The Naked Scientists

The Naked Scientists Forum

Author Topic: Why do Galaxies and Stars rotate?  (Read 25664 times)

Offline Mad Mark

  • Full Member
  • ***
  • Posts: 60
    • View Profile
Why do Galaxies and Stars rotate?
« on: 16/01/2007 21:24:25 »
What process starts Galaxies and stars to rotate?
A new star in a large cloud of gas would pull gas in from all directions so what decides its rotation?
And how does all the rotation in the Universe start?
Water down a plug hole I understand but in a early Universe with the first stars I don't. 


 

Offline thebrain13

  • Sr. Member
  • ****
  • Posts: 442
    • View Profile
Why do Galaxies and Stars rotate?
« Reply #1 on: 17/01/2007 05:45:32 »
There is no answer to that question, other than, because it was randomnly created that way.
 

Offline Soul Surfer

  • Neilep Level Member
  • ******
  • Posts: 3345
  • keep banging the rocks together
    • View Profile
    • ian kimber's web workspace
Why do Galaxies and Stars rotate?
« Reply #2 on: 17/01/2007 11:16:58 »
This is quite a regular question and should go into some sort of FAQ's group

There's no difference between the rotation of water down the plughole and rotating stars and galaxies.  It's all down to the conservation of angular momentum.  As anything collapses towards a point any residual velocity in a direction perpendicular to the line between the point and the object must increase to maintain the angular momentum. OK most of it cancels out but there is always a small residual that gets magnified.  In fact most gas clouds could not collapse into stars without getting rid of excess angular momentum by forming planets.  Most of the angular momentum in our solar system is concentrated in the planets if they and their angular momentum were incorporated into the sun it would be rotating so fast that it becomes unstable.

Angular momentum is one of the most important features in our universe it is the only thing that stops it all collapsing.  I am currently working on a presentation "angular momentum, the most powerful firce in the universe"  because it defeats even gravity and is one of the reasons why Swartschild point singularities DO NOT EXIST!  I am very supprised that the general scientific community continue to talk about point singulariries when the only sorts of black holes that can exist must be Kerr or rotating black holes.  These are much more complex and interesting thab the Swartschild sort and at the limit have linear ring singularities containing all the angular momentum in the hole
« Last Edit: 17/01/2007 11:26:01 by Soul Surfer »
 

Offline thebrain13

  • Sr. Member
  • ****
  • Posts: 442
    • View Profile
Why do Galaxies and Stars rotate?
« Reply #3 on: 17/01/2007 22:24:13 »
But soul surfer, the question is how did all this stuff START rotating, not why would this cloud, that is allready spinning, start spinning faster once it collapses.

Conservation of angular momentum can not explain how an object starts rotating to begin with.
« Last Edit: 17/01/2007 22:26:23 by thebrain13 »
 

Offline Soul Surfer

  • Neilep Level Member
  • ******
  • Posts: 3345
  • keep banging the rocks together
    • View Profile
    • ian kimber's web workspace
Why do Galaxies and Stars rotate?
« Reply #4 on: 17/01/2007 22:53:11 »
The initial expansion of the universe seems to have been quite smoth and uniform as the evenness of the cosmic microwave background shows but it did not totally lack any turbulence.  during the dark ages the universe had to cool enough for the first stars to form.  a lot of these exploded in supernovae and put shock waves and turbulence throughout the expanding universe it is the residue of this turbulence that leads to residual angular momentum.  when you conside the many orders of magnitude that a cloud with a few atoms per cubic metre and light years across has to contract to form a star a few light seconds across you dont need much residual rotation in the clous for it to be pretty quick for a star
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
Why do Galaxies and Stars rotate?
« Reply #5 on: 18/01/2007 21:00:14 »
But soul surfer, the question is how did all this stuff START rotating, not why would this cloud, that is allready spinning, start spinning faster once it collapses.

Conservation of angular momentum can not explain how an object starts rotating to begin with.

He has already explained you, actually:
Quote
As anything collapses towards a point, any residual velocity in a direction perpendicular to the line between the point and the object must increase to maintain the angular momentum. OK most of it cancels out but there is always a small residual that gets magnified.
If you have two masses separated in the void and let them attract each other gravitationally, it's extremely unprobable that they move exactly along the line between the two centers, because of the initial (not exactly zero) velocity of the two masses. So they attract and ineract moving one around the other, like the earth around the sun, or like two stars of a binary system.
« Last Edit: 18/01/2007 21:08:01 by lightarrow »
 

Offline thebrain13

  • Sr. Member
  • ****
  • Posts: 442
    • View Profile
Why do Galaxies and Stars rotate?
« Reply #6 on: 18/01/2007 23:02:43 »
I understand exactly what you guys are saying. But you two are neglecting a simple fact. The law you two are referring to is called conservation of angular momentum, not creation of angular momentum.

