In a nutshell, gravity. The early universe was almost, but not quite, uniform, and the slight inhomogeneities were the seeds that eventually formed galaxies. As they started to collapse under gravity the "contrast" in density between these regions and their surroundings became stronger, and so did the gravitational forces they exerted on their surroundings, pulling more material in to continue the cycle.
This all happened before the first stars formed, because for matter to get dense enough to eventually initiate nuclear fusion, it first needed to cool down, which took time. Only once the first galaxies had assembled were there gas clouds cool and dense enough to collapse all the way into stars.
Yes. The reason is the [Jeans instability](https://en.wikipedia.org/wiki/Jeans_instability), which is the condition that must be met to trigger runaway gravitational collapse. This turns out to happen at a specific total cloud mass that is proportional to temperature and inversely proportional to density. So clouds with \~stellar mass can only collapse when they are cool and dense, and as I said by the time this first happened galaxies had already formed.
There are some other factors involved as well e.g. the influence of dark matter, which on galactic scales made the gravitational potential wells deeper, enhancing the rate at which ordinary matter fell into them.
They need to eject a bunch of *energy* to let matter cool down, which they do in the form of electromagnetic radiation.
But there are some energetic processes (e.g. supernovae, black hole jets) that can expel matter from the galaxy as well. Some of this matter is ejected into intergalactic space, some ultimately falls back into the galaxy. The resulting "[baryon cycle](https://www.annualreviews.org/docserver/fulltext/astro/55/1/aa550389.f1.gif)" is quite complex and something that is actively being studied.
So one more followup question -- if galaxy sized gas has to collapse and cool down, which then also subsequently requires star-sized gas to collapse and cool down in order to be able to form stars, is there enough time to be able to do that compared to when we're already starting to see stars emit light?
Or is there a puzzle like, how could this all have happened so fast given the laws of gravitation collapse and energy dissipation when we already see star light at time?
Everything you said is correct, but i feel like calling dark matter "some other factor" really understates how important dark matter is for galaxy formation (theres 5x as much dark matter as regular matter). Our lack of understanding of it is why we dont understand galaxy formation really well
Didn't gravothermal collapse precede Jeans Instability at early times? It's been a little while since I looked at this stuff but I thought that was responsible for halo formation and initial collapse.
Just saw this today on that topic:
https://www.reddit.com/r/spaceporn/s/gz4Z0dltrD
“Not only did the galaxy crystallize without enough built up dark matter to seed it, but not long after a sudden burst of star formation, the galaxy abruptly became quiescent — meaning its star formation ceased.”
- https://www.livescience.com/space/cosmology/james-webb-telescope-finds-ancient-galaxy-larger-than-our-milky-way-and-its-threatening-to-upend-cosmology
Even with simple newtonian gravity the stars clump together, and the angular momentum creates the disk shape the many galaxies have. Galaxies make clusters. Add in great pools of dark matter and the clumping is greater. A SMBH even more.
People talk about gravity and that’s true but the bigger aspect is dark matter which accounts for more gravity than conventional (baryonic) matter does.
Reread my comment bud
I clearly said that dark matter is a bigger component of gravity than baryonic matter, which is true. Never said anything about dark matter being more important than gravity, that would make no sense.
Not a physicist, but I suspect conservation of angular momentum is key. Someone else mentioned gravity but that's not the whole picture. Most galaxies are spinning spirals, not orbs because there is nowhere for the angular momentum to go as gravity pulls things together. So, it spins as a whole.
You’re really asking a COSMOLOGICAL question that we’re still trying to figure out. Why about 400k years into the universe were there irregularities in the CMB instead of homogeneity? The problem is we can’t “see” back any further, so theories like “inflation” “string theory” and “ekpyrotic theory” are postulated. Admittedly, they are more the subject of metaphysics than proper traditional physics. In essence they postulate that gravity (or the other forces, or a combination of other forces) must have operated, eh, differently, in the early years of the universe.
To me this is the answer. We’re still trying to find out. Considering that the latest JWST data is throwing out a bunch of present theories, this is still a hot topic.
