[Dave Lev (OP)]

Please look at our moon.
The Gravity force of the Sun/Moon is stronger by more than a twice with regards to the Earth/Moon.
If the Moon had to chose its orbital path based on the sum of the force vectors acting upon it, it will had to orbit around the Sun instead around the Earth, however - this isn't the case.

Your understanding of orbital mechanics needs a lot of work.  The strongest force acting on the moon is from the sun, and that means that at all times, the moon accelerates towards the sun.  That's the implication of what I've said above.  I did not say the moon cannot orbit Earth.  The moon has no choice or preference about this.  It moves exactly as per forces as described by Newton.

O.K
We have to ask
"The sun attracts the moon with a force twice as large as the attraction of the earth on the moon. Why does the moon not revolve around the sun?"

Please look at the following article:
https://www.quora.com/The-sun-attracts-the-moon-with-a-force-twice-as-large-as-the-attraction-of-the-earth-on-the-moon-Why-does-the-moon-not-revolve-around-the-sun
"Here the moon and earth form a system, which is like a Binary system . If two astrologers rotate around their center of gravity together, then it is called binary system"
The answer is: "Binary system"
So, this binary system is a "local gravity force" that gives the moon/earth system the possibility to orbit around the Sun.
Hence, the noon is not their by itself, as the earth is not their by itself.
Now they both can orbit around the Sun.
But it is clear that without setting a binary system with the Earth - Just based on pure gravity force - the moon will prefer to orbit around the Sun.
It will never ever set a binary system with the earth if it was orbiting around the Sun.
Few words about binary-and-multi-star-systems:
https://stardate.org/astro-guide/binary-and-multi-star-systems
"A binary is a pair of stars that orbit each other. A multi-star system consists of three or more stars. The stars in a binary or a multi-star system all formed from a single cloud of gas and dust, so they are true “siblings.”"
The Earth & the Moon are real "siblings". This is a key element. I will explain later on why the current concept of how the Moon had been created is absolutely unrealistic:
https://en.wikipedia.org/wiki/Giant-impact_hypothesis
"The giant-impact hypothesis, sometimes called the Big Splash, or the Theia Impact suggests that the Moon formed out of the debris left over from a collision between Earth and an astronomical body the size of Mars, approximately 4.5 billion years ago"
I will also explain how the earth and the moon had been formed "from a single cloud of gas and dust, so they are true siblings". and why they set this binary system due to gravity force.
But in order to explain it, we must know how our galaxy really works.
The spiral arm is a key element in our discussion.
Therefore, understanding the gravity force between the objects in the arm is vital.
So, the Earth/moon is a binary system which holds them both while they orbit around the Sun.
In the same token, the arm is a multi-star-system.
Each star in the galaxy has its own virtual host point. Together, those host points set that "multi-star-system" which holds all the neaby stars in the arm.
So, the sun is not there by itself, (as the moon is not there by itself)
There is a local system that is bounded by gravity force.
Therefore, I call that Binary/multi-star-system as "local gravity force" which is the base for what we see.

[Halc]

We have to ask
"The sun attracts the moon with a force twice as large as the attraction of the earth on the moon. Why does the moon not revolve around the sun?"

As I've said, it does go around the sun, once per year, just like Earth.  So you want to know why the two (Earth and moon) don't separate due to the greater gravitational force of the sun.

Please look at the following article:
https://www.quora.com/The-sun-attracts-the-moon-with-a-force-twice-as-large-as-the-attraction-of-the-earth-on-the-moon-Why-does-the-moon-not-revolve-around-the-sun

Ouch.  This is why I rarely go to quora for answers.  I counted well over a dozen serious errors in that article.  This gem for example:
"Earth's mass and size are big so we do not understand the rotation of the earth. But this is not the case even between Jupiter and Sun. Here the sun does not turn around like Jupiter, and the Sun also does not turn around Jupiter!"
Apparently Earth is too large to understand if it orbits the sun or not, but Jupiter is small enough that it doesn't???  What?  It doesn't help that the guy apparently lacks English skills, but really, what was he trying to say with that statement?

"Here the moon and earth form a system, which is like a Binary system . If two astrologers rotate around their center of gravity together, then it is called binary system"

Do they have to be astrologers?  What if two physicists rotate around their common center of gravity?
If the two astrologers were anywhere near a significant mass like the moon, they would not orbit each other.  Binary systems are not always stable, and the one described there is incredibly unstable.  We need to ask why the Earth-moon system is more stable than the pair of astrologers.

The answer is: "Binary system"
So, this binary system is a "local gravity force" that gives the moon/earth system the possibility to orbit around the Sun.

Sure.  They're the only two significant masses in that system.  The sun's gravity does not act to separate them.  The non-uniformity of the sun's gravity does, and that non-uniformity is far less between the Earth and moon that is the acceleration of the moon due to Earth.  That's why the two stay together.

Hence, the noon is not their by itself, as the earth is not their by itself.

That article denies that the Earth is not by itself.  The orbit of Earth is depicted as a perfect circle, apparently unaffected by the pull of the moon.  Yet another mistake.  It also shows the moon curving away from the sun at times, another mistake if the sun's force is greater than the Earth's.  We can forgive these things since the diagrams are clearly not to scale.

But it is clear that without setting a binary system with the Earth - Just based on pure gravity force - the moon will prefer to orbit around the Sun.
It will never ever set a binary system with the earth if it was orbiting around the Sun.

Not true.  It is orbiting the sun, and yet it forms a binary pair with Earth.  The sun's gravitational field is too uniform at this radius to separate the two, just like the pull of the galaxy is far too uniform to pull the arms apart.  The pull from the sun goes up as the radius drops.  Venus has more acceleration than does Earth for this reason.  Not so with the galaxy according to your graphs.  The net acceleration (proportional to net force) actually goes down as the radius decreases.  This seems to be why galaxies form arms but new solar systems don't seem to.  Maybe they do at first.  Not like we can see that.