Consider a figure skater spinning with her arms extended, she has a certain degree of angular momentum, due to her push on the ice. Now she pulls her arms inward and is spinning faster now, but she still has the same amount of angular momentum. Her rotational speed increased, not her angular momentum.

Now lets consider our galaxy, Earth is 30,000 lightyears away from the center, and we rotate the center of our galaxy at the speed of 800,000 kilometers per hour. And our galaxy has a hundred billion stars. All of these numbers are pretty big, so that means that our galaxy must of started with an incredibly humongous buttload of angular momentum, not the small residual momentum that is left over after most of it is cancelled out.

And as I pointed out before angular momentum can not be magnified the same way rotational speed can. Also if the angular momentum contained in galaxies was random, you would expect to find galaxies with almost no angular momentum. Which is contrary to the observed, every galaxy has a buttload.
 

Offline ukmicky

  • Moderator
  • Neilep Level Member
  • *****
  • Posts: 3011
    • View Profile
    • http://www.space-talk.com/
Why do Galaxies and Stars rotate?
« Reply #7 on: 19/01/2007 01:40:48 »
Quote
so that means that our galaxy must of started with an incredibly humongous buttload of angular momentum, not the small residual momentum that is left over after most of it is cancelled out.

Angular momentum is energy and energy cant be destroyed it can only be converted into something else so how can it truly be cancelled out.

Basically the way i see it is like this our galaxy started out with bucket loads of fuel in the form energy held in the gas and dust clouds which formed after the big bang.
Super massive stars then formed capturing that energy and after a very short period of time went supernovae and gave ordered direction to all the gas and dust thrown out allowing  the universe for the first time to see  more order and less chaos

Those first massive stars also created massive blackholes which came together and then formed the supermassive blackholes which now occupy the centre of our galaxies. Those super massive black holes then captured most of the gas and dust surrounding them which was travelling in a more orderly manner turning all its motion into orbiting angular momentum as it all came together and formed the galaxy.
 

Offline thebrain13

  • Sr. Member
  • ****
  • Posts: 442
    • View Profile
Why do Galaxies and Stars rotate?
« Reply #8 on: 19/01/2007 04:49:09 »
Angular momentum is energy, that is true. Angular momentum can not be cancelled out. That is not true. Consider two earths, both spinning in opposite directions, both containing equivalent amounts of angular momentum. Now if they collide the opposite amount of angular momentum contained in each planet will be lost, the resultant planet is not spinning anymore, it contains no angular momentum. The angular momentum has cancelled out.

I do not understand how supernovas could possibly cause our galaxy to rotate. Supernovas blow stuff all over the place with no particular order. Unless supernovas push all the stars in one direction on one side of the galaxy, and the opposite direction on the other side, they cant realistically contribute to the overall angular momentum of a galaxy.

Saying supernovas release a ton of energy, which gets turned into another form of energy (angular momentum) and thats why galaxies rotate, is like saying, the heat energy from my furnace has turned into angular momentum so now my house rotates rapidly.
« Last Edit: 19/01/2007 05:11:26 by thebrain13 »
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
Why do Galaxies and Stars rotate?
« Reply #9 on: 19/01/2007 10:05:10 »
Also if the angular momentum contained in galaxies was random, you would expect to find galaxies with almost no angular momentum.
Galaxies with no angular momentum are impossible to exist now, because matter would have collapsed into a giant star that would have already exploded as supernovae. Maybe Quasars (objects that we see now but that actually existed billions of years ago) are just that.
 

Offline syhprum

  • Neilep Level Member
  • ******
  • Posts: 3818
  • Thanked: 19 times
    • View Profile
Why do Galaxies and Stars rotate?
« Reply #10 on: 19/01/2007 13:09:36 »
Angular rotation is always conserved but bodies rotating in opposite directions can be considered to have positive and negative values that add up to zero when they collide.
 

Offline thebrain13

  • Sr. Member
  • ****
  • Posts: 442
    • View Profile
Why do Galaxies and Stars rotate?
« Reply #11 on: 19/01/2007 21:00:00 »
Right a galaxy with too little angular momentum in respect with its mass would just collapse because the force of gravity would overwhelm the outward centrifugal force. But, saying a galaxy has to have a certain amount of angular momentum to exist, or at least exist as the way we see them, is not an argument for why they are like that.

