Can you please explain the reasons why galaxies have a spiral appearance?

Question

Do the energy forces on the outside of galaxies have the same amount of energy forces into the closer regions, or center of the galaxy. ie. Will something of the same size and mass carry the same force and gravitational pull in different areas of a galaxy?

Answer 1

There is no simple explanation for galactic structure and scientist are performing very complex calculations and simulations to understand what is happening.

There is one general phenomenon, though, which will shape galaxies and planetary system into flat disks: angular momentum conservation. If you have a more or less spherical cloud with a certain amount of angular momentum to begin with, the gravitational contraction will conserve that angular momentum. The closer the gas moves towards the center, the faster it has to spin. At some point it will spin so fast that the centripetal forces equal the gravitational attraction and the gas (and the stars forming from it) settles into relatively stable orbits. So this gets the disk shape roughly right.

The local gravity of the gas will then compress it into filaments and the differential rotation will pull these filaments into the different shapes we see. Calculating these non-linear interactions is very hard. For a comparison: Cassini keeps surprising the experts by showing ever new and unexpected details in Saturns' rings...

Trouble with all this hand-waving is: in order to conserve angular momentum, one needs some angular momentum to begin with. And it is not easy to explain where enough (local) angular momentum came from assuming the kind of density distribution we see in the early universe (the minimal model would be some shear motions between parts of the gas that filled the early universe).

I googled it and got a lot of interesting hits... from what I have seen I am not convinced there is ONE unique model that will explain everything that has been observed, especially not since dark matter throws a real monkey wrench into all the observations and models:

http://www.google.com/search?client=firefox-a&rls=org.mozilla%3Aen-US%3Aofficial&channel=s&hl=en&q=angular+momentum+conservation+shape+of+galaxy&btnG=Google+Search

Answer 2

jludvig has a good answer. There really isn't one good model that explains all the myriad shapes of galaxies and how they move.

By the way, dark matter/energy is no longer a theory without evidence. In 2006 two galaxies in collision caused the dark matter around both to glow; so it was no longer dark. Without dark matter, swirling galaxies (like our Milky Way) would soon fly apart under their own centrifugal forces. With dark matter, however, there is sufficient gravity pull from the center of mass of the galaxies to offset that centrifugal force.

Swirling galaxies are like ferris wheels in that the stars out on the rim of the wheel move at faster tangential velocities than stars near the center. Thus, in general, stars near the edge will have more kinetic energy than stars near the rotational center of a rotating galaxy, no matter what it's shape.

The exception to this may be when a star gets sucked into a black hole at the center of the galaxy. In which case, like a ship going down in a whirlpool, the star's velocity will speed up as it enters into the event horizon. This change in speed causes high energy photons to emit as signals that radio observatories have observed.

The angular momentum needed at big bang time to transfer to the swirling galaxies is a mystery. Until recently, the theory of the BB posited a homogenous primordial mix expanding into the void. And that homogenuity disallowed offset forces that are needed to impart angular momentum.

But recently, better measuring devices have found that deep space is not uniformly distributed. This suggests that the first seconds of the BB may not have been as homogeneous as they first suspected. If that be the case, then angular momentum could very well have sprung from that uneveness of the expansion.

Answer 3

See Newton's Laws of Motion

Answer 4

I think it's because of the black hole in the middle of them.