Few words about binary-and-multi-star-systems:
https://stardate.org/astro-guide/binary-and-multi-star-systems
"A binary is a pair of stars that orbit each other. A multi-star system consists of three or more stars. The stars in a binary or a multi-star system all formed from a single cloud of gas and dust, so they are true “siblings.”"

We are true siblings with many (all?) of the stars closest by, but we're not really a multi-star system.  Maybe we are.  Not sure.  The rule is not always true.  Some multi-star systems are made up of stars that didn't come from the same cloud of dust.  Maybe one system captured a passing star from somewhere else, a sort of adopted star, not a true sibling.  A single star probably cannot capture another, but any multi-star system is quite capable of it.

The stability of systems with more than two significant masses is low.  There are no 'host points', so the paths are unpredictable, and such systems are capable of ejecting stars from the family in a way that a binary system would not.

The Earth & the Moon are real "siblings". This is a key element. I will explain later on why the current concept of how the Moon had been created is absolutely unrealistic:

Well, other moons on other planets are not real siblings in that they formed from the same material.  So their history of being siblings doesn't explain the orbit or lack of it.  Yes, we went up there and found the moon to be the same stuff as us, thus forming the idea that it is a chunk of Earth knocked off or spun off.

"The giant-impact hypothesis, sometimes called the Big Splash, or the Theia Impact suggests that the Moon formed out of the debris left over from a collision between Earth and an astronomical body the size of Mars, approximately 4.5 billion years ago"
I will also explain how the earth and the moon had been formed "from a single cloud of gas and dust, so they are true siblings". and why they set this binary system due to gravity force.

Being formed by a whack is quite different than forming separately from a common cloud.  The latter hypothesis is unlikely.  Still, Pluto has a moon even more disproportionately large, and that one doesn't appear to be the same material.  Unclear on the history of those two.

But in order to explain it, we must know how our galaxy really works.
The spiral arm is a key element in our discussion.
Therefore, understanding the gravity force between the objects in the arm is vital.
So, the Earth/moon is a binary system which holds them both while they orbit around the Sun.
In the same token, the arm is a multi-star-system.
Each star in the galaxy has its own virtual host point.
Together, those host points set that "multi-star-system" which holds all the neaby stars in the arm.

You've not explained this host point mechanics, and thus it is just fantasy.  It can be trivially demonstrated with a simulation of 3 or more stars, none of which will exhibit motion about a host point. You can deny the Newtonian forces upon which that simulation would run, but you've not described new laws to replaces the ones you're denying.

Sure, arms hold themselves together via gravity.  That is pretty simple since force (towards galactic center) goes up at the outside edge of the arm, and force (towards galactic center) goes down on the inside edge.  This is very different from Earth/moon where the sun's pull is greater, not less, at the inside edge.  So arm stability can be explained without this host-point nonsense.

None of how the arms hold together is relevant to the question of the acceleration of the arm itself, which is totally unexplained in your posts.

[Dave Lev (OP)]

Sure, arms hold themselves together via gravity.  That is pretty simple since force (towards galactic center) goes up at the outside edge of the arm, and force (towards galactic center) goes down on the inside edge.

Dear Halc
If you agree with that, than this is all is needed to explain the spiral arm structure.
As I have stated, each star is connected in the arm by Newton gravity force.
All the stars have a similar orbital speed. (More or less)
Therefore, for any time frame, they all cross the same distance.
In order to achieve it, as they drift outwards, they also drift backwards.
That activity sets by definition the spiral shape of the arms.

That is the whole idea about spiral arms.
Now, let's try to find what our scientists have to say about: "How Our Milky Way Galaxy Got Its Spiral Arms"
https://www.space.com/24642-spiral-galaxies-milky-way-shape-explained.html
Dated - February 12, 2014
"The researchers found that the universe was a very chaotic place in its infancy. The first galaxies were disks with massive, bright, star-forming clumps and little regular structure. To develop the nice spiral forms seen today, galaxies first had to settle down, or "cool," from the previous chaotic phase. This evolution took several billion years."

First contradiction - "Massive disc galaxies" - If our universe was "very chaotic place in its infancy" how could it be that we have got immidiatly disk galaxies with massive, bright, star-forming clumps and little regular structure."
Why those massive disc galaxies had been developed in the chaotic Universe? How long it should take to set the first massive disc galaxy in that unclear process? How could it be that all the billions spiral galaxies had been formed from this chaotic Universe?

"Gradually, the galaxies that were to become spirals lost most of their big clumps, and a central, bright bulge would appear; the smaller clumps throughout the galaxy would begin to form indistinct, "woolly" spiral arms.
These arms would only become very distinct arms once the universe was about 3.6 billion years old. At that age, as the galaxies had a chance to settle down, the turbulence decreased, and new stars would form in a much quieter disk. "We can see the transition from the early chaotic state to the modern, relaxed state," said Bruce Elmegreen.
These first spiral galaxies were either two-armed structures or had thick, irregular spirals with some remaining clumps. More finely structured, multi-armed galaxies like the Milky Way galaxy and its neighbor Andromeda appeared much later, when the universe was 8 billion years old."

Second contradiction - "Age" - We see spiral galaxies at the most far end of the Universe.
The estimated age of many mature spiral galaxies is more than 13.2 Billion years.
So, how could we see today very mature spiral galaxies at estimated age of only 0.6 billion years, if based on this article we need several billions (3.6 Min) and also with the assumption that we have got "Massive disc galaxies" almost immediately from the chaotic Universe?

Third contradiction – "Random process" - They don't show exactly how do we get this spiral arms from  massive disc galaxy. They just say that "Gradually, the galaxies that were to become spirals lost most of their big clumps,.."
This is a random activity by definition. They don't say why they lost the big clumps and how they really got their spiral arms based on Newton gravity.

