Thread: Spiral arms of a galaxy

How a lay man can justify the appearance of spiral arms in a galaxy when there is different velocity for the rotation of stars at different distances from the center. They are likely to be merged together in few million years time! Please if the thought is appropriate.

Arms are not stars rotating together. They are density waves.

You may find the following sites to be of use:

Origin Of Spiral Arms | CAS CMS
The Origins of Spiral Arms


As you will see, Wayne's proposal of density waves is likely not the full story.

Tks, this did give me a good pic.

Originally Posted by sak

How a lay man can justify the appearance of spiral arms in a galaxy when there is different velocity for the rotation of stars at different distances from the center.

Ahh, but here's the rub - the velocity at different distances is not as different as you might think. Rather than the expected decrease in velocity the further from the centre, the velocity remains pretty constant. The stars out near the edge are going at a similar velocity to stars much closer the centre. Most of the disc rotates rather like a fixed disc! Hence the requirement for dark matter.

Speedfreak, perhaps it was late at night when you posted that. If the orbital velocity of a star far from the galactic centre is much the same as one close to the galactic centre, then the latter will overtake and pass the further star, since it has less distance to travel. For the galaxy to rotate like a disc the orbital velocity wold have to increase systematically with distance from the centre.

Yes, it was late at night! You are of course correct, and readers should ignore my statement about a fixed disc. But the rest of my post stands.

Originally Posted by John Galt

You may find the following sites to be of use:

Origin Of Spiral Arms | CAS CMS
The Origins of Spiral Arms


As you will see, Wayne's proposal of density waves is likely not the full story.

This give a good explanation!

Originally Posted by sak

How a lay man can justify the appearance of spiral arms in a galaxy when there is different velocity for the rotation of stars at different distances from the center. They are likely to be merged together in few million years time! Please if the thought is appropriate.

Good question. Let's look at the rotation rates of the Milky Way according to observations as calculated from relative blue and redshifts, as seen in posting #5. As we can see it is completely unlike planetary orbital velocities. Close to the galactic center stars are orbiting at their slowest rate and continue to rotate faster until the distance of the galactic bulge is reached. At this point the rotation rate remains generally close to a constant velocity. This is the opposite of Einstein, Newton, and Keplerean orbital mechanics. All mainstream hypothesis involve the injection of roughly ten time more dark matter than normal matter by mass, into the galaxy equation. Even then such velocities cannot at all be explained for the reason that you mentioned. If the stars of the galaxy are rotating at the observed rates, then it wouldn't take very many revolutions before spiral galaxies would lose their form. To explain this their are density wave models whereby an orbiting wave moves in the general form of the spiral and upon each rotation different stars become different parts of the density wave. There are a few such hypothesis. Another likely hypothesis, I think, is that they do not understand how it works at all

My own idea, for whatever it is worth, is that from our perspective we cannot properly determine the outer orbital velocities of orbiting stars in our galaxy (or other galaxies) and that they're presently underestimated. According to this idea all stars general would have a velocity according to their relative position in the spiral. Outer stars would orbit progressively faster. But the orbital rate of these stars would "drag" relative to their location in the spiral and in time the older galaxies would have greater drag resulting in longer tails of the spiral. In most galaxies that appear similar in age to the Milky Way, the most revolutions that their arms seem to make is about one revolution of curvature. I do not recall seeing any greater that this since older galaxies arms become smeared until they become solely disk-like and later an elliptical spheroid, if this evolutionary sequence is accurate.

Originally Posted by forrest noble

My own idea, for whatever it is worth, is that from our perspective we cannot properly determine the outer orbital velocities of orbiting stars in our galaxy (or other galaxies) and that they're presently underestimated.

What is your evidence for proposing this hypothesis?

Originally Posted by John Galt

Originally Posted by forrest noble

My own idea, for whatever it is worth, is that from our perspective we cannot properly determine the outer orbital velocities of orbiting stars in our galaxy (or other galaxies) and that they're presently underestimated.

What is your evidence for proposing this hypothesis?

Good question. I think from our perspective maybe 2/3 of the way out in the galaxy, the relative redshifts and calculations to determine orbital momentum would be very difficult. Maybe corroboration of sorts might come from Andromeda calculations but even that I think would be equally difficult concerning blueshift differentials.

Oddly, I consider MOND as evidence in support of the present orbital velocity determinations because of its very close match. But I also think that the primary MOND model has its own predictive inadequacies.

What I consider to be the failings of GR as described by dark matter models, I believe is evidence that the standard model of gravity has problems concerning galactic orbital mechanics/ stellar motions. I also have my own model of gravity that can be seen at pantheory.org (book pages 57A - 57E4), which is the basis for some my statements.