# Galaxy shape

Status
Not open for further replies.
T

#### Tarvaa

##### Guest
Hello,

Firstly, I must point out that I am entirely non-scientifically minded. So I need a qualitative answer, rather than a quantative answer. In other words, I would like to avoid equations and math jargon, if possible.

Secondly, I hope it is OK to ask a question in this area of the forum.

So, finally, my question is this: - Why are galaxies flat?

The Milky Way is a flat spiralling object in space. I assume (assumption is the mother of all _____) that it is gravity that that causes the spiral pattern, much akin to a plughole. There is a very heavy object in the middle that is slowly drawing things toward it. Nevertheless a very heavy object's gravitational effect would surely operate on 3 dimensions. Therefore, I would have thought that galaxies would be more 'puff ball' in appearance, with material drawing to a central point all points in the 3 dimensions that fall under the influence of the heavy object.

I wonder if you understand me!

Going back to the plug hole analogy, that spiral appears generally flat to us because the huge gravitational affect of the earth and the flat nature of the basin. But in the grandness of space, the cosmos, the is no such overriding gravitational influence except very close to the centre, and there is absolutely no basin.

I cannot understand why a galaxy appears to to be flat, or disc-shaped.

What do you think? I really have no idea?

thanks

S

#### SpeedFreek

##### Guest
Hi, and welcome to the SDC (space dot com) forums!

So, what happens when you combine gravity with rotation?

T

#### Tarvaa

##### Guest
Gravity plus rotation? Honestly, I have no idea.

Do you mean the heavy object with a huge gravitational affect is rotating? I suppose that result in microscopic variations of the gravity field and over time flatten an array of objects. Am I close?

S

#### SpeedFreek

##### Guest
A large object is held in a spherical shape because gravity acts equally in all directions. But if that object is rotating, the sphere flattens into an oblate spheroid, like the Earth, where the circumference around the equator is larger than the circumference around the poles. The larger the rotation, relative to the size, and the flatter the sphere becomes - it turns into a disc-like shape.

We do see elliptical galaxies, which look relatively spherical, like balls of stars, but we also see galaxies like our own, whose stars have been spread out into a disc-like shape because the galaxy is rotating.

M

#### MeteorWayne

##### Guest
I moved this topic to Ask the Astronomer, a better fit.

Actually, only the disk of the galaxy is flat. There is also a thicker central disk, large spherical central bulge, globular clusters orbiting the "center", and a large spherical amount of dark matter sorrounding the while shebang.

It has to do with angular momentum. Any rotating structure tends to flatten out. For dense objects like planets and stars Even cloud droplets), the gravity is stronger than the angular effects, and it becomes a sphere. For less dense objects, the angular effects whin out.

MW

Welcome to Space.com

C

#### csmyth3025

##### Guest
Tarvaa has a good question - and I think it applies to planetary systems as well as galaxies. As I understand it, planetary systems form as a result of the collapse of a large molecular cloud.

When all the collapsing is done, the star and it's planets have some sort of rotation (angular momentum). Is this always the case? That is, do stars always have angular momentun?

Is it possible that material in the pre-stellar molecular cloud could fall in from every direction straight towards the center of the proto-star? Would such an outcome (everything falling straight towards the center) be such an unlikely event that it almost never happens?

Do molecular clouds always have some intrinsic angular momentum?

Chris

M

#### MeteorWayne

##### Guest
The chances of there being NO angular momentum are astronomically ( :lol: ) small. If there was none, the mass, whatever it was, would collapse into a sphere. The lack of such outcomes shows how rare (or nearly impossible) it is.

E

#### emperor_of_localgroup

##### Guest
csmyth3025":p9iardvv said:
When all the collapsing is done, the star and it's planets have some sort of rotation (angular momentum). Is this always the case? That is, do stars always have angular momentun?

Is it possible that material in the pre-stellar molecular cloud could fall in from every direction straight towards the center of the proto-star? Would such an outcome (everything falling straight towards the center) be such an unlikely event that it almost never happens?

Do molecular clouds always have some intrinsic angular momentum?

Chris

My first puzzlling question is also related to this thread of shape. Why everything in 'space' is rotating or spinning? Linear movements are understandable because of gravity. Is it possible to trace most rotations and spins of larger objects to apparently circular motions of electrons around nuclei?

The shape of a galaxy is probably (I took CM long ago) determined by the 'theory of central force' and 'many body systems', where one of the bodies is the massive center of the galaxy. A star revolving around the galaxy-center at close to 90[super]O[/super] will eventually align with the plane of the galaxy because of a non-zero component of gravity force towards the plane.

Has someone here given angular momentum the credit for central bulge of a galaxy? Is angular momentum a force?

Z

#### ZenGalacticore

##### Guest
While a large spiral galaxy like ours is "flat" relative to its diameter, it's still on average about 10,000 light-years thick. That's pretty thick.