Conclusions:
I have set a simple explanation why 400 Billion spiral galaxies have got their spiral shape from day one based on Newton gravity force.
Our scientists (even at 2014) try to find an explanation which is not correlated to the universe age time frame. This random activity can't be the base for all the billions spiral galaxies that we see in our universe.
I have set my explanations about the spiral arms structures which is much more relaibale from the last "story" from our scientists.

[Halc]

If you agree with that, than this is all is needed to explain the spiral arm structure.

It is far more complicated than either of us can explain.  You have a bowl of cake batter with a bar of dark batter across the middle.  You stir once or twice and you get a spiral structure something like the galaxy.  You stir 40 times and that structure fades to either super-thing spirals or just one color.  How are the arms still there after 40 laps with all the parts moving at different angular speeds around the galaxy?  Our own arm seems to be one of the ones that is being thinned to the point of nonexistence, possibly explaining why we've not been disturbed by the material in a thicker arm which would have disrupted life formation.

As I have stated, each star is connected in the arm by Newton gravity force.
All the stars have a similar orbital speed. (More or less)

Too much speed, but yes, more or less the same.  Not the same angular speed, which would be needed to keep the arms intact indefinitely, but just similar linear speed.

Therefore, for any time frame, they all cross the same distance.

Exactly, but the stars close to the center of the galaxy have far less distance to go, so they go around far more often than the stars at the far ends of the arms that might yet to have gone around even 10 times since they have so much further to travel to achieve one lap.

In order to achieve it, as they drift outwards, they also drift backwards.
That activity sets by definition the spiral shape of the arms.

That drifting outwards and back defines the clumping of the arms.  The spinning at different angular velocities of each end defines the spiral shape, just like the cake batter.

The speed at which everything moves is unexplained by your assertions.  You never address this issue.

Now, let's try to find what our scientists have to say about: "How Our Milky Way Galaxy Got Its Spiral Arms"

Your last link was hardly to a scientist.  This one here is at least a magazine article which presumably has some standards beyond what quora obviously has.

https://www.space.com/24642-spiral-galaxies-milky-way-shape-explained.html
Dated - February 12, 2014

I like this one, reporting on doing the research the correct way:  Look into the past as see how the shapes evolved.  It's sort of like watching an animation of our own galaxy over billions of years, except all the frames have been dropped on the floor and need sorting.
The article is more about how the research was done than what the results of that research were.

"The researchers found that the universe was a very chaotic place in its infancy. The first galaxies were disks with massive, bright, star-forming clumps and little regular structure. To develop the nice spiral forms seen today, galaxies first had to settle down, or "cool," from the previous chaotic phase. This evolution took several billion years."

First contradiction - "Massive disc galaxies" - If our universe was "very chaotic place in its infancy" how could it be that we have got immidiatly disk galaxies with massive, bright, star-forming clumps and little regular structure."
Why those massive disc galaxies had been developed in the chaotic Universe? How long it should take to set the first massive disc galaxy in that unclear process? How could it be that all the billions spiral galaxies formed from this chaotic Universe?

Hard to parse your comments here.  You're suggesting a contradiction, but I don't see one identified.  Higher density matter tends to group due to its mutual attraction.  That forms galaxies.  If the matter has any angular momentum, then the clump will naturally form a disk shape, just like what happens with new solar systems forming out of a gas cloud.  The disk shape is the only way to preserve angular momentum of a compressing structure.

Second contradiction - "Age" - We see spiral galaxies at the most far end of the Universe.
The estimated age of many mature spiral galaxies is more than 13.2 Billion years.
So, how could we see today very mature spiral galaxies at estimated age of only 0.6 billion years, if based on this article we need several billions (3.6 Min) and also with the assumption that we have got "Massive disc galaxies" almost immediately from the chaotic Universe?

Nothing said that very mature spiral galaxies were of age 0.6 billion years.  That would be a young galaxy still lacking a mature spiral structure.  From where are you getting this claim of a spiral galaxy that is only 0.6 BY old?

Third contradiction – "Random process" - They don't show exactly how do we get this spiral arms from  massive disc galaxy. They just say that "Gradually, the galaxies that were to become spirals lost most of their big clumps,.."
This is a random activity by definition. They don't say why they lost the big clumps and how they really got their spiral arms based on Newton gravity.

Missing details is not a contradiction, just not a full description.  Such details might not be clear.  They only looked closely at about 41 galaxies to get an idea of the various stages of the process.  Nobody has a video of the evolution of any one galaxy as it goes through the process.

Conclusions:
I have set a simple explanation why 400 Billion spiral galaxies have got their spiral shape from day one based on Newton gravity force.

You have no such thing.  You just assert it, and the data from the Hubble image clearly shows that assertion to be wrong since no young galaxy has arms, and thus they're not there from 'day one'.

The article you link suggests no explanations whatsoever.  It just reports data: This is what apparently happens.  Given that, find a theory that explains it, but no such theory seems to be proposed in that article.  You have asserted having an explanation, but none is provided.  You just assert the current observed picture, and fail to explain simple observed speeds of the arms.  So your explanation falls completely short of the current views on galaxy dynamics, which are admittedly only best models at the time.  Those models are always changing, especially since none of them really provides a full explanation.

You can accept it or reject it.

And so I have.

[Dave Lev (OP)]

The speed at which everything moves is unexplained by your assertions.  You never address this issue.

Yes, I can explain it.
However, I need to see the real data.

Please look at the "Observed rotation curve" in the following article?
http://ircamera.as.arizona.edu/astr_250/Lectures/Lecture_22.htm
Observed rotation curve:
We see that at about 0.2 KPC the orbital velocity is maximal - 260 Km/sec
Than it goes down to about 190 Km/sec at 3KPC.
From this point it goes up to 210 Km/sec at 4KPC
and to 230 Km/sec at 7 KPC.
Than it starts to go down again to 200 Km/sec at 10KPC
Goes up to 235 Km/sec at 13 KPC and stay at that velocity at any further distance.
Is this correct?