Here, I'll use another analogy, saying that galaxies start with random amounts of angular momentum, but only the ones with the right mixture exist right now, is the same as saying people are born with random amounts of hearts, yet people only have one heart because if they had zero our multiple hearts they wouldn't be here.

I can defeat that argument by saying, well if that were the case where are the giant piles of dead people who werent born with one heart?

So my question to you is where are all these vastly unstable galaxies? Where are the galaxies that dont spin, where are the galaxies where all the matter falls right to the middle and then it explodes outward only to repeat the process?

Also, if galaxies were created with random amounts of angular momentum, some galaxies would have much larger black holes in the center of them, in comparison to the overall mass of the galaxy. But that is not observed, black holes always make up very specific amounts of the mass of an entire galaxy.
 

Offline Soul Surfer

  • Neilep Level Member
  • ******
  • Posts: 3345
  • keep banging the rocks together
    • View Profile
    • ian kimber's web workspace
Why do Galaxies and Stars rotate?
« Reply #12 on: 20/01/2007 11:38:46 »
part of the answer to your last question is that they have all happened in the universe's past that's what quasars are.

However to return to the main thrust of the question. I think you are visualising the big bang as a load of sub atomic particles expanding from a point in straight lines away from the centre  this was not the case that would be what you could call a cool big bang.  Our big bang was a Hot big bang in which the subatomic particles were all jostling together and bouncing off each other at an incredible pace as space expanded about them allowed them to cool so there was always lots of angular momentum about.  the explosions and shock waves from the first supernovae added more coherent movements in general area and tended to push things around a bit.

There are also rules (like the kinetic theories of gases) about how the angular momentum distributes itself in gravitating systems.

Look up the virial theorem on google  that should help.
 

Offline thebrain13

  • Sr. Member
  • ****
  • Posts: 442
    • View Profile
Why do Galaxies and Stars rotate?
« Reply #13 on: 21/01/2007 07:08:28 »
Virial theorem, hot cool big bang, talk about overcomplicating a very straight forward problem. It doesnt matter how the angular momentum is distributed, Im asking how it got there in the first place. And a hot big bang and supernovas create random amounts of angular momentum.

If no one has an explanation for how galaxies obtain relatively non-random amounts of angular momentum, Im going to have to say that this is just another example of where modern physics falls short.
 

Offline Soul Surfer

  • Neilep Level Member
  • ******
  • Posts: 3345
  • keep banging the rocks together
    • View Profile
    • ian kimber's web workspace
Why do Galaxies and Stars rotate?
« Reply #14 on: 21/01/2007 11:29:58 »
Consider a blob of cold gas around a light year across starting to collapse to form a star.  All the atoms are moving randomly, so considering the angular momentum of each atom with respect to the point in the middle to which the cloud will collapse and resolving the velocities of the atoms into those towards or away from the centre of the cloud and those going round the cloud it is easy to see that a supstantial fraction of the total energy in the cloud is in the form of angular momentum.  Now this angular momentum will always be retained by the cloud.  The most likely value for the angular momentum is zero but it is a statisical process and it is very unlikely that it WILL be zero it will be somewhere around the standard deviation of the velocities.  this when contracted down by a factor of many thousands will mean that the rotation speed will be very fast.

I have tried hard to explain the processes to you as clearly as possible. Any attempt to create a computer model the process will show the rotations developing.  You appear to be determined to find a lack of understanding in this process.  If that is so I can only suggest that the person lacking the undersatanding of the process is you and not the rest of the scientific community.  Try to think it through a little more thoroughly.
« Last Edit: 21/01/2007 11:49:16 by Soul Surfer »
 

Offline Mr Andrew

  • Sr. Member
  • ****
  • Posts: 206
  • God was primitive man's attempt at Physics.
    • View Profile
Why do Galaxies and Stars rotate?
« Reply #15 on: 21/01/2007 23:46:02 »
I agree with thebrain13...it's perfectly clear how spinning galaxies can form.  But his question I believe is how does all that angular momentum get there in the first place.  Well...it's pretty simple:

Assume that you have a gas cloud.  Now for all intensive purposes, assume that there is no gravitation momentarily.  All of the particles are moving in different directions...they have linear momentum.  Now put a source of gravity somewhere in our gas cloud, like a partially-formed star, and all the particles start to spiral towards it...Tah Dah, the linear momentum was converted to angular momentum.  The real question now is, where did all of that linear momentum come from and why isn't it all in the same direction radially outward from the big bang point?  Well, according to the hot big bang theory, the particles were already moving in different directions.  It is akin to taking a balloon of gas molecules (the kinetic molecular theory of gases states that they have random motion) and suddenly decreasing the atmospheric pressure so it expands.  The molecules retain their random motion but their container is now bigger.  Keep decreasing the pressure on the outside and do it very rapidly and you've got a mini, hot big bang.  Now, why it was a hot big bang and not a cold one, I am not entirely sure.  I would seem it had to be or else where would all the random motion come from?
 