R

#### ramparts

##### Guest
emperor_of_localgroup":lwc47rmz said:
My first puzzlling question is also related to this thread of shape. Why everything in 'space' is rotating or spinning? Linear movements are understandable because of gravity. Is it possible to trace most rotations and spins of larger objects to apparently circular motions of electrons around nuclei?

You're thinking too much :lol: Galaxies don't care what electrons do around nuclei, and electrons don't really follow circular orbits anyway due to quantum uncertainty. It's just a result of having a lot of bodies which gravitate. Gravity pulls two bodies together, but if you have more than two bodies around, things won't get pulled in straight lines. It's this leftover lack of straightness which turns into rotation. So let's say you have a test particle far away from two stars, one of which is heavier It'll get pulled towards both, but more towards the heavier one, and (depending on the initial conditions) will likely end up in some sort of highly elliptical orbit around the star.

Compounding this is the fact that gravity only pulls things in straight lines if they have no previous motion, but after 14 billion years, nothing in space lacks motion any more This is why gas clouds that lead to stars with planetary systems have so much angular momentum, forming disks which eventually coalesce into planets.

The shape of a galaxy is probably (I took CM long ago) determined by the 'theory of central force' and 'many body systems', where one of the bodies is the massive center of the galaxy. A star revolving around the galaxy-center at close to 90[super]O[/super] will eventually align with the plane of the galaxy because of a non-zero component of gravity force towards the plane.

Has someone here given angular momentum the credit for central bulge of a galaxy? Is angular momentum a force?

Well, sure people have given angular momentum "credit" - angular momentum factors into everything here! Bulges are believed to form as a result of galactic mergers. Since two disk galaxies which merge are almost never aligned, stars and gas get thrown around every which way, resulting in a much more spherical shape (this is also the process which leads to elliptical galaxies, which are basically just big bulges without disks). Then newer gas on the outskirts of the galaxy forms the disk outside the bulge post-merger.

S

#### Saiph

##### Guest
it's because of gravity we have rotation, it favors rotation over linear motion by far.

If two stationary objects use gravity, they will fall towards each other in a straight line. But if one of them has a small sideways motion, it'll miss, and be swung round, beginning the rotation of the system. And believe me, it's very rare to find any object without a small sideways motion compared to another. Throw in another object, like rampart says, and it's essentially guaranteed.

Now, a system ends up rotating in a plane, because there is a net motion inside the system, things are moving slightly more one way than any other, and as the system shrinks due to gravitational contraction and things begin to spin more and more, this becomes more apparent.

Things flatten due to this overall rotation. An object spinning in the same direction as the overall system will collide with fewer other particles, and be left alone. Something rotating perpendicular to the plane will have more encounters, and more chances to be diverted, struck, slowed, etc. This causes them to fall inwards more so than their counterparts moving w/ the spin. I.e. objects perpendicular to the spin of the system are filtered out, leaving a flatter system.

One other thing to keep in mind is this happens much quicker in systems with lots of small objects, (gas clouds w/ trillions of individual atoms) than with a system with fewer, more massive systems (galaxies, stars, clusters, etc) due to the momentum exchanges required. Its a lot easier to shift an atom than a star, and in a gas cloud there are far more atoms than there are stars in a gas cloud. So if you have large spherical star system (like a galactic core) it will flatten over time...but it can take a very long time, compared to a gas cloud of the same (or larger!) size.

C

#### csmyth3025

##### Guest
So planetary systems wind up with a net angular momentun, like our solar system, even though the constituents of the precursor molecular cloud may have seemed to be mostly in random motion.

As I understand it, the axis of rotation of stars and the corresponding axis of rotation of their planetary systems are randomly distributed relative to the axis of rotation of the galaxy in which they're imbedded. Despite this individual randomness, almost all these stars share the same direction of motion around the central massive object of the galaxy and are thus part of the overall angular momentum of the galaxy.

I've noticed in Deep Field photographs of galaxies that they seem to be randomly oriented. Do the galaxies that make up galactic clusters tend to have an overall angular momentum around the cluster's center of gravity?

Is there a scale at which there is no net dicernable angular momentum (superclusters, for instance)?

Chris

M

#### MeteorWayne

##### Guest
You misunderstand chris. In any system with more than a single member, there will be a plane where there is more angular momentum...after that it's all simple physics.

N

#### neilsox

##### Guest
Likely the central bulge of a galaxy is an oblate spheroid rather than a perfect sphere.
Very rarely we see an object traveling in very close to a straight line. This happens if it it is on a collision course with the net byro center. If there is no mass at the byro center the object does a sling shot maneuver which with very rare exceptions results in a curved path. Sorry I don't know how to pronounce or spell byro center = the center of gravity of two or more astronomical bodies = likely not the exact definition either. Help! Neil

S

#### Saiph

##### Guest
that would be barycenter

C

#### csmyth3025

##### Guest
As I understand it, one of the bedrock assumptions of the Standard Model is that at very large scales the universe is homogenous and isotropic. Can the universe be homogenous and isotropic at these scales if there is a net angular momentum to the movement of its constituent parts?

