# Angular Momentum in Galaxies: Thread A

Discussion in 'Astronomy, Exobiology, & Cosmology' started by Erring Flatley, Mar 16, 2005.

1. ### Erring FlatleyErring FlatleyRegistered Senior Member

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A: For any person to answer: Why do galaxies as they spin not collapse into a "blackhole" like water down a drain?

3. ### blobranaRegistered Senior Member

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Hum,
They do eventually....

The actual formation (and feeding processes) of galactic black holes early on in the history of the universe created a shock wave that blast away the black holes food....thus delaying the collapse of the galaxy into a singularity.

Every time a Black Hole eats, it moves the food further away...

5. ### LucasRegistered Senior Member

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447
This is the same as: why don't the planets of the solar system collapase into the sun?
The answer is that they follow geodesics, as the stars of the Milky Way do

7. ### Lord_PhoenixNew World OrderRegistered Senior Member

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Umm interesting

8. ### TruthSeekerFancy Virtual Reality MonkeyValued Senior Member

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Because they spin. There are two forces here. Gravity brings everything closer, but since it is rotating, it doesn't fall.

Ahhh... it's kinda hard to explain without a picture. But imagine that there are two vector forces, equally strong, that form a 90 degree angle. It pushes inside, than pushes out, and never fall. Another way to see this is by attaching something to a rope and than swinging the rope above your head. Keep swinging...

9. ### Erring FlatleyErring FlatleyRegistered Senior Member

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OK, so if the solar system is very stable and it obides by the same rules as galaxies, then galaxies are very stable as well and do not collapse into black holes "like water down a drain." Is that true?

Last edited: Mar 18, 2005
10. ### James RJust this guy, you know?Staff Member

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Stars don't fall into the centre of galaxies for the same reason that Earth doesn't fall into the sun.

Earth doesn't fall into the sun because it is in orbit around the sun. The Sun's gravity causes the Earth's path to deviate from a straight line (which is the path it would have if the Sun wasn't there). However, it essentially does not change the Earth's speed. If the Earth didn't have tangential speed around its orbit, then it would fall into the Sun.

Think of throwing a ball sideways (parallel to the ground) with some speed from a high tower. The faster you throw the ball, the further away from the tower it will land. If you throw it fast enough, it won't land at all, because the Earth's surface will "curve away" from it at the same rate as it falls. The same idea applies to any orbit.

11. ### Erring FlatleyErring FlatleyRegistered Senior Member

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That is absolutely true. And, you threw the ball at a sufficient speed to overcome the pull of gravity. What force is throwing the stars into orbit in a galaxy? If the stars condensed from widely dispersed matter then this matter is falling toward the center of the galaxy. Something, some force, like the energy of your arm with the ball, must be causing the stars in a galaxy to move into orbit. They cannot fall into orbit by gravitational attraction.

Last edited: Mar 18, 2005
12. ### blobranaRegistered Senior Member

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Hum,

Gas clouds are huge and stars are small compared to them. And like an ice skater spinning with arms outstretched, spins faster when the ice skater draws in their arms, it’s the same with a shrinking gas cloud. Small perturbations and turbulences get magnified as the gas is drawn towards a forming star nucleus.
(I have ignore such things as magnetic field influences, but the same process can be applied to galaxy formation)

13. ### TruthSeekerFancy Virtual Reality MonkeyValued Senior Member

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That's what I said....
Thanks for the "tangential speed"... I couldn't remember that word....

14. ### TruthSeekerFancy Virtual Reality MonkeyValued Senior Member

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Well... maybe they are still "falling" into the centre of the galaxy.....
Like... ok.... imagine a gigantic drain. You throw a tiny ball far, far away. What will it do? It will start circling, and circling and circling. It will stay in the outer parts for a long time, and slowly come closer, until some day it falls in the hole. The same thing for the galaxy. A star is attracted to the galaxy and comes with a certain tangential velocity. This is all the force it needs to keep rotating, until some day, the spiral galaxy becomes eliptical, and than it collapses into a quasar....

I think this is why the rotating velocity of the stars reach a point where it becomes constant throughout the galaxy. As time passes, it will slow down and form an eliptical galaxy.

