How do star clusters become spherical, how they form and live over billions of years?

imagine the person who doesn't know much about space observing the night sky through the telescope for the first time and then he or she stumbles upon this I'd be surprised if I were them yes globular clusters are beautiful and interesting objects both for amateur astronomers and astrophysicists for instance they can tell us a lot about star formation and evolution as well as about the early universe but what caught the eye of our unexperienced observer was their spherical shape at the same time we already know that many of globular clusters are ancient not much younger than the universe itself some of them are more than 12 billion years old which means those are quite stable systems but why do they have such a shape what holds them together why don't they collapse or dissipate what is going on inside clusters that's what we're going to talk about today globular clusters are groups of hundreds of thousands or even millions of stars bound by gravity and they have a distinct spherical shape and all of those stars are in relatively small volume several tens - a couple of hundreds of light-years across central regions of globular clusters can contain hundreds of times more stars than we have in our solar neighborhood and stellar density increases in the center there are about 116 known globular clusters in our galaxy of course they exist not only in the Milky Way they are found in small dwarf galaxies as well as in the larger massive ones m87 has about 15,000 globular clusters besides shape they also got a peculiar location in the galaxy in the Milky Way they are not in the Galactic disk but rather mostly in the halo and they also could be in the Galactic bulge but as they are very slowly orbiting the galactic center they can pass through the Galactic disk not all of them formed in our galaxy Palomar 12 cluster probably had been a part of the Sagittarius dwarf galaxy the Milky Way satellite galaxy and later was captured by our galaxy as I've said many of globular clusters are older than 10 billion years most of the stars are probably formed from the saying giant molecular cloud at about the same time of course in reality is not as simple as that stars of multiple generations could be found in globular clusters also there are anomalous stars like blue stragglers which I already talked about in a separate video I might get back to stellar generations in globular clusters in future videos globular clusters consist mostly of old low mass red and orange stars there is almost no interstellar gas hence no active star formation I should mention that there is still no definitive answer to the question how globular clusters form but there are some morals or even groups of models according to one set of models globular clusters began forming in the early universe during or even before realization the massive clusters was determined by the genes mass when regions of higher density in the early universe surpassed genes mass they started to fragment and form stars also Dark Matter halos could be involved in other models clusters formed during early stages of galaxy formation gasps and young galaxies could condense into clouds of mass comparable with globular clusters also collision of gas clouds could lead to formation of clusters not all of the clusters are that old they could form recently or even might still be forming they can form when galaxies collide interstellar gas is compressed and clusters form in the regions of higher density one such example is antennae galaxies this is the Hubble Space Telescope image these are two interacting galaxies and we see here many young globular clusters the biggest obstacle in creating the one model is uncertainties in initial conditions but probably in future scientists will get there with new evidence but let's get back to the shape speaking of distinct shapes in the universe there are two main options quite flat disks for instance earth for instance our solar system other planetary systems and spiral galaxies also there are spheres planets stars globular clusters and elliptical galaxies in both cases there's symmetry and systems are in equilibrium at least for some time and there's a question why in certain cases an object or system would have an irregular shape while in others there is symmetry and why sometimes at the disk and other times it's a sphere let's look at our solar system because of some instability and the region of higher density or some outside influence like a supernova shockwave a part of a giant molecular cloud starts to collapse under its own gravity particles in the cloud are moving in random directions but the whole cloud had some initial spin and one direction begins to dominate and one distinct rotation direction appears the cloud collapses further and just like a skater or me who brings arms closer because of conservation of angular momentum rotation speed increases and the cloud becomes even flatter we can see that effect rotating a ball of clay fast enough and also on a larger scale because our planet rotates it's polar radius is 20 kilometers shorter than equatorial radius a much more extreme example of this is how Mia a dwarf planet it has enough mass to be spherical but it looks like this because of very fast rotation so as a result we get a protoplanetary disk of gas and dust with a star at its center and planets form in that disk and later they mostly maintain that our in tation the similar thing happens in galaxies but on a different scale why don't clusters become a disk in the case of the solar system we have a relatively dense gas where particles collide quite often as a result the ones that move in opposite directions slow down after the collision but the ones that are moving in the same direction accelerates and because of the initial spin it winds over and the cloud becomes flat but in the case of clusters bar so if the cloud collapse in two separate stars and when there is no more gas even if stars interact it happens rarely and they mostly continue to move in their initial paths to sum up in the case of a planetary system the first thing that forms is a disk of gas and dust and only then planets form in that disk first structure than objects and in the case of clusters first stars form and then later a structure of cluster first objects and then they settle down in a structure but how do they settle down when there's so many objects stellar dynamics is quite complicated this is not two or even three bodies in simple modal stars in clusters were compared to molecules in gas of uniform temperature stars are moving in random directions interacting and not necessarily physically colliding when two stars get closer they influence each other gravitationally they exchange energy and this slowly leads to equilibrium in a system this takes tens of millions of years and under gravity a cluster becomes spherical in the case of some elliptical galaxies there can be another way to the shape they can form when two spiral galaxies collide spirals used to have order and most stars were moving in the same direction and plain but collision disrupted orbits of stars and instead of beautiful spiral structure we get this that's what's going to happen to our own galaxy and Andromeda in a few billions of years we can also make another comparison with a different type of clusters open clusters which are also groups of stars that formed at about the same time but they can be all kinds of irregular shapes well first open clusters are much smaller and less massive they contain weigh viewer stars only hundreds maybe thousands of stars also they're less massive and stars are farther away from each other which means they interact and exchange energy less frequently and that doesn't lead to equilibrium which makes globular clusters spherical plus in our galaxy open clusters are concentrated in the Galactic disk where they are subjected to much stronger external forces like tidal forces and their gravity is way weaker to resist that influence as a result they get distorted and deformed and even completely dissipate and stars are no longer bound by gravity open clusters could last for hundreds of millions of years but that's not nearly as stable as globular clusters stability of globular clusters also depends on velocity of stars if the star is fast enough it can leave the cluster but when they're slower gravity holds them together globular clusters are also subjected to external forces but their gravity is strong enough to resist and stars are held together for billions of years but they still lose some stars that are too fast from time time and other questions why so many stars so close to each other don't collapse on a very basic level we can compare it to our solar system why doesn't the earth fall into the Sun because it's moving if I set the speed to zero it immediately crashes into the Sun obviously in the case of globular clusters it's way more complicated because the number of stars is huge but anyway random orbit Sanders is gravity but still something is happening when two stars interact they exchange energy the star of lower mass speeds up and the higher mass star slows down because of that more massive stars migrate to the center of a cluster and lower mass stars to the outer regions the process is called mass segregation a similar thing happens to the blue stragglers that migrate to the center actually there is way more to talk about another process that is called called collapse I haven't mentioned possible intermediate-mass black holes in globular clusters and other interesting aspects so I think I might talk about them in future videos links to all of the source materials are down below in the description and if you enjoyed the video leave a like comment and subscribe bye [Music]

Channel: Cosmos:elementary

Views: 1,648

Rating: 5 out of 5

Keywords: space, science, astronomy, universe, cosmos, space videos, globular clusters, star cluster, open cluster, stars, shapes in the universe, flat planet, galactic disc, flat galaxy, elliptical galaxy, spiral galaxy, formation of solar system, big bang

Id: xFzswOzuPRU

Length: 10min 27sec (627 seconds)

Published: Thu Feb 20 2020

Reddit Comments

[u/CHUVASCH1]

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[u/kvashenov]

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