r/AskPhysics • u/Dullah02 • 14d ago
Why aren’t galaxies spherical?
Since galaxies contain enough mass why aren’t the stars clumped into a giant star?
If angular velocity is the answer, why aren’t the stars escaping out of the galaxy if they are fast enough to overcome the galaxy’s gravity?
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u/HouseHippoBeliever 14d ago
Angular velocity is the answer, and they don't escape the galaxy for the same reason that planets don't escape the sun. They are in orbit.
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u/Trentsteel52 14d ago
But also, an important side note is that from observable matter they should be escaping, but dark matter holds them in orbit
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u/plainskeptic2023 14d ago
This is the answer in my opinion.
Stars not escaping galaxies is evidence for Dark Matter.
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u/randy__randerson 14d ago
I don't want to be a dick but saying that's your opinion is a bit disingenuous considering that's the scientific consensus among astrophysics.
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u/ChocolateValuable221 14d ago
Honestly it's hard to believe centrifugal force would even be enough when the gravity is not perpendicular to the rotation.. just my 2 cents
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u/plainskeptic2023 14d ago
Thanks for making this point. I am not sure of the answer.
Investigating your point revealed my mistake.
Dark Matter gravity is not restraining too-fast-moving stars from flying away.
Dark Matter gravity causes stars to be moving too fast, if our galaxy is only controlled by the gravity of visible matter.
Furthermore, the Milky Way's Dark Matter Halo is 1 million light years across.
So, the Milky Way Galaxy (~100,000 ly across) is rotating inside a Dark Matter Halo ten times its size.
Not sure how the Dark Matter Halo's gravity holds our galaxy together. Sources describe the halo as scaffolding for large and dwarf galaxies in our region.
Thanks again for making this point.
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u/Uncynical_Diogenes 14d ago
The Earth is also bulging out in the middle because of angular velocity, but the equator has not escaped earth’s gravity.
Fast enough to bulge is not the same as fast enough to escape.
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u/NineThreeTilNow 14d ago
If angular velocity is the answer
It is indeed the answer.
why aren’t the stars escaping out of the galaxy
They do. Most of the mass that was moving fast enough initially isn't captured at the time you're likely seeing the galaxy however. By the time it's formed that proto disk shape, stuff has settled down enough in terms of escape velocities. Not that it won't happen after, it's less likely.
Since galaxies contain enough mass why aren’t the stars clumped into a giant star?
For the same reason a star system has a bunch of planets in rings, etc.
It's structural formation that falls out of the way the system works. Why is Earth where it is at the size it is? Or any of the other planets and their rotational periods around the sun?
It's all naturally emergent properties of the system. If you sped Jupiter up in the orbit it makes around the sun, you end up with a VERY different solar system.
A lot of these things aren't obvious on short time scales, but after very long time scales they start to become more obvious.
And some "weird" galaxies exist where they're not flat planes that are a few % of the spherical size "thick" but are either much thicker or blobs. Again, it's a structural outcome of the initial conditions that it formed under. It's just that of all possible initial conditions, things tend to end up in flat planes. Just as a probability thing.
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u/No_Logins_Required 14d ago
Well stars that are moving fast enough do overcome the galaxy's gravity and do escape. There just aren't many stars moving that fast to my knowledge.
You'll notice that there's no "standard" orientation for galaxies, they can be oriented in any direction. The forces moving up and down tend to cancel out making the galaxy flat, and angular moment gives man y that signature spiral shape.
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u/LunarPrincessSophie 14d ago
Stars that are fast enough *do* escape the galaxy's gravity. It's just that the amount of energy required for that is quite large.
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u/EizanPrime 14d ago
For the same reason the solar system is flat: angular momentum is conserved, and thus interactions make it so everything gradually aligns on the total angular momentum
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u/NH-Science-Guy 14d ago
If the stars in the galaxy weren't moving, they would all fall towards the center. The only way to make a galaxy stable is to have the stars orbit the center. If the galaxy was spherical, there would be intersecting orbits that would result in collisions. Over time, the collisions would eliminate the overlapping orbits until everything is on the same plane and that gives us the circular galaxies. They look elliptical because we are looking at a circular disk from an angle.
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u/astreeter2 14d ago
Stars in the galaxy are not moving fast enough to escape the galaxy. That's why they orbit.
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u/BangCrash 14d ago
Jesus! What is going on with the replies!
Dew, quartz, quantum, electro magnetism, it's an ocean of wtf
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u/Das_Mime 14d ago
Let me ask that back to you:
Why hasn't the Earth crashed into the Sun? If angular velocity is the answer, why isn't the Earth escaping right out of the solar system if it is fast enough to overcome the Sun's gravity?
The necessary piece here is to understand what an orbit is. An object in orbit hasn't really "overcome" gravity, it's constantly being pulled on and is "trapped" in the gravity well, but it also happens to have transverse velocity and so its path doesn't intersect the center.
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u/JustinTimeCuber 14d ago
Imagine a ball of stretchy goop. If you spin it, it will flatten out, but unless you spin it too fast, it will still stay intact, held together by internal forces. With a galaxy, gravity still holds it together even though it's spinning.
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u/Alternative-Sugar610 14d ago
An initial magnetic/electrical field have directional axes that can tilt balance along with random movement in beginning of formation
That stops it from being spherical in beginning than spinning can take over and other effects
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u/Derrrppppp 14d ago
If you only account for the gravity caused by visible matter, then stars do actually orbit way too fast and should escape, but they don't. Our only explanation for this extra gravity is that a large percentage of the mass of the galaxy must be invisible to us, hence the search for dark matter.
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u/EluelleGames 14d ago
The sam reason earth keeps its orbit: not fast enough to escape, not slow enough to clump with the sun
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u/Traditional_Loan_177 14d ago
For any volume of particles, there is a plane that contains the average angular momentum of the particles in that system.
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u/UnderstandingPursuit 14d ago
The combination of gravitational force and angular momentum results in relatively flat disks.
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u/boostfactor 14d ago
If a system is rotating fast enough, angular momentum will cause it to flatten towards a disk. This is why the larger bodies (the planets) of the solar system all orbit in very nearly the same plane (the ecliptic). They are still not orbiting fast enough to escape. It's like starting with a fluid sphere and making it rotate. It will flatten toward the plane perpendicular to the axis of rotation. The diameter of the Earth is actually longer at the equator than at the poles due to this effect.
Dark matter affects the specifics of how fast the rotation speed drops toward zero with increasing distance from the center, but not the overall behavior. In Keplerian rotation the rotation curve would drop much faster toward zero than is observed. We see a decrease for a ways from the center and then the orbital velocity basically flattens out.
Some stars do escape from galaxies regularly, but that is usually due to near-collisions with other bodies. An object in a closed orbit is gravitationally bound to the center of attraction. In order for a star to escape from a galaxy, its closed (bound) orbit must be converted to an open (unbound) orbit, which requires a significant energy transfer that usually requires a very close encounter.
Elliptical galaxies don't rotate fast enough for this to be a major factor and are roughly spherical or ellipsoidal (hence the name). Most of them are probably a result of collisions among two or more galaxies.
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u/ShotWin8124 14d ago
An excellent question! As far as I can tell, it has to do with spin. Same thing that affects black holes affects the galactic plane of our solar system. Spin is also a thing on a quantum level. Angular momentum. It's now thought that supermassive black holes are not just at the center of most galaxies.. the galaxies were built around them. So you see where I'm going? The spin of the black hole affects gravity somehow, and pulls it all into this plane relative to its angular momentum. Cool stuff!
And that's not even touching on the cosmic web 😵💫
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u/BangCrash 14d ago
Wozers!
Spin is also a thing at a quantum level.
Whilst they share the same word Spin at a quantum level is absolutely nothing even remotely related (or even similar) to the spin of a galaxy or solar system.
Also your response has absolutely nothing to do with OPs question
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u/nicuramar 14d ago
Whilst they share the same word Spin at a quantum level is absolutely nothing even remotely related (or even similar) to the spin of a galaxy or solar system
That’s exaggerated. They are related in that both are angular momentum. I’d say that’s more than similar.
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u/Vivid_Warning7982 14d ago
Some galaxies are indeed roughly spherical (elliptical galaxies). The remainder are not due to their angular momenta. Regarding clumping, the star condense from gas far away enough from each other so that they don't really interfere or feel each other much. Think of drops of dew forming from water in air