It's hard to explain without graphs......

As for star formation, the gas clouds are way too far apart from the centre to fall into it. Remember that as stars get closer to a black hole, the speed of rotation increases dramatically.

Also, keep in mind that the gravity that keeps the galaxy together is not the gravity of the hole in the centre, but a combination of the gravity of the hole in the centre with the gravity of the stars around.....

15. ### blobranaRegistered Senior Member

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@TruthSeeker

Hum,
You seem to imply that as star orbits in spiral galaxies decay the galaxies evolve into/form elliptical galaxies.
While this is correct; you forgot to mention that the main reason that they evolve into ellipticals is due to intergalactic collisions and disruptions.

Our own spiral galaxy will collide with the andromeda/m33 galaxies and become ellipicals. And as you say the galactic black holes will be able to feed again and create quasars.

(It just gives a bit of meaning to why we see quasars so far away and the changing population ratios of spirals/ellipticals…)

16. ### Erring FlatleyErring FlatleyRegistered Senior Member

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Please see the Hubble ultra deep field images (HUDF) at: http://imgsrc.hubblesite.org/hu/db/2004/07/images/a/formats/full_jpg.jpg Looking at the galaxies in these earliest images of galaxy formation, we see galaxies that are markedly lumpy. The lumps are star clusters that are drifting together to form a galaxy. This shows space matter drifts together at a small level locally then these lumps drift together to form larger lumps that drift together to form even larger lumps until finally enough matter combines to form a star which in turn form star clusters that in turn form galaxies. Galaxies do not form from gas clouds directly but but through a process of coagulation. In the final galaxy for some matter to be thrown into a true orbit by actions such as star collisions or nova would require other mass to be thrown focibly toward the center of galaxial mass. We do not see this. We see all the stars in orbit. All observable matter that was spiralling into the galaxial center of mass is put into orbit. This requires a force working against the force of gravity toward the galaxial center of mass.

Last edited: Mar 18, 2005
17. ### TruthSeekerFancy Virtual Reality MonkeyValued Senior Member

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Yes, indeed.

However, I wonder if the spiral galaxies could actually naturally evolve into elliptical galaxies. Do they always become elliptical because they collide?

I think the answer is completely in the distribution of mass in a galaxy. Spirals have a very wide distribution, stars are very spread out away from each other. There must be a reason why they arrange in such a format. Well, first of all, if we analize the arms, we see that they sort of "rotate". The arms themselves don't, but the stars do. This might be due to gas clouds in the arms, which would create a greater density, slowing down the passage of the stars. This would form the arms.

The key here is the question: how do the arms rotate. They probably rotate, and in a very slow rate, because of the high difference between the mass and density of the objects in the centre of the galaxy compared to the clouds. So if it rotates "inwards", the galaxy is collapsing. Certainly it wopuld take billions or even trillions of years for the galaxy to "collapse" and form an eliptical galaxy. The other way would be if they rotate outwards. In this case, the gravity of the centre is not strong enough to keep the galaxy together. We don't see that in the skies, so I would think they actually go inward.

I think the key here is the density of gas clouds in spiral arms. The mystery, is why they form those spirals. It might be just simple concentration of mass.

But my point is that galaxies could evolve in a very simple way, desribed below:
?---> spiral-->eliptical-->quasar

In the case of galaxies colliding, it would be:
spiral+eliptical= irregular-->eliptical-->quasar

The key is in the distribution of mass, which can be very scattered, particularly if we are talking about gas clouds in spiral ams.

This would not only explain the differences in the rotation speed of galaxies, but also how they form and what is dark matter (i.e. gas clouds not dense enough to create light).

In elipticals and quasars, there wouldn't be much gas distribution, because the gas is too compacted. In spirals, the gas would be more scattered, which would explain why it has arms and so on.

That would also explain why we don't see many spirals in the beginning. Of course, to explain the existance of spirals, we would need to investigate wheter galaxies can explode (I'm sure I read about exploding galaxies somwhere)....

