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krrazypassions's avatar

Why is the Milky Way galaxy so flat? Are most of the other galaxies flat too?

Asked by krrazypassions (1350points) June 24th, 2011

Milky Way galaxy is a spiral-shaped galaxy which would appear flat when viewed by its sides. Why is it flattened like a disc? Is this flatness related to the flatness of the solar system, in which all planets orbit the sun in the same plane?

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23 Answers

wundayatta's avatar

I think it would be flattened for the same reasons the orbits of the planets tend to be in the same plane: gravity. All the stars are pulling on each other, and eventually they arrive at a position where they have pretty much, on average, created the shortest distance between themselves, which, it seems, is in a plane.

ETpro's avatar

Centrifugal force spreads the galaxies out. Gravational attraction between the interasting stars flattens them.

PhiNotPi's avatar

Centrifugal force. It started out as a massive spinning cloud of gas. The equator of the cloud has a stronger outward pull, and bulges out more. There is not a lot of gravity to keep it together, so it spreads out into a plane. Anything not in the plane tends to get pulled into the plane with time (or gets sucked into a black hole).

poisonedantidote's avatar

5,878,625,373,183,607 Miles. Hardly wafer thin :P

krrazypassions's avatar

@wundayatta Again, like the planetary orbital plane , is the plane in which the galaxy flattened decided by the motion of the galaxy in the local cluster or perhaps the universe?

PhiNotPi's avatar

@poisonedantidote It may be very thick, but we are talking about proportions. To the bacteria on a CD, the CD seems very thick. When we scale the galaxy to the size of a CD, we are much, much smaller than the bacteria

poisonedantidote's avatar

@PhiNotPi I’m just pulling your legs

krrazypassions's avatar

@ETpro @PhiNotPi ok, its perfectly fine to picture the effect of the centrifugal force due to the spinning of the galaxy about its axis… but what determines direction (rather the plane) of the spin of the galaxy? is that plane perpendicular to the forward motion of the galaxy in the universe?

gasman's avatar

When a galaxy (or solar system, for the matter) forms it acquires some random angular momentum local to the area of the formation, so it takes on a preferred axis of rotation and flattens perpendicular to this axis as others noted above. Many galaxies have only a small angular momentum and hence are spherical or nearly so.

What’s important is that the disk-shaped galaxies, taken in large number through large volumes of the universe, are randomly oriented. On a cosmic scale space is isotropic: there is no preferred direction, axis, or plane—this only occurs locally.

krrazypassions's avatar

@gasman But the galactic halos are spherical and they are said to contain the dark matter.. so only the non-dark matter flattens leaving the majority of dark matter as a sphere? They know there is something spherical around the flat galaxies due to gravitational lensing..

PhiNotPi's avatar

Also, there is no direction of foward motion in the universe. If something is moving through the universe, it is impossible to tell if you are moving and everything else is standing still, or vice versa. To have motion, you must have a frame of reference.

gasman's avatar

@krrazypassions Interesting—I didn’t know about dark matter haloes.

@PhiNotPi But there IS absolute acceleration, right? Mach’s principle: You can tell if you’re rotating in space because the “fixed” background of stars will appear to rotate in the opposite direction. It isn’t strictly relative.

PhiNotPi's avatar

@gasman I was talking about linear motion, not rotational motion. If you were moving through space (zero acceleration), it would be impossible to tell if you were moving or staying still.

Zaku's avatar

The galaxy is not a centrifuge. The only force at work is gravity from the mass of the matter in the galaxy. As with any collection of objects, the 3D weighted average of their positions in the center of gravity, about which objects will orbit (or if they have no lateral velocity, crash through the middle, perhaps getting caught up there). Matter that has close enough vectors to each other will orbit each other indefinitely until disturbed by some outside mass, and the more mass involved, the wider the range that objects can get caught up, which tends to group things together. This tends to result in a sphere in the middle, and a ring of matter around that, grouped by matter which had similar initial velocities, in general. Spirals are common shapes, but not the only type.

Anyway, the shape of the galaxy will result from the velocities of all the matter before it coalesced into its current shape.

gasman's avatar

@PhiNotPi Actually there is a CMBR rest frame (cosmic microwave background radiation) but you don’t hear much about it. If you’re moving in this frame, CMBR will appeared blue-shifted ahead of you and red-shifted behind you. So are all inertial frames equivalent or not?

ETpro's avatar

@Zaku I do not understand why centrifugal force would not apply to spinning galaxies. Can you explain that or cite your source?

Zaku's avatar

@ETpro Well, centrifugal force is not a force, but inertia, experienced as the contrast/resistance to some other force pulling towards a center of motion. The only actual force at work is gravity. It’s not like there are two forces – it’s that there is momentum in one direction, and acceleration from another direction, resulting in curved motion. So it’s not that centrifugal force doesn’t apply; it is that centrifugal force is actually the same force as the gravitational force, which when all of the attractions are added together, results in one pull towards the galactic center of mass.

Zaku's avatar

By the way, apparently there is no agreed answer to why some (many) galaxies are spiral-shaped, or even what the spiral “means” in terms of what all is there and what it is doing. A new theory was just presented a couple of months ago.

krrazypassions's avatar

@Zaku Hey can you tell us about that new theory please?
yeah centrifugal force isn’t real force- its a virtual one- i heard that from Feynman in his first ( i think) lecture of the Messenger Series (The Character of Physical Law).. have you guys seen those 7 lectures- they are available on the web since Microsoft’s Project Tuva.. Classic lectures!

Zaku's avatar

@krrazypassions Ok, well here are two theories:

1) Older theory (~50 years from original idea) Spiral galaxies are actually spirals of density wave concentrations, where it’s not actually the shape of the most stars you are seeing in a spiral, but the shape of the areas where the most new bright stars are as a result of the pattern of where mass is densest moves through the galaxy. See this article for another wording of that, with some pictures.

2) In the new theory, the arms really are the positions of the general star densities, but they are constantly forming and dissipating, though it is so slow (galactic time scale) that we haven’t been able to observe enough time to see it yet.

ETpro's avatar

@Zaku Seems to me centrifugal force is at work in all galaxies, but unless the angular momentum of most of the material entering the nascent galaxy is aligned remotely near to a single plane, it doesn’t work to flatten the galaxy, and you end up with an elliptical or even nearly spherical galaxy. But in all cases, centrifugal force is in operation due to the inertia of the orbiting object, which wants to travel in a straight line taking it slowly away from the galactic core, balanced by the gravitational well of the core bending the object’s path in toward itself.

What I was getting at in my first post was that when most of the material drawn into a forming galaxy enters somewhere near a single plane, which is often the case, you wind up with a spiral galaxy that flattens because centrifugal force keeps everything in a circling orbit that’s relatively near the orbital plane of other orbiting objects, and those objects interact gravitationally over time to more closely align their orbital planes with other object nearby in the main plane of rotation.

You see the same thing at work in the orbits of the planets and asteroid belts around our Sun, and the rings of the 4 ringed planets.

Zaku's avatar

@ETPro When I read the way you explain it, with centrifugal force flattening a galaxy, it makes me think you are conceiving of it in a way that I don’t think is accurate. What I think might be accurate, and what might be at work in solar system formation, is that a concentration of mass gathered more mass together in the same plane, due to gravitation with itself, inertia, and collisions cancelling opposite momentum. But I don’t think that centrifugal force (which is simply the effect of gravity having a net force towards the center of galactic mass, and not in the direction of the stars’ motion) would have a flattening effect. What might would be a concentration of mass in the plane, simply because the mass started out that way due to whatever orientation existed before.

Of course, in the theories I mentioned above, even that is not what is going on. In the density wave theory, the spiral shape indicates the positions of density waves starting new bright star formation, rather than concentrations of where the mass is. Meanwhile, in the theory published two months ago, the arms are actually sort of being launched out of the galactic core.

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