of studying and observing star clusters go all the way back to the second century, as the first star cluster was cataloged by the Roman astronomer Polamy in 15080. That cluster was Omega Centauri. However, if you notice, it was named as if it was a star in the constellation Centaurus. This is because to the unaided eye, it appears to be just a bright point of light without any extended feature. The image on the left shows what Omega Centauri is like when observed through a powerful telescope, but the image on the right shows what it's like when seen using just your naked eye. Now as you can see, there are many stars shown in this picture, so in an astronomy edition of Wes Wally, can you tell which one is Omega Centauri? I'll give you five seconds to try to find it. Okay, so it's actually right here circled in red. You would agree that it's really hard to distinguish a star from a star cluster, right? Hence, you can't really fault the early astronomers for misidentifying these objects. Later in the talk, I'll teach you an easy way to find Omega Centauri in the night sky. Like many first-generation scientific instruments, early telescopes won of too high a quality and lacked the capability to resolve star clusters easily. Mind you, telescopes were invented in the early 17th century, so even through them, star clusters looked like fuzzy clouds of light, similar to what's shown in the figures below. Therefore, in the early days of telescopic astronomy, star clusters were often referred to as nebulae or clouds. In 1764, Charles Messier, the same guy who created the Messier Catalog of Deep Sky Objects, was the first person to observe individual stars in a globular cluster. You might notice that this is the first time I'm using the term globular cluster. I'll go through more about this later, but all you need to know now is that it's just a type of star cluster. So over several centuries, astronomers have studied star clusters in great detail, and we have pleased to get a pretty good idea of how these clusters have formed. Clusters are formed when very large clouds of gas in dust, also known as giant molecular clouds, contract to form stars. You might be wondering why the entire cloud can't just contract to form one single massive star, but the chances of this happening is very unlikely, since every giant molecular cloud contains many localized dense clouds of gas in dust. It is because of this reason that astronomers believe that most stars are formed as part of clusters. Our sun was also thought to be part of a cluster, but sadly it has since dispersed shortly after the sun was born. As an illustration, the pictures below show the remains of giant molecular clouds in which the young stars shown were formed. So I'm now going to talk about the two different types of star clusters. The first one is open clusters. These are loosely distributed clusters which contain around 10 to 100 stars, and they are relatively young, about a few million years old, which is young in astronomical terms. Open clusters tend to be found within the disk of the galaxy as shown over here. As such, they're mostly found along the same plane in the sky. On the other hand, you have globular clusters. These are tightly bound clusters which is what you would call a ball of stars. They normally contain around tens of thousands to hundreds of thousands of stars, which more than open clusters. And they are relatively old, at least a few billion years old. They tend to be found all over the galaxy in a halo of bulge as shown in this diagram. The sun's location is also shown here for reference. Of course, star clusters are not just pretty balls of stars in the universe. In terms of its age in astronomical research, they are useful specimens as they can allow us to trace the star formation history in galactic evolution over time. One of the main tools used to do this is the colored magnitude diagram. So what is this diagram about? Now, I must ask those who are allergic to graphs and plots to turn away as I'm going to talk about them in the next few slides. The colored magnitude diagram, or CMD for short, is a plot of observational data which shows how stars in a star cluster can be described in terms of their brightness in a y-axis and color in the x-axis. In the CMD example shown on the right, each dot represents a star. When all the stars are plotted together in one diagram, you get this shape over here. Different parts of the diagram represent different stages of life for the star. For instance, you have stars in the main sequence, which is a relatively early stage in a star's lifetime, found in this region over here. But as you move up, you will get stars in the red giant branch, which is a much later stage in a star's lifetime. Now, the next thing you can do is to determine the overall characteristics of a particular star cluster. The light blue line that is fitted to the star cluster is actually a model, which allows us to determine useful information, such as the ion abundance, age, and distance to the cluster. And finally, with proper calibrations, the CMD can be converted to a Hertzsprung-Russell diagram, such as the one shown on the left, to determine the luminosity and temperature of the stars. In