So..... we had the radiation era, than "gas cloud" era and than we see mostly quasars (because concentration of matter was great), than those quasars explode, the matter comes back together, which form spirals and in some cases elliptical galaxies, than the whole process starts over.

Huuumm... looks like I solved a puzzle....

This is the simplest explanation I can think of.

18. ### blobranaRegistered Senior Member

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@Erring Flatley
>>we see galaxies that are markedly lumpy.

@TruthSeeker
>>The key here is the question: how do the arms rotate.

Hum, I think the key word here is <b>den<font size=2>si<font size=3>t<font size=4>y-w</font>a</font>v</font>es</b>…
The spiral arms and lumpy bits of the galaxies only outline the brightest stars, forming an illusion that the mass is concentrated there.
It is possible that the density waves (<i>that are powered by supernova shock waves, etc</i>) can sometimes travel in an <i>opposite</i> direction to galaxy rotation.

19. ### James RJust this guy, you know?Staff Member

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Dust particles which form galaxies start with small, random speeds in a variety of directions. In any cloud, overall there will be a small imbalance in a particular direction, just due to random processes alone. Conservation of angular momentum as the dust cloud collapses causes that initial small imbalance to eventually result in the entire galaxy spinning at a reasonable speed.

There's no special force which causes a galaxy to spin - or a planet for that matter. The principles are the same in each case.

20. ### TruthSeekerFancy Virtual Reality MonkeyValued Senior Member

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Yeah, yeah, yeah....
But nobody knows yet what creates the density waves. And that's the question I tried to answer in the latest post...

21. ### blobranaRegistered Senior Member

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Hum,
Well yeah, it seems a bit unclear still, but it is generally assumed that when a supernova explodes a large bubble is formed; this will in turn compress interstellar gas, forming new stars that explode, creating new bubbles....and so on...

From this picture of the local Orion arm, roughly 250ly across, that the sun is currently entering, we can see that it is full of bubbles.

(image is 500pc across, with the Sun at the centre)

The whole arm will move slowly towards the right as the shockwaves encounter new hydrogen/dust clouds...

Last edited: Mar 20, 2005
22. ### Erring FlatleyErring FlatleyRegistered Senior Member

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Yes, but the point I am trying to make is that all of this matter is on a falling trajectory when it starts as scattered space matter, and some force must move it out of the falling trajectory into an orbit. If you look at the HUDF images (http://imgsrc.hubblesite.org/hu/db/2004/07/images/a/formats/full_jpg.jpg) even the star clusters from which galaxies are formed are made of stars that do not fall all the way to center of mass of the star cluster in their falling trajectory. As the HUDF images show it, inter-galactic matter coagulates into larger and larger lumps until it becomes stars. The stars gather into star clusters, but unlike the inter-galactic matter the stars do not fall to their collective center of mass. Something, a force, causes the stars to halt in their fall and move into orbit in a star cluster. And once several star clusters fall together, again some force prevents the collection of stars from collapsing to the collective center of mass, resulting in a collection of stars orbiting about a center of mass but not falling into it, that is, forming a stable galaxy. There is some force preventing both star clusters and galaxies from falling into their center of mass. Once a collection of matter begins falling toward their collective center of mass, which will become the center of mass of a galaxy, is there a play between future galaxies to give them all, the necessary angular momentum to form galaxies. That is the point of looking at angular momentum. So, the question is: is their a force causing a collection of spacial mass to have the necessay angular momentum to form galaxies and is there a force preventing galaxies from collapsing inward? The answer to the latter is obviously yes from me. And if that answer is yes, is the answer to angular momentum also yes? Are there two unaccounted for forces acting here to promote the formation of galaxies as opposed to a universe that is in a constant chaotic boil of collapsing and exploding galaxies? It seems very unlikely to me that a repulsive force acting between stars, by itself would be sufficient to prevent collapse and bring the formation of a galaxy. To me something must start all that mass spinning and then something must stop it from reaching the ultimate result of that collapsing spin.

23. ### TruthSeekerFancy Virtual Reality MonkeyValued Senior Member

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Huuummm... interesting....
I didn't learn that in astronomy class...
Does that increase a lot the probability of a supernova occurring close by?