It seems to me that at some point the movement of galaxies and galaxy clusters must become random, with no net angular momentum, in order to satisfy this assumption.

Chris

S

#### SpeedFreek

##### Guest
Are you talking about the apparent "movement" of the clusters due to the expansion of the universe, or the movements that occur within clusters due to peculiar motion?

C

#### csmyth3025

##### Guest
SpeedFreek":1ozzwufr said:
Are you talking about the apparent "movement" of the clusters due to the expansion of the universe, or the movements that occur within clusters due to peculiar motion?

I'm talking about the peculiar motion between galactic superclusters. It seems to me that at very large scales the motion of these groupings must be random relative to each other such that they possess no net angular momentum - even though they are all moving away from each other due to the expansion of space. If not, I would think that the universe would look different depending on which direction you looked.

I think this randomness is called "the end of greatness" and is believed to occur at a scale of roughly 300 million light years.

Chris

M

#### MeteorWayne

##### Guest
Yes, averaged over large areas there is no net motion to the Universe.

C

Thanks MW

Chris

E

#### emperor_of_localgroup

##### Guest
ramparts":1zyyfy8p said:
Gravity pulls two bodies together, but if you have more than two bodies around, things won't get pulled in straight lines. It's this leftover lack of straightness which turns into rotation.

This sentences probably sum up the whole story of why all space objects are revolving around something. But spinning may be caused by more than one reasons. Slight variations of gravity pull on different parts of an object, electric/magnetic fields, and I'll still keep electron's motion in atoms as a long shot option.

But it is surprising, of the billions of galaxies we have not found a single spherical or sem-spherical galaxy. On some rarest occasions initial distribution of matters could have canceled out all mutual forces making a spherical galaxy.

IMO, angular momentum does not produce the shape of a galaxy, but the shape of a galaxy produces a distinct angular momentum.

Some of you may know the correct answer to this question, but I do not.
Do the planets revolve around the sun in the same direction as the direction of spin of the sun? Do any of our planets revolves in opposite direction? Can we make the same statement about galactic center and revolving stars?
I know a few planets spin in directions opposite to sun's spin.

M

#### MeteorWayne

##### Guest
emperor_of_localgroup":2z7tqcmk said:
But it is surprising, of the billions of galaxies we have not found a single spherical or sem-spherical galaxy. On some rarest occasions initial distribution of matters could have canceled out all mutual forces making a spherical galaxy.
That's not entirely true. a sphere is just an elliptical galaxy with low eccentricity

Some of you may know the correct answer to this question, but I do not.
Do the planets revolve around the sun in the same direction as the direction of spin of the sun?
Yes
Do any of our planets revolves in opposite direction?
No
Can we make the same statement about galactic center and revolving stars?
No, the alignement of stellar spin and planetary orbits in the galaxy has shown no preferred plane or direction in our limited database
I know a few planets spin in directions opposite to sun's spin.
Yes, a handful have been found like that. The likely cause is planetary orbits that have been shifted by interactions with other massive bodies such as a large planet, or a binary companion which may have been ejected from the system.

MW

R

#### ramparts

##### Guest
And if you think about it, the idea of there being a "net motion" doesn't make much sense for a universe which is (as ours is) of constant density on large scales. In what direction would that net motion be? Unless the net motion is accelerated (i.e. caused by a gravitational pull, which can't happen in a constant density universe), then you can always find a frame in which the net motion is zero.

C

#### csmyth3025

##### Guest
Even though the matter of the universe may, indeed, be randomly distributed and have random peculiar motion on large scales, it still seems hard to imagine how a galaxy 100,000 light years across such as ours can acquire the tremendous angular momentum it has.

I suppose this is no harder to imagine than is the notion that our galaxy is only one out of more than 80 billion galaxies in our observable universe. Just about all of these, I believe, have their own angular momentum - with orientations that are, overall, randomly distributed.

When you look at the "big picture", you have to admit that the angular momentum of our galaxy - as mind boggling as it might be - is only a very small drop in the (very big) proverbial bucket.

Chris

K

#### kk434

##### Guest
The galaxy rotation curve is "non Newtonian" and DM plays a huge role in the motion of the stars around the galactic center, All those unknows makes theorys about the shape of a galaxy very speculative. So i dont think that there is a definitive anwser to the original post.

Status
Not open for further replies.

Replies
15
Views
952
Replies
0
Views
266
Replies
1
Views
319
Replies
0
Views
336
Replies
8
Views
684