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Galaxy rotation velocity question


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Let's assume for the sake of this question there is no dark matter. Is the velocity of the stars around the galactic center of say Andromeda so fast that they will be eventually fling away and Andromeda will break up?....thanks,

Rich

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Ok, now add dark matter...are the ones further out still going too fast?

Thanks,

Rich

Depends how much dark matter you add, but if you add the right amount, then they are just fine too. Typically DM dwarfs the stuff you can see. E.g.,

mw_halo.jpg

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So then in the present situation, now, are the further stars out moving too fast,  that they will they break away, or are they balanced so that they will remain in orbit about the galatic center?

Thanks,

Rich

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But if they are moving at the right speed, why add dark matter? It sounds as though they are moving too fast, so dark matter is added to slow them down. But you can't slow a star down by adding something that isn't there, and if it's there, then the star is slowed down already, so how do you know it was moving too fast in the first place...like I said. I'm confused....

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The stars further out are moving too fast if you ignore dark matter. The galactic disk would be unstable, and stars would be flung out. Dark matter is added so there is more mass, and so more gravity to hold them in their orbits. The stars can go around faster if there is a stronger restoring force. 

You can spin a rock on a rope faster around your head with a stronger string sort of thing.

Basically to keep in orbit too forces have to balance, that of gravity pulling it in, and centripetal pushing it out.

So you have

F = Gm1 m2 / r2  being the force of gravity - m1 is the mass inside the orbit of the star m2 at distance r.

F = m2v2/r being the centripetal force a star moving at velocity v experiences.

These must be set equal to keep the star where it is 

Therefore:

Gm1 m2 / r =  m2v2/r 

which simplifies to

Gm1 / r  =  v2

For a star in a fixed orbit r is fixed, G is a constant so can't change, which means the thing controlling the velocity v of the star is the amount of mass inside its orbit. We can estimate this from the number of stars, and there isnt enough to satisfy the observed velocity. Therefore there must be more "dark" (unseen) matter to compensate - or alternatively the equations that work so well on earth and the solar system do not apply on galactic scales.

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But if they are moving at the right speed, why add dark matter? It sounds as though they are moving too fast, so dark matter is added to slow them down. But you can't slow a star down by adding something that isn't there, and if it's there, then the star is slowed down already, so how do you know it was moving too fast in the first place...like I said. I'm confused....

Dark matter simply (perhaps not so simply :grin: :grin: ) adds mass and so adds gravity it doesn't slow them down.

In effect the galactic mass is increased, just that a lot of it you cannot see (well so far), so you now have the mass and gravity of the galaxy is such that the speed the outer stars are moving at is corret to maintain the orbits and not for them the go off on a trip through the cosmos on their own.

These observations were how DM came to be considered, Zwicky.

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Ok, thanks, I follow the algebra, that makes sense. But if you look at a galaxy, like Andromeda, it looks as though the stars are being "flung out" of their orbits from the galatic center.  What I'm saying is maybe they aren't in a fixed orbital distance, but are breaking away. It looks like we don't want them to break away, so we add dark matter to hold them in. Why not just let the equation proceed without adding DM and see what happens? Which we have come full circle and are back to the original statement. I say there is no dark matter and Andromeda is flinging apart, unless we "help" it with DM. I don't understand why we just don't accept what we observe, and let the galaxies fling apart...

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Why not just let the equation proceed without adding DM and see what happens?

If you remove the DM that we currently see as being present (this is by no means certain at the moment) then the Milkyway, Andromeda etc that we see today would have long since dissipated away. Hence why we are currently adding DM into our equations.

It maybe that our current theory of gravity is not taking into account hidden variables, stuff that we don't yet know about. But for the time being, adding DM to the universe is currently our best guess at what is apparently occuring.

Edit .. Maybe they wouldn't have dissipated, but the orbit speeds around the galaxy would be very different to what they currently are without our DM addition.

Edited by Cath
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Let's assume for the sake of this question there is no dark matter. Is the velocity of the stars around the galactic center of say Andromeda so fast that they will be eventually fling away and Andromeda will break up?....thanks,

Rich

Yes. That's how dark matter was first detected. The rotation speeds of galaxies are so fast that they should break up so there had to be an unseen element that was preventing that break up.

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We can work out the approximate age of Andromeda, and even some of the stars in the outer reaches, and its clear they are billions of years old, and the flinging out time scale would be on the order of a few million years, so they really should have gone by now several times over. 

Additionally, from rotational stability dynamics, a spinning mass of particles such as the stars in Andromeda would become unstable and the disk would start to break away from the smooth disk, forming clumps and oscillations (like a badly balanced wheel) even if the velocities are right for the mass - there is a certain ratio where disks can be kept stable, and the MW and Andromeda exceed that without adding DM, so that's another good reason we think there is DM present. (There are several more).