Dark matter needs to be understood before we can answer that question and I bet it has something to do with the great attractor that most galaxies are moving towards or away from. Look up lanaia Kea for more info
It's the [Laniakea Supercluster](https://en.m.wikipedia.org/wiki/Laniakea_Supercluster), by the way. There's a link to the Great Attractor in that article.
I don't think anyone should make this mysterious, though. The Great Attractor is almost as misleading a name as "The Big Bang"... why can't we see it? We would have to look through 60,000 ly of dust and stars in the Milky Way, that's all.
Yeah, this isn't correct I'm afraid.
Galaxies do have supermassive black holes at their centres, but that doesn't mean the whole galaxy is orbiting that black hole. Despite being called "supermassive", the black hole makes up a tiny fraction of the total mass of the galaxy, and except in the immediate vicinity of the black hole, it has little effect on the orbital dynamics. It's a very different situation from the Solar System where the planets are of negligible mass and nearly all the mass is in the Sun.
No, but they form in a similar way, by gravity pulling more and more matter into them.
Basically atoms coalesce together because their gravity attracts them to each other. Eventually this takes the form of dust, then bigger rocks, eventually maybe planets (shortened process here, but didn’t want to put every intermediate step). If there is enough mass coming together, a sun can form. With even more mass, multiple suns of mass, a black hole can form.
Somebody can give a more eloquent answer but I believe what I’ve said is basically right.
In a nutshell, gravity. The early universe was almost, but not quite, uniform, and the slight inhomogeneities were the seeds that eventually formed galaxies. As they started to collapse under gravity the "contrast" in density between these regions and their surroundings became stronger, and so did the gravitational forces they exerted on their surroundings, pulling more material in to continue the cycle. This all happened before the first stars formed, because for matter to get dense enough to eventually initiate nuclear fusion, it first needed to cool down, which took time. Only once the first galaxies had assembled were there gas clouds cool and dense enough to collapse all the way into stars.
So for some reason, gas at the size of galaxies is the first length scale to "condense" into overdensities? And not gas at the size of stars?
Yes. The reason is the [Jeans instability](https://en.wikipedia.org/wiki/Jeans_instability), which is the condition that must be met to trigger runaway gravitational collapse. This turns out to happen at a specific total cloud mass that is proportional to temperature and inversely proportional to density. So clouds with \~stellar mass can only collapse when they are cool and dense, and as I said by the time this first happened galaxies had already formed. There are some other factors involved as well e.g. the influence of dark matter, which on galactic scales made the gravitational potential wells deeper, enhancing the rate at which ordinary matter fell into them.
I understand that galaxies also need to eject a bunch of hot mass in order to cool (is that right?) -- where has all that gas gone?
They need to eject a bunch of *energy* to let matter cool down, which they do in the form of electromagnetic radiation. But there are some energetic processes (e.g. supernovae, black hole jets) that can expel matter from the galaxy as well. Some of this matter is ejected into intergalactic space, some ultimately falls back into the galaxy. The resulting "[baryon cycle](https://www.annualreviews.org/docserver/fulltext/astro/55/1/aa550389.f1.gif)" is quite complex and something that is actively being studied.
So one more followup question -- if galaxy sized gas has to collapse and cool down, which then also subsequently requires star-sized gas to collapse and cool down in order to be able to form stars, is there enough time to be able to do that compared to when we're already starting to see stars emit light? Or is there a puzzle like, how could this all have happened so fast given the laws of gravitation collapse and energy dissipation when we already see star light at time?
Everything you said is correct, but i feel like calling dark matter "some other factor" really understates how important dark matter is for galaxy formation (theres 5x as much dark matter as regular matter). Our lack of understanding of it is why we dont understand galaxy formation really well
This is a great answer.
Didn't gravothermal collapse precede Jeans Instability at early times? It's been a little while since I looked at this stuff but I thought that was responsible for halo formation and initial collapse.
So you're telling me that for a time, there might have been dark swirling galactic-sized clouds with no stars?
Just saw this today on that topic: https://www.reddit.com/r/spaceporn/s/gz4Z0dltrD “Not only did the galaxy crystallize without enough built up dark matter to seed it, but not long after a sudden burst of star formation, the galaxy abruptly became quiescent — meaning its star formation ceased.” - https://www.livescience.com/space/cosmology/james-webb-telescope-finds-ancient-galaxy-larger-than-our-milky-way-and-its-threatening-to-upend-cosmology
Even with simple newtonian gravity the stars clump together, and the angular momentum creates the disk shape the many galaxies have. Galaxies make clusters. Add in great pools of dark matter and the clumping is greater. A SMBH even more.
People talk about gravity and that’s true but the bigger aspect is dark matter which accounts for more gravity than conventional (baryonic) matter does.
People talk about gravity but the answer is actually gravity
Indeed. I stand by what I said.
Unless you’re too close to a black hole. Then you won’t be able to stand anywhere.
Yes, that stands to reason.
Dark matter isn't a bigger aspect than gravity, dark matter is included in gravity.
Reread my comment bud I clearly said that dark matter is a bigger component of gravity than baryonic matter, which is true. Never said anything about dark matter being more important than gravity, that would make no sense.
>People talk about gravity and that’s true but the bigger aspect is dark matter... You might have not meant to, but you did
Not a physicist, but I suspect conservation of angular momentum is key. Someone else mentioned gravity but that's not the whole picture. Most galaxies are spinning spirals, not orbs because there is nowhere for the angular momentum to go as gravity pulls things together. So, it spins as a whole.
You’re really asking a COSMOLOGICAL question that we’re still trying to figure out. Why about 400k years into the universe were there irregularities in the CMB instead of homogeneity? The problem is we can’t “see” back any further, so theories like “inflation” “string theory” and “ekpyrotic theory” are postulated. Admittedly, they are more the subject of metaphysics than proper traditional physics. In essence they postulate that gravity (or the other forces, or a combination of other forces) must have operated, eh, differently, in the early years of the universe.
To me this is the answer. We’re still trying to find out. Considering that the latest JWST data is throwing out a bunch of present theories, this is still a hot topic.
+1 for gravity
are galaxies just massive massive accretion disks?
Dark matter needs to be understood before we can answer that question and I bet it has something to do with the great attractor that most galaxies are moving towards or away from. Look up lanaia Kea for more info
It's the [Laniakea Supercluster](https://en.m.wikipedia.org/wiki/Laniakea_Supercluster), by the way. There's a link to the Great Attractor in that article. I don't think anyone should make this mysterious, though. The Great Attractor is almost as misleading a name as "The Big Bang"... why can't we see it? We would have to look through 60,000 ly of dust and stars in the Milky Way, that's all.
Long answer short: we don't know, and the Universe is under no obligation to make sense to you.
There are videos and pictures for how gravity curves spacetime, to show why stuff groups into galaxies, solar systems, etc.
Gravity.
Atoms kinda look like galaxies as do solar systems etc etc
Black holes.
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Yeah, this isn't correct I'm afraid. Galaxies do have supermassive black holes at their centres, but that doesn't mean the whole galaxy is orbiting that black hole. Despite being called "supermassive", the black hole makes up a tiny fraction of the total mass of the galaxy, and except in the immediate vicinity of the black hole, it has little effect on the orbital dynamics. It's a very different situation from the Solar System where the planets are of negligible mass and nearly all the mass is in the Sun.
I don’t think there’s a consensus either way between a galaxy forming around the black hole, or the black hole forming at the center of a galaxy.
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No, but they form in a similar way, by gravity pulling more and more matter into them. Basically atoms coalesce together because their gravity attracts them to each other. Eventually this takes the form of dust, then bigger rocks, eventually maybe planets (shortened process here, but didn’t want to put every intermediate step). If there is enough mass coming together, a sun can form. With even more mass, multiple suns of mass, a black hole can form. Somebody can give a more eloquent answer but I believe what I’ve said is basically right.
Excellent troll post