this diagram, the more luminous stars are at the top, while the hotter stars are on the left. So as you can see, star clusters can be used to measure a variety of useful properties that will help understand the nature of the universe better. So getting the technical aspect of studying star clusters out of the way, let's move on to something more fun, which is observing star clusters themselves. Today, I'll be introducing three famous clusters that you can observe in Australia. The first common star cluster is Pleiades, which is also the inspiration for the Subaru group. Even though it's actually a northern star cluster, you can still observe it in Australia in the summer. This beautiful open cluster is sometimes referred to as the Seven Sisters, as this is because usually, only seven prominent members can be seen with a naked eye. In total, there are more than 100 members in Pleiades. You might be wondering why there is a cloud-like feature surrounding this cluster. This is caused by what we call a reflection nebula, but the actual scientific term should be a scattering nebula, as the dust scatters the light from the stars to form this bluish cloud. This is also the same reason why the sky is blue. So here's how to find Pleiades. Starting from the Orion's belt, follow where it points until you reach Aldebaran, an orange star in the constellation Taurus, which you can't miss. Then, Pleiades will be found further down in the same general direction. The second common star cluster I'd like to introduce you is the Jewelbox cluster. Early astronomers thought that it resembled a box of jewels, which is why it is named as such. Just like Pleiades, this is also an open cluster with about 100 members inside. What makes this cluster very pleasing to the eye is the contrast of colour caused by the red supergiant star surrounded by many blue dwarf stars. So where is the Jewelbox cluster found? This is pretty easy. Starting from the southern cross, also known as crux, hits slightly southwest from the left star of the cross, which is Mimosa, and you should be able to spot the Jewelbox cluster. And last but not least, the final star cluster I'd like to introduce you is none other than Omega Centauri, which is the largest globular cluster in the Milky Way. Estimated to contain about 10 million stars, Omega Centauri is speculated to be the core of a dwarf galaxy that was disrupted and absorbed into the Milky Way. So how do you find Omega Centauri? As you can see, there are two main ways of finding it. The first one involves tracing the stars Hadar and Epsilon Centauri, which point towards the cluster. The second one traces the right and top star of the southern cross, which point towards the cluster as well. So here's a quick summary of what I've covered today. Star clusters are groups of stars that are gravitationally bound. They can be classified into two main groups, open and globular clusters. They are useful in astronomical research, and some of the common star clusters you can observe in Australia include Pleiades, Jewelbox cluster, and the Omega Centauri. And lastly, I hope you found at least one thing that's interesting about these objects, and I strongly encourage you guys to join the star clusters gang. With that, I've come to the end of my talk. Any questions? Okay, since there are no questions yet, I'll just talk about some things that we can observe currently at this time in Australia. So the first thing I would like to introduce you is the southern part of the sky, which involves, which includes cracks, the southern cross, Centaurus, Carina, Vela, and some of the other constellations shown in this picture over here. So as you can see, this is based on the Stellarium app, which is one of my favorite software. And if you notice, the time below is actually it shows what the sky looks like at about 7.30pm. Okay, sorry, we have a question over here. So can star clusters be mistaken for small galaxies that are really far away? So the thing about galaxies is that usually they have a certain kind of feature, it's like they have a spiral, they are sometimes spirals, they are lenticular, they are elliptical, and usually star clusters can only be observed for nearby galaxies. So that means if you look at like far away objects, like you look at distant galaxies, most of the time they won't be star clusters because the stars are too faint or they are too far away to be resolved. So there might be a chance that you could mistake star clusters for small galaxies, but it's usually pretty obvious. Okay, so what makes the sky blue? Okay, so this was a point that I mentioned previously regarding the PADs or their reflection nebula. So the reason why the sky is blue is because, as you know, the light is comprised of many different wavelengths, right? You have red, orange, yellow, green, blue, sorry, yeah, so all these clusters, all these colors, sorry, and so the thing about blue is that because it's the shortest wavelength of light and based on this thing called Rayleigh scattering, only the shortest wavelength of light gets scattered the most as compared to the longer wavelengths such as green, orange, red. So that is the reason why blue is, blue is seen very strongly because it's