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Our object today is Messier 12, also known as NGC 6218, and we're here to talk about the mystery revealed in the title of this article: why is the mass function of NGC 6218 flat? When we talk about globular clusters, we mean a mass, in this case 200,000 stars, that are located in the same region of space, physically connected, gravitationally bound, and very, very old; These are some of the oldest objects in the outer reaches of galaxies, with stars formed shortly after the Big Bang. So what do we do with such an object? One of the first things we do is measure the masses of all the stars in a globular cluster and categorize them. This forms the mass function.
It shows that we have this many bright stars, this many medium-sized ones, and this many dim ones. We do this because it gives us a tool to understand, first, what the composition of the cluster is now, and second, it gives us clues about how it formed and what has happened to it since then. And when we do this for M52, we find something strange. Here's what the mass function of Messier 52 looks like. You see four different curves here because they are constructed from observations of four different parts of the cluster. So we have an idea of what it looks like in the core and in the rings that spread outward. They are simply shifted up by an
arbitrary amount so that we can see them one above the other. The X-axis is plotted as magnitude, and this is, of course, a strange way in which astronomers measure brightness: a smaller number here means a brighter star. And since these stars are the same distance apart, it also means that the star is more massive. So, this is where this mystery arises. Viewers may not quite understand what the flat nature of this mass function means, but let me explain it using the example of other mass functions from other clusters. So, again, here are some different mass functions. This time we have mass here and quantity here, and we've arranged them vertically with an offset to see the difference. And our object NGC 6218 is down here. You can see how
flat it is compared to the other clusters shown as examples. So, what does this mean? In simple terms, this means that for every high-mass star, there are the same number of low-mass stars. And this is somewhat puzzling, given our understanding of how globular clusters form and how stars evolve. Because if we assume that all the stars in a globular cluster formed a very long time ago, many billions of years ago, and have evolved in isolation since then, we know that a certain fraction of massive stars should have formed along with a much larger fraction of low-mass stars. Over time, all of these massive stars will burn brightly for a short period before ending their lives as a white dwarf or in a supernova explosion. So, going back to the present day, looking at this
very ancient fossil of galaxy formation, we should expect to see significantly more low-mass stars compared to massive ones, because the massive stars should have already evolved. But we see something completely different. We see a very flat mass function, which means we are missing many low-mass stars. We expect to see a phenomenon called mass segregation in a globular cluster. This means that over time, as these stars orbit around a common center of mass, they gravitationally interact with each other because they are packed into a fairly small volume of space. And every time they interact, statistically speaking, they exchange
energy and momentum. Therefore, after each such interaction, the massive star will transfer kinetic energy to the low-mass star. As a result, the orbits of massive stars will shift closer to the center of the cluster. Low-mass stars, having more energy, will orbit closer to the outer parts of the globular cluster. But mass segregation helps us predict that if you moved all the low-mass stars to the periphery of the cluster, they would become very vulnerable to being pushed out. So if there's an external force acting, or simply interactions with other stars in the vicinity, and especially if there's an even larger external influence, like the gravity of the galaxy itself, then you can actually remove those
stars through tidal disruption. And that's exactly what we think could have happened in this case, because we know that globular clusters, although many of them are observed in the wider halo of the galaxy, themselves orbit the center of the galaxy and can even pass through its disk, experiencing enormous tidal forces that can form a stream of stars that will eventually fill the halo of the galaxy. The authors of the original article I cited here believe that during the existence of this cluster, it lost up to a million stars, which now fill the halo of our galaxy.
Orbital calculations predict that it comes very, very close to the center of the galaxy, at 600 parsecs. This may be difficult to comprehend, but if we remember that the nearest stars to us only a few parsecs away, it provides a yardstick for assessing the immediate surroundings of our solar system. 600 of these units is not too many on the scale of a galaxy that is tens of thousands of parsecs across. So this is an object that is penetrating the disk of the galaxy very close to the galactic center and will experience the tidal effects of the All of these low-mass stars, due to mass segregation, are already on the outskirts of the cluster, so they will be vulnerable to being pulled in during many passes.
Looking into the future, it is predicted that instead of a globular cluster's long lifespan of 20 billion years, it probably has only 4.5 billion years left before it is completely destroyed by repeated passes through the disk. . Venus is here. I like this oversized image of the Earth. What's really interesting about this vast Earth is that you can see Australia here, called New Holland. This is its original name. Oho. It's a cute little moon here.