If so, how could it be that we don't see the real orbital velocity near the SMBH?
We know that the orbital velocity of the plasma is 0.3 c (speed of light)
At the end of this article it is stated:
"Consider a star whose proper motion has been measured to be equivalent to 1000 Km/sec and which lies only 0.01 pc from SgrA*.
So, why we don't see the real velocity at the center?\
Why it shows that the orbital velocity at the center is almost Zero?

Why they also give this information without any connection to the galaxy shape?
I would expect to see measured velocities per radius in the following segments:
1. Accretion disc (What is the Min radius and maximal radius. What is the orbital velocity at each radius?
2. Bulge - Radius range and velocities
3. Bar - Radius range and velocities
4. Ring - Radius range and velocities
5. Disc/spiral arms - Radius range and velocities
6. From the end of the disc - Radius and velocities
Would you kindly direct me to the real measurements of orbital velocities Vs radius in the Milky Way? (From 0 - 20K Pc)
I will give full explanation once I have the real data.

[Dave Lev (OP)]

Thanks

Your link above seems to be fairly in line with similar graphs I've seen.

So as you agree on this graph, let's set the correlation with the structure.
In order to do so, please look at the following diagram of the Milky way structure:
http://www1.ynao.ac.cn/~jinhuahe/know_base/astro_objects/galaxies/Milky-Way-Galaxy-files/logarithmic-spiral-pattern-before2001.JPG
We can see clearly that the radius of the ring is 3Kpc.
Now, please remember that the minimal orbital velocity was exactly at that radius.

Than it goes down to about 190 Km/sec at 3KPC.

So, the orbital velocity at the ring is 190 Kpc.
That shows that at the end of the bar, the orbital velocity is minimal. That evidence contradicts your following message:

The ends of the bar are moving faster than anything else, just like the ends of the thrown spanner are moving faster than the rest of it.

So, the end of the bar has the minimal orbital velocity.
This by itself must set a big red light. How could it be???
In any case, the spiral arms starts exactly at that ring (3KPC).
We don't see the end of the arms but we can assume that it ends at about 15 KPC (at about 45K light year).
Let's focus on the Bar and the bulge:
As we move inwards from the ring the orbital velocity is increasing.
It almost goes to 260 Km/s at 0.5KPC.

I get that at about 0.5 KPC.

However, as we get closer to the center from that radius, the orbital velocity gets down.
At about 0.1 KPC the orbital velocity is almost Zero!!!
This is a very important segment in the galaxy.
There is a meaning for that activity.
Our scientists focus only on the orbital velocity of the stars in the arms, while they ignore completely other important segments..
That shows that their knowledge in spiral galaxy is very poor.
I can explain each segment.
However, do you agree by now that there is high correlation between the galaxy structure and the orbital velocity?
Do you agree that your following message is incorrect?

Why they also give this information without any connection to the galaxy shape?

It seems there is little connection.  Objects at radius X move at the same speed regardless of being in an arm, near one side or the other, or between them.  The arm might affect regular wobble in and out, but not the tangential speed around the galaxy.

[Dave Lev (OP)]

No, it means that not a lot of pop-science web sites focus on that part.  I haven't tried much to actually see what the consensus is about how things move in that inner 3 kps, and how the bar keeps its shape.  Such a thing is fairly common, hardly a weird anomaly of just our own galaxy.

Please look at the following image from NASA (which is identical to the one that you have offered)
https://solarsystem.nasa.gov/resources/285/the-milky-way-galaxy/
It shows clearly that at 3KPC there are two sides of the ring
The one which is closer to the Sun is called: Near 3KPC Arm
The one which is farther from the Sun is called: Far 3KPC Arm
Together they set the ring.
The end of the Bar is directly located just between those arms on each side.
How can you claim that:

The arms seem not to be continuous tubes of stellar material, but rather chunks arranged sort of like fallen dominoes...

Well I don't see it from that data.

The ring I think is way outside, like maybe 40 kps or something, a halo of gas not particularly dense enough to form luminous objects.

The data is clear. It is time for you to agree with the evidences.
There is a clear ring at 3KPC!!!
This ring has a great impact on the activity of the galaxy.
I will explain it later on.
But first - do you agree that :
1. There is a ring at 3KPC?
2. The orbital velocity at that ring is minimal - 190 Km Sec?
3. The Bar ends exactly at that ring?
4.The arms starts to form exactly from that Ring?
Why do you disqualify even the clear evidences which me & you represent?
Why are you so negative?
Would you kindly look again at the evidences and try to understand that spiral galaxy is not just spiral arms?
Would you kindly try to be more cooperative and help us to highlight the real activity at spiral galaxy?

[Dave Lev (OP)]

The one which is closer to the Sun is called: Near 3KPC Arm
The one which is farther from the Sun is called: Far 3KPC Arm
Together they set the ring.

No, they set the inner portions of a spiral.
If you want to call it a ring, fine, but the two arms do not connect.  Each spirals out into the two major arms.  The image clearly shows this.  I notice both arms get noticeably thicker at the point halfway around when the pass relatively close by the opposite end of the bar from which they appear anchored.

No - None of them spirals out. Our scientists call them 3KPC as they stay at 3KPC.
Yes - They are connected at their edges to other spiral arms, but they together set a ring or almost a ring.
I hope that the following image can give you better overview
https://crossfithartford.com/dummies_diagram_of_plant_the_milky_way.php
We can see the two 3KPC arms.
One is called near 3KPC arm and the other Far 3KPC arm.
Why do we call them both 3KPC arm?
Don't you think that the name of the 3KPC represents their radius???
It also seems that both of them are connected/almost connected.
In the following image it is quite clear:
https://solarsystem.nasa.gov/resources/285/the-milky-way-galaxy/
We see that in one side they are connected at a point which is written as galactic bar, while on the other side it is called long bar. At those points we also see the connections to the main spiral arms.
In the image it looks like elliptic ring shape.
However, don't you agree that the mane - 3KPC arm proves that this arm is located all the way at a radius of 3KPC?
They both stay there by themselves.  None of them is part of another spiral arm.
Therefore, if one arm covers the upper half/almost half of the cycle, while the other arm covers the other half/almost half of the cycle, don't you agree that in the total we get a ring/almost ring
You might call it as upper 3KPC arm + lower 3KPC arm.
However, even if you don't see a perfect ring, it is still a ring (or almost a ring...)
We know that the orbital velocity goes down to its minimal value exactly at 3KPC.
Therefore, this ring is a key element in the activity of the galaxy.

[Dave Lev (OP)]

Please look at the following diagram about: NGC 1398: AN UTTERLY PERFECT SPIRAL GALAXY

https://www.syfy.com/syfywire/ngc-1398-an-utterly-perfect-spiral-galaxy

We clearly see a ring at the center (at the edge of the bar which isn't fully clear).
It is also stated:
"NGC 1398 is a barred spiral, with that rectangular-shaped feature running across the core and ending at the inner "ring." This is a common feature in spirals — the Milky Way has a big one — and they can affect how the stars and gas move around the galactic center (I describe this in the earlier post on NGC 1398)."
I have no idea what is the radius of that ring or its width, but it shows that there is "inner ring" in barred spiral galaxy as at the Milky Way.
It is quite clear that this "inner ring" has a significant effect on "how the stars and gas move around the galactic center" as I will explain later on.

[Dave Lev (OP)]

As I said, I can consider that a ring if you wish, but I see a clear spiral there
Great!  Other people call it a ring. Let's go with that.

Thanks!!!
So, the ring has almost a perfect cycle shape around the Bar. (Although if we look carefully inside the ring, we see clearly that it has a spiral shape.)
Once we agree on that ring, we can now start looking at the structure/velocity of barred spiral galaxy

[Dave Lev (OP)]

Let's start with the following question:

What kind of gravity force that ring could generate?
Let's assume that there is no SMBH at the center.
Let's also assume that we set Billion of stars at a perfect ring of 3KPC and hold them ALL at their current position for very long time.(Please don't tell me what will happen to the ring after one minute. This is a theoretical question about stars ring)

What kind of gravity force we might find inside the ring?
Do you agree that all the stars together will set an equivalent gravity force on any object which is located inside that ring?
Therefore:
1. If an object is located directly at the center - Do you agree that the impact of the gravity force from the stars in the ring at a distance of 3KPC in one side should cancel the impact of the stars in the other side of the ring? Therefore, the net gravity force on that object is - Zero?
2. If an object is located at 1.5KPC from the Center (Let's set it close to 12 (in clock) - The impact gravity force of the stars in 3 and 9 should cancel each other? However, the distance to 12 is now 1.5 KPC while the distance to 6 is 4.5KPC.
Therefore, do you agree that the net gravity force in the direction of 12 is quite strong?
3. If an object is located very close to 12 (let's say at 2.9 KPC). - The impact of the gravity force in 12 is almost maximal (as the distance is just 0.1KPC). However, the impact of 6 is minimal (as the distance is 5.9KPC). Remember that 3 cancel the impact of 9.
Therefore, do you agree that the impact of the ring gravity force is higher as the object is closer to the ring?

 
 

[Halc]

Let's start with the following question:

What kind of gravity force that ring could generate?
Let's assume that there is no SMBH at the center.
Let's also assume that we set Billion of stars at a perfect ring of 3KPC and hold them ALL at their current position for very long time.(Please don't tell me what will happen to the ring after one minute. This is a theoretical question about stars ring)

Without reading further, a hoop like that is in equilibrium and can orbit itself, with or without a central object.  The mass within (not part of) the ring will contribute to higher speed of the ring, but a ring by itself will have some minimum speed.  It isn't totally stable, but stable enough, just like our asteroid belt, which has a central mass.  I can think of no example of a ring on its own.

What kind of gravity force we might find inside the ring?

A hollow sphere would have a uniform field.  A body inside one would be weightless, but a ring will attract to itself.  I would fall outward down to the nearest part of the ring if I found myself stationary within it somewhere.
A stationary 'me' would have no orbit or radius, so this isn't even in violation of Newton's law that says mass outside my radius has no overall effect on my orbital speed.

Do you agree that all the stars together will set an equivalent gravity force on any object which is located inside that ring?

I don't know what you mean by 'set an equivalent gravity force' here. Equivalent to what?

Therefore:
1. If an object is located directly at the center - Do you agree that the impact of the gravity force from the stars in the ring at a distance of 3KPC in one side should cancel the impact of the stars in the other side of the ring? Therefore, the net gravity force on that object is - Zero?

At the center, yes.  Net of zero by symmetry.

2. If an object is located at 1.5KPC from the Center (Let's set it close to 12 (in clock) - The impact gravity force of the stars in 3 and 9 should cancel each other?

There will be no net pull to either side, yes, again by symmetry.  The stars at 3 and 9 on the ring are both 'south' of the object that is 1.5kpc north of the center, so there is a net pull towards the center from those two stars.

However, the distance to 12 is now 1.5 KPC while the distance to 6 is 4.5KPC.
Therefore, do you agree that the net gravity force in the direction of 12 is quite strong?

Stronger than any of the others, yes.  You need to integrate the vectors on the entire ring to get the net force on the object.  Taking these single samples will not work.

3. If an object is located very close to 12 (let's say at 2.9 KPC). - The impact of the gravity force in 12 is almost maximal (as the distance is just 0.1KPC). However, the impact of 6 is minimal (as the distance is 5.9KPC). Remember that 3 cancel the impact of 9.

Yes on the 12max/6min, but no on the 3/9 cancelling.  Almost the entire ring pulls the 2.9kpc object downward.  This is why there is zero net force in a sphere.  To compute the net force of the ring, you need to integrate at least the vertical force vector over the entire ring.

Therefore, do you agree that the impact of the ring gravity force is higher as the object is closer to the ring?

Not because of your faulty argument above, but yes, it is.  Your argument above would have there being a net force in a sphere, and we know that's false.

[Dave Lev (OP)]

Without reading further, a hoop like that is in equilibrium and can orbit itself, with or without a central object.  The mass within (not part of) the ring will contribute to higher speed of the ring, but a ring by itself will have some minimum speed.  It isn't totally stable, but stable enough, just like our asteroid belt, which has a central mass.  I can think of no example of a ring on its own.

Thanks
The activity of the ring is clear. It is also clear that it has a great impact on any object which is located inwards (or outwards).
Let's start by looking inwards:
Let's set back  the SMBH at the center of the galaxy.
We have already found that the impact of the Ring at the center is virtually Zero.
Therefore, any object which is located very close to the center is directly affected by the SMBH. It might be a star, gas cloud or even Atom.
However, as we move further away from the center, the gravity force of the ring is more relevant.
Please remember that the SMBH mass is estimated to be in the range of few millions Sun mass. However, the total estimated mass of the ring is in the range of several billions of sun mass.
Based on that knowledge, let's look again on the orbital velocity at the range of Zero to 3KPC (about 10K LY).
We know that the orbital velocity of the accretion disc is 0.3 speed of light.
The orbital velocity of S2 which is located at a distance of 2-10 Light days from the SMBH is 5000 Km/s
If we get to a distance of 0.1 KPC (326 LY) from the center the orbital velocity gets down to 150 Km/s.
However as we get further away from this point, the orbital velocity stars to increase.
At 0.5 KPC the orbital velocity gets to its pick of almost 250 Km/sec.
So far so good.
We can assume that this increase in the orbital velocity is directly affected by the ring.
However, as we go further away from the center and get closer to the ring the orbital velocity goes down again to its minimal value at the ring - 190Km/s. This is unexpected phenomena.
Why the orbital velocity goes down again as we get closer to the ring?
In order to answer this question - Let's assume that we have only one object which orbits inside the ring.
So, we have the ring, we have the SMBH but only one star which orbits there.
We can easily calculate the estimated orbital velocity of this object at any radius from the accretion ring up to 3KPC.
However, as there are millions (or billions of stars) in the bulge + in the Bar, they also set a gravity force on each other.
Therefore, this could be the answer why we see that the velocity is going down as we get closer to the ring.
There is another factor.
The ring is connected to spiral arms.
So, it is used as a frame which holds the spiral arms and set their orbital velocity.
Therefore, it is clear that the ring is loaded by those spiral arms. This load could also have some impact on the orbital velocity of the ring and the orbital velocity of stars inside the ring.
Do you agree with that?
Please also be aware to a very critical issue.
There is no spiral shape inside the ring!!!
The spiral shape is very clear in the ring itself and in the spiral arms.
Therefore, it is clear that the ring has a significant impact of the spiral arms.
I will discuss about it later on.

[Dave Lev (OP)]

You treat the ring and arms as solid objects, talking about loads being put on them and such.  They're not.  They're collections of detached objects which don't alter paths if loads are put on other parts.  You can't alter the speed of the ring by putting a force on part of the ring.  That works only if it is an object.

I disagree with you,
The ring is an object by itself as the Earth is an object by itself.
The Earth is made by many different of Atoms and molecular that are all bonded by gravity force.
In the same token, the Ring is made by many Stars and dust that are all bonded by gravity force.
In one hand you agree that spiral structure start exactly at the ring:
https://phys.org/news/2018-01-image-hubble-barred-booming-spiral.html

agree that the spiral seems to end at 3 kps, not spiraling all the way into the center.  There are some galaxies that do that, but they're in the minority.

But on the other hand you don't see the great impact of the ring.
Why is it?
Let's look carefully at the following image:
https://en.wikipedia.org/wiki/Spiral_galaxy#/media/File:PIA19341-MilkyWayGalaxy-SpiralArmsData-WISE-20150603.jpg
https://phys.org/news/2018-01-image-hubble-barred-booming-spiral.html
https://www.syfy.com/syfywire/ngc-1398-an-utterly-perfect-spiral-galaxy
We see clearly that the arms are connected exactly at the opposite sides of the ring.
In some galaxies it is quite difficult to see the ring, but we see clearly that spiral arms are connected exactly at the opposite sides of the ring and directly to the bar.
This connection between the Arm to the Bar is very important.
It shows that the spiral arm is an extension of the bar.
So, we can think about it as a long arm. Outside the ring we have the spiral shape. Inwards the ring we have the bar.

If we look carefully we in all the spiral galaxies in the Universe - we can also see that all of them have a very symmetrical view.
Spiral arms starts exactly from the opposite sides of the ring - The arm at one side is very similar to the arm in the other side. So in all the galaxies we mainly see two main arms. We might see other arms in the galaxy, but all of them are connected to those two main arms (or near by the conection point)  in a symmetrical view.
We can also ask: Is it feasible to have a spiral galaxy without a ring?
The answer is clearly - Yes, but a spiral galaxy without a ring is quite different from a one with the ring
Please look at the following image:
http://hubblesite.org/image/1636/news_release/2005-01
Please see the ratio between the bar radius to the arm maximal radius:
Do you agree that the ratio is about 1:3?
In the Milky Way the bar gets to 3 KPC while the spiral arms gets to 45 KPC.
So the ratio in the Milky way (with a ring) is 1:15.
Hence, as the ring is more developed, the ratio is higher.
Do you agree that without a ring (or with minimal ring) there is a maximal load (or maximal spiral arm size/radius) that spiral galaxy can take?

Therefore, we have to ask the following:
1. How could it be that the bar is connected to the ring while at the same spot of the connection we clearly see the starting point of the spiral arms? Our scientists only try to explain the orbital velocity in the arm, but they ignore completely the Bar. How could it be that they don't see that the bar is directly connected to the arm?
2. Why there is no spiral shape in the Bar?
3. How could it be that at the ring, the orbital velocity is at its minimal value (190Km/s)?
If we move inwards to the bar (let's assume in the Milky Way - 500 Light years inwards from the ring), the orbital velocity is higher (at about 200 Km/s), while if we move outwards into the spiral arms at the same distance, the orbital velocity is higher again (at about 200 Km/s).
4. Why spiral galaxies have a symmetrical view?
5. What is the real impact of the ring?

[Halc]

You treat the ring and arms as solid objects, talking about loads being put on them and such.  They're not.  They're collections of detached objects which don't alter paths if loads are put on other parts.  You can't alter the speed of the ring by putting a force on part of the ring.  That works only if it is an object.

I disagree with you,
The ring is an object by itself as the Earth is an object by itself.
The Earth is made by many different of Atoms and molecular that are all bonded by gravity force.
In the same token, the Ring is made by many Stars and dust that are all bonded by gravity force.

If that were true, I would immediately drop straight through and fall to the core of the Earth because that is where gravity is trying to take me.  The fact that it doesn't happen means that the Earth is bonded by forces other than just gravity, forces that do not exist between the various objects in the ring.

Why should I continue this thread when you post something that obviously wrong?

We see clearly that the arms are connected exactly at the opposite sides of the ring.
In some galaxies it is quite difficult to see the ring, but we see clearly that spiral arms are connected exactly at the opposite sides of the ring and directly to the bar.
This connection between the Arm to the Bar is very important.

There are no connections.  These are not solid objects.  Two smears of dust that intersect do not make physically connected dust.  If that connection is important, then your idea really falls flat, because there's no connection.  Everything in those pictures is free-falling objects interacting only by gravity, much in the same way that the atoms of Earth or a spanner are not.

Please look at the following image:
http://hubblesite.org/image/1636/news_release/2005-01
Please see the ratio between the bar radius to the arm maximal radius:
Do you agree that the ratio is about 1:3?

About that, yes, but only if you don't include the dimmer outer parts of the arms that they cropped off the image, but you're including those parts when you say the milky way goes all the way to 45 kps:

In the Milky Way the bar gets to 3 KPC while the spiral arms gets to 45 KPC.
So the ratio in the Milky way (with a ring) is 1:15.

If you look at the Milky Way image in your first link of the prior post, the bar ratio seems to be about 1:5 or 1:6, and a similar ratio for that 'perfect spiral' galaxy you link.
So the ratio is higher, but not a lot higher.

Do you agree that without a ring (or with minimal ring) there is a maximal load (or maximal spiral arm size/radius) that spiral galaxy can take?

I don't know what 'load' is.  If you're treating the arms or ring as solid objects, then the question is nonsensical.
OK, you say it is a maximal galactic radius, but that is a function of how much mass and angular momentum the galaxy has.  Mass holds it together, and angular momentum flattens and spreads it out.  These numbers are different for different galaxies.  The symmetry of the galaxy has a lot to do with its recent history.  That perfect spiral galaxy has not been disturbed in quite some time, where our galaxy has been continuously eating small snack-size neighbors, and is soon to be eaten itself by a larger fish, all of which will at least temporarily destroy that nice bar/ring/ arm picture of both galaxies.

Our scientists only try to explain the orbital velocity in the arm, but they ignore completely the Bar. How could it be that they don't see that the bar is directly connected to the arm?

Probably because it isn't.

3. How could it be that at the ring, the orbital velocity is at its minimal value (190Km/s)?

Perhaps because there is not much additional mass inside the ring to make it go faster.
Maybe the velocity in some places is not tangential.  The graphs do not show velocity, only speed.

[Dave Lev (OP)]

If you look at the Milky Way image in your first link of the prior post, the bar ratio seems to be about 1:5 or 1:6, and a similar ratio for that 'perfect spiral' galaxy you link.
So the ratio is higher, but not a lot higher.

Thanks
So, we agree that with a ring the ratio between the bars radius to the maximal arm radius is higher.
If so, why do we ignore the outcome of this evidence?

Our scientists only try to explain the orbital velocity in the arm, but they ignore completely the Bar. How could it be that they don't see that the bar is directly connected to the arm?

Probably because it isn't.

Why do you claim that it isn't?
Please look again at the following image:
https://en.wikipedia.org/wiki/Spiral_galaxy#/media/File:PIA19341-MilkyWayGalaxy-SpiralArmsData-WISE-20150603.jpg
Please advice if you agree with the following:
1. The starting point of the Soutum-Centaurus arm is located at the same spot where the Bar ends?
 2. Exactly at the other end of the bar we see a symmetrical view. The starting point of Perseus arm is located at that spot.
3. Those two arms seems to be main arms due to their length and width.
4. The ring cross exactly at those  two edges/spots of the bar connections/locations with the those two main arms.
5. Based on this image, it is clear the bar is orbit in clock wise.
6. Near the Soutum - Centaurus arm starting point to the end bar location spot, we also see a starting point of a smaller thinner arm. However, it is located a little bit further from the clock wise orbital direction of the bar.
7. In full symmetrical view, we see another thinner arm which its starting point is located near the other Bar/main arm spot. This arm also starts a little bit further from the clock wise orbital direction of the bar.
8. So do you agree that we see clearly a symmetrical view from both end sides of the Bar?

Don't you think that there is a meaning for what we see?

As you don't like to call this spot as a "connection" point, do you estimate that the bar can move faster/slower than the starting point of the main arms and one day it should be disconnect from the arm?
If so, can you please offer an image of a Barred spiral galaxy with two main arms, while its bar (ends) had been totally disconnected from those two main arms?

[Halc]

So, we agree that with a ring the ratio between the bars radius to the maximal arm radius is higher.
If so, why do we ignore the outcome of this evidence?

Evidence of what?  A general trend?  No.  The cherry-picked dataset of 3 is not evidence of that.  A trend can be found in a survey of a lot of galaxies.
In that small dataset, I see a tightly wound set of arms, and a loosely wound set.  The one is so tight that it almost looks like a ring, just like toilet paper looks like concentric circles even though it isn't.
I suspect that the loose galaxy had less angular velocity.  Perhaps such slowly rotating barred galaxies generally have larger bar-to-galaxy ratios, but only a larger survey would be evidence of that.