Offline thebrain13

  • Sr. Member
  • ****
  • Posts: 442
    • View Profile
Why do Galaxies and Stars rotate?
« Reply #16 on: 22/01/2007 01:53:08 »
This is how I see it, there is no theory for how the rotation of galaxies, planets, stars, galaxy clusters, etc, can start other than randomness. I disagree that we observe random amounts of angular momentum in our universe.

And I know there is virtually no chance for any object to have zero momentum, but that doesn't mean that it cant be close, and it should be closer. (and by closer I mean ang./mass) If you flip a coin a hundred billion times heads is going to appear very near 50% of the time. The more flips you have the closer it will be to 50% on average. So if the amount of angular momentum is given to a hundred billion stars (the size of a galaxy) all random, then why do larger galaxies possess more angular momentum per mass than smaller ones?

And I am not going out of my way to not understand, I just think that theory is false. [^]
« Last Edit: 22/01/2007 03:58:39 by thebrain13 »
 

Offline Soul Surfer

  • Neilep Level Member
  • ******
  • Posts: 3345
  • keep banging the rocks together
    • View Profile
    • ian kimber's web workspace
Why do Galaxies and Stars rotate?
« Reply #17 on: 22/01/2007 14:58:46 »
Thats where you are wrong with your thinking.  It's your understanding of the statistics.  If you consider a random process like your coin flips  If I do them in sets of 10, 100, 1000,   up to billions  and do each of the sets many times the standard deviation will appear to get closer to 50% as the numbers in the sets increase but if you consider the number of flips either side of the exact 50% value that you are most likely to observe in any given set as the number of samples in that set increases you will find that the number of flips either side of the exact 50% value you are most likely to get in any st will increase as the number of flips in the set increases and this will continue to increase without limit however many samples you get in the set.  So the most probable residual angular momentum in a cloud of gas always increases as the cloud gets bigger.
 

Offline thebrain13

  • Sr. Member
  • ****
  • Posts: 442
    • View Profile
Why do Galaxies and Stars rotate?
« Reply #18 on: 22/01/2007 18:41:01 »
Right, your saying that if you flipped a coin 100 billion times, the chance of the two tallies of heads and tails is more likely to have a bigger difference. Like, it wouldnt be that unlikely for heads to come up 1000 more times than heads.

However, lets say heads represents ang.(angular momentum) in a clockwise direction, and tails represents ang.mom in a counterclockwise direction. And each coin contains mass. If we flipped a coin five times the coin on average would spin fast. Because even though the most angular momentum the whole system could contain would be five, all heads or all tails, the mass of the coins is five so any deviation makes a big difference, in how much ang./mass the system has. which would be representative of the percent of heads to tails there were, 100% any coin would be the fastest 50% would be the very slowest.

Now if you flipped a hundred billion coins, you would most likely have more total ang. on average than if you flipped five. Lets say after you flipped a coin 100 billion times heads came up 1000 more times than tails. The speed in which it spinned, or its ang./mass, would be 1000/100,000,000,000 or 1/100,000,000 very small. compared to if you flipped it five times it would be 1/5, 2/5, 3/5, 4/5, or 5/5 all much larger numbers.

And the fact is larger galaxies not only have larger amounts of ang., they have more ang./mass. Which is opposite of how a random system would work.
 

Offline Soul Surfer

  • Neilep Level Member
  • ******
  • Posts: 3345
  • keep banging the rocks together
    • View Profile
    • ian kimber's web workspace
Why do Galaxies and Stars rotate?
« Reply #19 on: 22/01/2007 23:49:41 »
OK you really want to get down to the detail.

I presume that you are now happy with individual stars sources of angular momentum bur want galactic rotation to be explained.  I have never found the following clearly written down in text books bur Its the way I have analysed the system for more than 20 years.

I need to clarify one thing about simple gravitational collapse  (where angular momentum is not a critically limiting feature).  This starts from the inside out i.e. a small dense body forms first because although the gravity is weaker the bits have less far to fall.

Firstly lets deal with globular clusters.  I see these are the result of  a large cloud of cool gas of many thousands or millions of solar masses starting to collapse and forming a big central star before the rest of the cloud has had time to collapse onto it this rapidly runs through its life and explodes violently sending out a turbulent shock wave that compresses the already dense gas and causes it to  collapse locally form many stars.