scattered by the atmosphere, by the air molecules in the Earth's atmosphere, and yeah, that is why the sky is blue. So which kind of cluster, which kind of star cluster do you like the most? Wow, okay, I think it's pretty obvious. I really like the Omega Centauri because of the fact that it has a lot of star and it really is one of the most prominent star clusters that you can observe in the night sky. So all you need to do is just even a small pair of binoculars, you can actually see that fuzzy ball of the nature of the star cluster and just knowing that it contains like 10 million stars is really amazing. Yes, okay, so how can we find them if you do not know where they are? Oh, so going back to the introduction, like the definition of star clusters, so they are actually defined by an obvious concentration of stars relative to the surrounding background. So it's actually pretty easy to find star clusters because of the fact that imagine it's, if you want to find one star or a few stars in a field, it's pretty hard. But if you have a group of them clumped together, you can think of it as like combining the brightness into a smaller, into a concentrated area, which makes it easier to spot. So I would say that star clusters are relatively easy to find in our Milky Way at least. Okay, did Hubble telescope lead to many discoveries? Yes, Hubble telescope definitely did make a lot of discoveries, but I don't think they made a lot of discoveries pertaining to star clusters because Hubble telescope, one of the main features of the Hubble telescope is actually to probe very deep sky objects. That means looking at other galaxies and other very faint galaxies. So usually you don't get these, the Hubble telescope would not actually find star clusters in other galaxies because as mentioned earlier, star clusters to be able to resolve star clusters in external galaxies is really, really hard and because they are so far away. So I think there were other telescopes which studied like star clusters in our Milky Way, but not so much for the Hubble space telescope. So what is the difference between open and globular clusters? Yep, so I think the main difference, as I mentioned, there are three main differences. So the first difference is obviously the number. So open clusters have fewer stars, globular clusters have many stars. And open clusters tend to be young, tend to be younger, whereas global clusters tend to be older. And open clusters tend to be found in the arms of the disc of the galaxy. That means if you imagine if you imagine a galaxy as a pizza, a full pizza, open clusters will be located in the pizza itself, whereas for global clusters will be located around the pizza. Okay, do other galaxies have star clusters? Definitely. So previously I mentioned that it's really hard to observe star clusters in other galaxies, but for nearby galaxies such as the Andromeda galaxy or the large and small Magellanic clouds, you can observe star clusters in them, even though it requires more observing, it requires a longer observation time to actually capture the detail of these clusters. So we do know that most galaxies or all galaxies should have star clusters. Right, how many black holes are there in the universe? This is a tough one. I'm so sorry. I'm not sure about this, but I must say that almost every galaxy has a supermassive black hole in the center. So a supermassive black hole is like a one gigantic black hole, which sucks all the stars and gas inside. But obviously it doesn't do it in one shot. It sort of grows in a spiral. It forms a disk. Yeah. Okay, next question. Is it true that the Andromeda galaxy is moving towards us? Yes it is. So if you know most of the galaxies are actually moving away from us, and this is because of the expansion of the universe. And that is why a lot of things are actually moving away, and this is called redshift. But surprisingly or quite expectantly, as strong Andromeda galaxy is actually moving towards us, because it's pretty close to the Milky Way. And therefore the Milky Way is able to exert some gravitational influence on the Andromeda galaxy. Okay, how far apart do they need to be to stop being a cluster? Oh yeah, so you mentioned that you thought that previous was a constellation, and you didn't realize it was an actual cluster. So yeah, that's a good question. So I mentioned earlier that star clusters are groups of stars that are gravitationally bound. So when the stars do not orbit around the common center of mass, do not orbit around, are not like clustered around each other, they would not be considered star clusters. And in terms of its actual spacing, in terms of the distance, I can't really give an accurate answer to you because sometimes that if you think about it, if you have big stars, the stars can be quite far apart and they are still gravitationally bound. But if they are small stars, that means these stars have to be closer together to be gravitationally bound. So it varies. What are the subsets of different colors? I'm not sure whether I understand this question correctly, but let me just go to the diagram which shows the Hertzsprung-Russells diagram. Okay, so it's over here. Yes, so if you look at the diagram, stars are comprised of many different colors. You have blue stars which are hotter stars and it goes all the way to red stars which are cooler stars. So the thing about this spectrum is that within each color, you have like as you can see, there is O, E, A, F, G, K, M. But within each letter, there is also like O1, O2, O3, all the way to O9 and it goes to D1 to D9. So there are subsets within each color. Can a supernova possibly destroy a star cluster? Yes, it can. And surprisingly, actually most star clusters, I would say as most open clusters, actually their lifetimes are pretty short because as you can see, they are much further spaced apart. And therefore, if they have certain gravitational perturbations from let's say like supernova or even massive objects or massive objects coming close to the star cluster, it will actually disperse and cease to become a cluster. How does Omega Centauri compare to 47 Toucani and are they similar in size and age? All right, I think from what I know, 47 Toucani is not as big but it still contains lots of stars because it's also a globular cluster just like Omega Centauri. Yes, I would say they are similar in size and age. They are probably at least 10 billion years old. The next question, does that mean that star clusters are star clusters not galaxies? Okay, yes, most of the time star clusters are not galaxies. The only exception I would say is Omega Centauri. The only example I can think of because Omega Centauri was actually speculated to be a remnants of a 12th galaxy that was actually destroyed by the Milky Way. So as you can see, the Milky Way is actually a cannibal in terms of stars. But yes, most of the time star clusters are distinct from galaxies. Yeah, so the next question is, are many clusters close together physically or do they only appear close? Yeah, so clusters to be considered a star cluster, the stars need to be gravitationally bound. So if you see a group of stars like one night, if you see a group of stars that see close, you can't actually confirm if it's actually a star cluster unless you observe their motion. Unless you observe how are they moving or if you observe their kinematic data. That means you see whether they are actually orbiting around the center of mass. So I must actually correct myself actually. When I looked at the when I introduced star clusters to you guys in the beginning, I said that it's an obvious concentration. So obviously this is not super accurate because obviously some stars can be clumped together or group close together but they're actually not considered star clusters. So what the proper definition is that you need to actually look at the kinematic information about individual stars in a star cluster to actually properly define it to be a star cluster. Okay, the next question is why is the universe a disk? Okay, so I don't think the universe is a disk. I would say that the universe is the shape of the universe is actually quite debatable but it's definitely not a disk because if you think about it, all the things in the universe are actually expanding. So you don't actually get a disk like what you would see at a galaxy unless you're talking about galaxies. Then I would say that galaxies are shaped like a disk because of its angular momentum because of its conservation of angular momentum. So the next question is why are hot stars blue and not red? Yes, so this one actually got to do with something a bit more complicated and it's got to do with this law called Planck's law. So let me just go to the slide. Yes, so if you think about it, what they found a few hundred years ago is that for hotter objects they tend to emit more radiation of course but not just that they also emit radiation that is closer to that there is shorter wavelength as compared to a cooler object and that is the reason why hotter objects are considered blueish in color like hotter stars whereas for cooler stars like they will appear reddish. What's the coolest thing you have learned about the nature or history of the universe from studying star clusters? Oh, what a big question. Okay, I would say that actually like for my honors that because I'm currently a PhD student but last year I also looked at I'm doing my honors project I also looked at star clusters and one thing I found it amazing is that because global star clusters especially global clusters these clusters are very tightly bound and because these clusters are so old they are like 10 billion years or even older. So what I found out or what the study was about was that I was actually looking at these clusters and seeing how it traces the galactic interaction so between the large Magellanic clouds and large Magellanic clouds and the small Magellanic clouds. So star clusters like I said it's not just pre-balls of stars but they can actually be used to determine very interesting things about things that are much bigger than themselves which are like galaxies. So that's one of the cool things that I've learned. Okay, can a star cluster orbit another star cluster? Yes it can I'm not so there is one object which I'm aware of which is called the double cluster in pursuers in the conservation pursuers but I'm not sure if it's actually a physical like they are actually