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O.k ill add my two pence in for whats its worth.

Galaxies are bound by gravity. In the centre due to the prescence of black holes and the dense cluster of stars the gravity is quite strong and therefore stars are bound even at the rotation velocities we observe. However as you move outwards the the mass of the galaxie becomes less as star density is less and therefore gravity becomes weaker. Therefore for the stars still to be bound and not shoot off with momentum we would expect the stars to have to be rotating slower than those of the central area. Of course this isnt what we see, the stars actually rotate at the same velocity as those nearer the core. Hence there was a need to explain what  gravitational force was binding the galaxies. Of course the current answer is dark matter.

There is however more to dark matter than simply stopping galaxies momentum tearing themselves apart. Currently our best understanding of the working of galaxies come from computer modelling. And when it comes to galaxies without including dark matter then the computer says "nooo". It seems dark matter plays two roles in galaxy behaviour. In all the models run galaxies without dark matter simply rip themselves apart, and perhaps more improtantly it seems that if dark matter is not factored in then galaxies just dont form correctly in the first place. Without dark matter models struggle to get gravity to form galaxies at all or end up with creations that simply dont represent our observations, and you know what they say about observations in science?.

In terms of M31 i think there is some distortion of the disk but suspect this may be more likely explained by past encounters with satelite galaxies rather than the galaxy self destructing.

Now of course i may well be wrong on all of the above and im sure Julian will correct me if so.

Just a thought though as we cannot see or detect dark matter then the amount required to contain galaxies is purely subject to our current understanding of gravity? Is it possible that dark matter is not based on current gravitational calculations and therefore the amount required is considerably different?

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I think based on what has been said here that the galaxies are spinning apart, no dark matter is involved, and that the reason galaxies haven't degraded apart yet is that they aren't billions or even millions of years old, and haven't had the time needed. This shortened  time theory eliminates the need to insert the undetected and never observed DM. It reminds me of the complicated epicycles early astronomers inserted into movements of the celestial sphere to account for the planets wandering about the cosmos.

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But if they are moving at the right speed, why add dark matter? It sounds as though they are moving too fast, so dark matter is added to slow them down.

The above may explain your confusion - the greater the mass the higher the orbital speed. Dark matter doesn't 'slow down' the stars, it's required to explain their high observed velocities. Either dark matter is present in large quantities or gravity/general relativity breaks at large scales. This readable article may help, it explains the evidence for dark matter and why it is seen as the most likely explanation.

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Yes there are definately similarities with those ealry astronomers as they also chose to continue with their beliefs despite a stack of evidence suggesting otherwise being placed in front of them. I dont quite get it, there is absolutely nothing wrong with questioning current theories but you originally asked a question and then when your presented with some sound reasoning as to why science currently holds that dark matter exists (and not just on bonding galaxies but other related problems) you simply dismiss it on nothing more than your experience of a few casual visual observations. If your going to challenge current thinking then at least try to have a sound argument to make a basis.

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I think based on what has been said here that the galaxies are spinning apart, no dark matter is involved, and that the reason galaxies haven't degraded apart yet is that they aren't billions or even millions of years old, and haven't had the time needed. This shortened  time theory eliminates the need to insert the undetected and never observed DM. It reminds me of the complicated epicycles early astronomers inserted into movements of the celestial sphere to account for the planets wandering about the cosmos.

So how would you say they are?  6000 years?

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Yes there are definately similarities with those ealry astronomers as they also chose to continue with their beliefs despite a stack of evidence suggesting otherwise being placed in front of them. I dont quite get it, there is absolutely nothing wrong with questioning current theories but you originally asked a question and then when your presented with some sound reasoning as to why science currently holds that dark matter exists (and not just on bonding galaxies but other related problems) you simply dismiss it on nothing more than your experience of a few casual visual observations. If your going to challenge current thinking then at least try to have a sound argument to make

The above may explain your confusion - the greater the mass the higher the orbital speed. Dark matter doesn't 'slow down' the stars, it's required to explain their high observed velocities. Either dark matter is present in large quantities or gravity/general relativity breaks at large scales. This readable article may help, it explains the evidence for dark matter and why it is seen as the most likely explanation.

I appreciate the multiple theories on the existence of DM, I'm just adding another theory that doesn't need DM. I asked questions to make sure these galaxies are breaking apart, which from the responses they are.

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Ah, 6000 years eh

This is an astronomy forum, not a religious forum.

I'm not the one who started the religion posts..see cgarry's post...if the galaxies aren't billions or millions of years old, and that lines up with someone's religion....so be it....follow the truth where it leads....

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