With very large clouds there are secondary and subsequent sets of shock waves which after a few hundred million years  (hundreds of generations of high mass stars) form the well known spiral arms of spiral galaxies.  The compression waves will be more violent on the leading edges of any initial rotation and will tend to work more strongly there this tends to bias the rotation in favour of any pre existing rotation.  This should produce the spinning up effect you are looking for

These shock waves only affect the gas and dust because once a star has formed it is effectively decoupled from the interstellar gas.  The very earliest generations of stars probably have strongly elliptical orbits while later stars like the sun have reasonably circular orbits.

Big elliptical galaxies are much more complex structure formed by the merging of many standard spiral galaxies and probably gave streams of stars going around in all sorts of different orbits.  some of them will probably be retrograde to any main rotations because of the statistics and there is not even a rotational axis like a spiral.
 

Offline thebrain13

  • Sr. Member
  • ****
  • Posts: 442
    • View Profile
Why do Galaxies and Stars rotate?
« Reply #20 on: 23/01/2007 03:40:51 »
I dont understand what you mean. Explain, compression waves will be more violent on the leading edges, and how this produces a spinning up effect.
 

Offline Soul Surfer

  • Neilep Level Member
  • ******
  • Posts: 3345
  • keep banging the rocks together
    • View Profile
    • ian kimber's web workspace
Why do Galaxies and Stars rotate?
« Reply #21 on: 23/01/2007 08:53:04 »
The way I see it is the rotational velocity of galaxies over much of the range of the spiral arms is around a few hundred Km/sec with some shear.  By the time they are big enough to have a significant effect the blast waves from supernovae are of a similar speed so in the direction of rotation of the galaxy the velocities of the gas are doubled and in the other direction they are reduced to zero.

I have tended to assume that the most new stars would be formed where the waves are pushing faster and outwards and this would propagate the blast wave.

OK I may be wrong and its the gas that has been stopped and starts to collapse towards the centre that creates the stars.  The only texts that I have read on this subject suggest that astronomers are reasonably sure that galaxy spiral arms are shock waves of new star formation but they are non committal about which way they are propagating.
« Last Edit: 23/01/2007 08:55:38 by Soul Surfer »
 

Offline lightarrow

  • Neilep Level Member
  • ******
  • Posts: 4586
  • Thanked: 7 times
    • View Profile
Why do Galaxies and Stars rotate?
« Reply #22 on: 23/01/2007 10:13:29 »
I have to correct myself: galaxies with no angular momentum are not so rare:
http://en.wikipedia.org/wiki/Elliptical_galaxies
 

Offline thebrain13

  • Sr. Member
  • ****
  • Posts: 442
    • View Profile
Why do Galaxies and Stars rotate?
« Reply #23 on: 23/01/2007 18:29:36 »
Well all the planets and stars in this type of galaxy still contain angular momentum, otherwise they would just fall to the center. Elliptical galaxies are formed when multiple galaxies fall into one another, previously containing relatively opposite quantities of angular momentum.

For example, if there was a solar system that joined our own, containing the same amount of mass but has an opposite relative value of angular momentum. You could say that the total angular momentum in our new solar system is zero, but if you add up all the amounts in every planet and star it would be double.

I know that technically I contradicted myself, but the heart of the question remains. So for the case of elliptical galaxies I will ask the question, how did the galaxies that formed the elliptical ones end up with that angular momentum?
 

Offline Soul Surfer

  • Neilep Level Member
  • ******
  • Posts: 3345
  • keep banging the rocks together
    • View Profile
    • ian kimber's web workspace
Why do Galaxies and Stars rotate?
« Reply #24 on: 24/01/2007 19:11:32 »
You have to remember that even in very dense clusters stars are so small and tightly bound that it is almost impossible for them ever to collide and affect each other  (although planetary systems are much more likely to be disrupted by near approaches of stars).  It is quite possible for a galaxy to have a non coherent angular momentum  ie rotations in all directions.  It is also even harder for a star and a black hole to collide because they are much smaller.  You could start with a galaxy that had very little angular momentum that every star had a randomly orientated very elliptical orbit going down into the dense centre where the is probably a black hole stabilising the structure.  Eventually weak interactions between the stars would change the orbits increase the net angular momentum of the stars to comply with the virial theorem.
 

The Naked Scientists Forum

Why do Galaxies and Stars rotate?
« Reply #24 on: 24/01/2007 19:11:32 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums