Galaxies

[Allister]

Here is a picture of Galaxy types...

Why are some galaxies elliptical, some spiral and some barred?

 

[Chesterton]

You know how water spirals down a drain as it's getting sucked in? Some believe every galaxy has a massive black hole at it's center. If that's true, perhaps the effect would be similar to the drain in a bathtub. The water forms a spiral around the gravitational core. (The forward motion of the matter is fighting the pull of gravity, and the gravity is winning.)

Water alone tends to evenly distribute itself, but if the water were unevenly distributed matter at the outset (if you were really, really dirty when you took the bath), you might see the equivalent of ellipticality (elipticalness?) or even barring.

 

[Wiccan_Child]

The most popular hypothesis is that bars start out as stellar nurseries that are slightly longer in one direction. The 'longer' ends create more stars than those on the rest, which elongates the nursery even more. Effectively, a slight irregularity in the shape of the nursery at the centre of a galaxy creates has a feedback-loop that further accentuates the irregularity.

As for spirals, well, I once saw a computer model of two amorphous clouds (of galactic proportions) zoom at one another. They didn't collide head on, but their gravitational attraction was great enough to pull them both round and into one another, creating a nice spiral arms and the like. So spiral galaxies seem to be the result of two irregular galaxies colliding.

It would also explain bars: the slight irregularity that 'seeds' a bar could well be the point where the two galaxies first meet (insofar as two diffuse clouds can meet at a definite point at all), since this would have twice the stellar density than normal.

But I'm a theorist of the quantum variety. Wait till Chalnoth comes along for the proper answer

 

[Chesterton]

But also, why aren't almost all galaxies (at leasy roughly) spherical? It seems gravity would shape all matter into spheres, as with stars and planets, rather than disks. Maybe all galaxies are in the process of becoming spherical?

 

[Wiccan_Child]

But also, why aren't almost all galaxies (at leasy roughly) spherical? It seems gravity would shape all matter into spheres, as with stars and planets, rather than disks. Maybe all galaxies are in the process of becoming spherical?

One word: rotation. A rotating sphere doesn't stay round for long.

 

[Chesterton]

One word: rotation. A rotating sphere doesn't stay round for long.

What do you mean by that? Are you saying the opposite of what I suggested - that all spheres are in the process of becoming disk-shaped? (So someday, the Flat Earth Society will be right? )

 

[Allister]

One word: rotation. A rotating sphere doesn't stay round for long

The planets and the stars haven't turned into disks

hmmmm??

 

[Avatar]

The planets and the stars haven't turned into disks

hmmmm??

If you aren't orbiting the gravity well you fall into it. If the sphere of independant objects is rotating (around a gravity well) in one direction it has to flatten. Gravity rules.

 

[Allister]

So why have the planets and moons not flattened?

 

[OldManAnon]

So why have the planets and moons not flattened?

Because the planets themselves are the independent objects. When they as a group orbit the sun they don't flatten - but their plane of orbit does.

That's the problem with your thinking. The Galaxies that flatten are analogous to our solar system, not our planets.

 

[Chesterton]

Because the planets themselves are the independent objects.

But they didn't start their lives as independent objects. They started out as unaggregated matter, the same as galaxies.

When they as a group orbit the sun they don't flatten - but their plane of orbit does.

But then why do the orbits of unconnected independent objects align themselves into a plane?

 

[Avatar]

But then why do the orbits of unconnected independent objects align themselves into a plane?

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[Wiccan_Child]

The planets and the stars haven't turned into disks

hmmmm??

Not yet . A more accurate description would be that a rotating spherical fluid will tend to flatten about its plane of rotation. Solid planets are not fluid, so do not experience this (though the Earth has an equatorial bulge because it was fluid once). Stars, gas giants, and galaxies are fluid, but the former two have something the latter one doesn't: internal pressure. The 'flattening force' from rotation is counteracted by a 'expansive forces' from pressure or thermonuclear effects.

 

[Wiccan_Child]

What do you mean by that? Are you saying the opposite of what I suggested - that all spheres are in the process of becoming disk-shaped? (So someday, the Flat Earth Society will be right? )

Not quite. Only rotating fluids with no internal pressure keeping them spherical (â la stars and gas giants).

 

[ragarth]

Give this 5 minutes of thought: The earth is a solid object, the force of gravity is not sufficient to collapse it's matter and form a neutron mass, and it's angular momentum is not enough to overcome the force of gravity. Regardless though, the earth's crust has a bulge, and the atmosphere bulges around the equator as well.

Galaxies are not solid, they are particulate. Let's imagine three stars orbiting the super giant blackhole of our theoretical galaxy. A orbits at the disc, B orbits perpendicular to the disk, and C has a small orbit around a pole.

C right off the bat is not possible due to the laws of physics, it'd either fly off, attain an irregular orbit somewhere around perpendicular, or fall into the core.

B and A stand a chance of interacting. If there are many variations of B and A we'll see them gravitationally effect each other, changing each other's orbits as they approach each other and over time homogenize their orbits towards a disc shape either by casting off stars, sending stars towards the black hole, or reaching equilibrium with each other in the form of a disc. How does the disc represent equilibrium? Because it represents the point at which there's minimum chance of stars interacting gravitaitonally- perpendicular orbits represent a potential interaction while parallel orbits represent a much lower potential for interaction- it's the lower energy state of orbiting bodies.

 

[TheBear]

Why are some galaxies elliptical, some spiral and some barred?

Because that's the way God made 'em. Who are we to question the mind of God?

 

[juvenissun]

Because that's the way God made 'em. Who are we to question the mind of God?

Not to question, but to understand. God encourages us to do that. It is the same when a student asked a question to teacher. Keep asking, and God WILL give you A surprising answer.

 

[Kharak]

Just don't mention the hidden mass messing with their rotation speeds . . . Oops.

 

[Naraoia]

What do you mean by that? Are you saying the opposite of what I suggested - that all spheres are in the process of becoming disk-shaped? (So someday, the Flat Earth Society will be right? )

The planets and the stars haven't turned into disks

hmmmm??

I think it depends on the speed of rotation (which depends on I have no idea what ). Saturn is much flatter than Earth, and I think there's an asteroid/dwarf planet/something in the solar system that's visibly flat because it spins so fast... *looks* It seems I was thinking of the dwarf planet Haumea. It does rotate very fast and it's probably an elongated ellipsoid, though its shape has only been determined indirectly, from the way its brightness fluctuates. That's what Wikipedia says, anyway.

 

[Allister]

Thanks for all the replies.....

One other question...for the mathematicians...

If I am standing still on the earth, and the earth spins on its axis, the earth rotates around the sun, the solar system moves within the galaxy, the galaxy is spinning and the galaxy is moving in X direction..... how fast am I moving...



If I am standing still; how fast am I moving.... I'm sure that one might even stump Gollum.