gravitationally bound or they just look like two clusters very close to each other so I have to check it out. Okay thanks for all your questions guys yeah I didn't expect to receive so many questions on this field. Can planets orbit stars in the star cluster? Yes it can but I would say that usually these planets would be a bit in terms of their orbits they would be a bit unstable because they are always prone to having being affected by the gravitational influence of other stars. So in terms of it being habitable for life I wouldn't say that these stars are or these planets found in star clusters are actually they have a very high chance of containing life. Yep okay the next question is is it established that black holes are behind the formation of galaxies especially since ages okay especially since their ages are similar and black holes are located at the center so this is a very interesting problem it's like a chicken and egg problem so which came first right so some people thought that it was black holes that led to the formation of galaxies or that or and some people thought that it was the formation of galaxies that led to black holes so there isn't there isn't a I can't really answer this because I'm not very familiar with this field but I would say my guess is that it's that galaxies themselves which form black holes first yeah is there anything about star clusters that totally do your mind I think I mentioned it like a few questions before like talking about how clusters can actually allow us to study like the interaction between two galaxies yep are there elements still being made in the universe yes yes they are so actually elements are things in terms of in terms of the the formation of different elements you have many different processes so in the form in doing the big bang hydrogen helium and lithium was formed but most of the other elements were formed in stars themselves so they are definitely like elements being formed like right now because of how the temperature and pressure of the of the star actually compresses and forces nuclei and and atoms together so that they will form like new elements yeah can the sun orbit a sun are you I'm guessing you're referring to a star right can a star orbit another star yes they can and that is and they are called double double stars right okay um next question what is the universe expanding in tune hmm do you think that there are other dimensions whoa this is a this is a tricky question so it always it is always taught that um when the universe it's expanding it's actually expanding into an empty space but um because the universe it's it's not like just a simple three-dimensional object it's actually more than that it's like four-dimensional or even more so when you say that when I say the universe is expanding it's actually um expanding into nothing in that sense yeah I know it's really hard like I I can't even there is no good analogy to actually explain this but um you can't think of it as like a balloon expanding into the surrounding air yeah even though a balloon is actually used a lot in in the universe expansion analogies yeah um do you think that there are other dimensions I think so like if you're talking about like other than three dimensions definitely there is um I believe there is this there is a spacetime I'm not sure if there is actually a dimension higher than that um I know it has been proposed a lot in a lot of theories and even if you watch if you watch movies like interstellar um they will also talk about like the fifth dimension and things like that but currently from what I know I I think that there are four dimensions will the sun explode okay the sun will not explode because of its mass the mass is not too um is it the mass is not large enough um so only massive stars which are more than eight solar masses that means eight times the mass of the sun will actually explode into a supernova so what is the the end stage of our of our sun you might ask in five billion years time it will actually um puff up its outer layers of gas in a very peaceful manner it would it wouldn't explode and it will form this thing called a planetary nebula yeah okay um I will just answer two more questions and then uh we will we will wrap it up so the second last question is how many star clusters are there in the Milky Way um I'm not too sure but I think it's about a hundred plus you know maybe if if you consider global clusters and open clusters I think there is about yeah there is a few hundred plus um open clusters in the Milky Way okay and the last one is um would love to hear a little bit about your PhD topic uh yeah so I just started my PhD a few months ago and I'm actually looking at the Magellanic clouds which are the two nearby interacting um 12 galaxies of the Milky Way and I'm just seeing I'm just actually investigating some of the chemical abundances of the Magellanic clouds to to try to understand more about its um evolution and history yeah all right thank you so much for all your questions and uh so sorry about the um technical difficulties that that that we faced earlier but I hope that you guys actually learned something about um star clusters today right thank you so much and uh see you again bye