5. Based on this image, it is clear the bar is orbit in clock wise.

The bar and arms are not clearly in orbit.  The shape of them seems stable, but the image does not show motion, and the ones that do show motion do not show motion that corresponds with that rotation curve.  Thus I do not see the motion of either bar nor arms qualifying as 'orbital motion'.  Yea, they twirl around, but that's just spinning, not necessarily orbiting.

What I see in the arms is that telltale fallen-dominos signature, where material seems grouped in diagonal clumps that overlap. I can really see it in that 'perfect spiral' galaxy you linked, especially in the outer parts of the arms. It is a familiar image:
https://en.wikipedia.org/wiki/Wake#/media/File:Bodensee_at_Lindau_-_DSC06962.JPG
The duck wake appears completely stationary to a camera that is following the duck, and has that overlapping fallen-dominos signature, not just one clean wave that angles out.  The material of the waves does not follow the wave/duck however.  The water is moving in a completely different direction than is the wake.  The wake appears to be a solid object connected to the duck, but it is not connected at all.
The galaxy appear to work exactly that way.  Apparent motion of the arm waves are just wave action, not actual motion of the stars that make them up.  You cannot deduce the arm motion from the motion of the component stars and vice versa.

8. So do you agree that we see clearly a symmetrical view from both end sides of the Bar?

Almost all galaxies look little different if rotated 180 degrees, so yes, I agree with this one.

Don't you think that there is a meaning for what we see?

You don't identify what meaning you expect me to get from that, but probably not the sort of things you've been pushing like any of it behaving as a solid.  There are simply no forces that could account for that.  A spanner holds its shape because there are very much forces that connect the components.  Those forces are not gravitational.

As you don't like to call this spot as a "connection" point, do you estimate that the bar can move faster/slower than the starting point of the main arms and one day it should be disconnect from the arm?

You would have images of galaxies then where that happens.  No, the [main] arms always start at the bar ends, just like the wake always starts at the duck despite not being connected .

[Dave Lev (OP)]

As you don't like to call this spot as a "connection" point, do you estimate that the bar can move faster/slower than the starting point of the main arms and one day it should be disconnect from the arm?

You would have images of galaxies then where that happens.  No, the [main] arms always start at the bar ends, just like the wake always starts at the duck despite not being connected .

Thanks

So now we agree that the main arms always starts at the bar end!!!
But why is it?
Let's start with the Bar:
What is the Bar?
Based on Wiki:
https://en.wikipedia.org/wiki/Barred_spiral_galaxy
"The creation of the bar is generally thought to be the result of a density wave radiating from the center of the galaxy whose effects reshape the orbits of the inner stars. This effect builds over time to stars orbiting further out, which creates a self-perpetuating bar structure."
As the bar had been set due to "density wave", can we claim that it is an object?
If the ring isn't an object than how could it be that the bar is an object?
If it isn't an object, how can we compare it to duck?

Now, let's look at the spiral arm? What is spiral arm?
https://earthsky.org/space/galaxies-spiral-arm
Astronomers believe that galaxies have spiral arms because galaxies rotate – or spin around a central axis – and because of something called “density waves.”
So, the bar is there due to "density wave", while the spiral arms is there due to the same idea of "density wave".
Hence, how could it be that the bar has a totally different shape and structure from the spiral arm while they both had been created based on the same concept of density wave?
How could it be that the "second density wave" (spiral arm) always starts at the end of the "first density wave" (Bar)?
Why just at 3KPC (exactly at the ring) the "first density wave" (bar) had been changed its shape to "second density wave" spiral arm?
If it is all about "density wave", can we calim that the ring is also there due to density wave?
Why the galaxy needs the Bar or the ring?
Why the spiral arms can't start directly from the Bulge or even directly from the SMBH?

[Halc]

As the bar had been set due to "density wave", can we claim that it is an object?
If the ring isn't an object than how could it be that the bar is an object?

Waves are not objects.  A density wave moves through water at about 1500 m/sec despite no bit of water moving at that speed.  Waves don't have mass of their own, and thus are not subject to forces like objects are.  A wave traveling from the depths to the surface for instance is not slowed by gravity at all.  It is a mistake to treat waves as objects like the spanner where the whole spanner reacts if you exert force at only one point of it.

If it isn't an object, how can we compare it to duck?

I was comparing everything to the wake, not the duck.

I don't know the dynamics of stellar motion within the bar.  I've seen a few pictures of paths of a single star in simulations, and the stars seem to stay within the bar, but move up and down the length of it. That wouldn't happen if it was a solid object.  But still, the star stayed in the bar (while stars in the arms seem not to stay in their arms), so in that way the bar does resemble a sort of liquid object like a blob of water floating in the space station.  The water holds itself together despite minor disturbances, but it also doesn't form a bar shape since the forces are different than the gravitational ones acting on the galactic bar.

Translation, the bar may well be an 'object' of sorts, a bit like the duck, but a liquid one at best, not a solid one. I'm willing to entertain that so long as you don't treat it like a solid spanner. The bar doesn't look like a wave, even if it formed due to density waves from the center.  The arms on the other hand look very much like density waves, like the wake of the duck.

Astronomers believe that galaxies have spiral arms because galaxies rotate – or spin around a central axis – and because of something called “density waves.”

Look up 'density wave theory' on wiki.  There are some really good illustrations that show what they're talking about.
Here's a gif showing how a wave forms moving at one speed when all the material moves at a different speed.
https://imgur.com/gallery/dtb8WrD
Follow any particular star and it doesn't stay with either arm.

So, the bar is there due to "density wave", while the spiral arms is there due to the same idea of "density wave".
Hence, how could it be that the bar has a totally different shape and structure from the spiral arm while they both had been created based on the same concept of density wave?

I don't understand the dynamics of the bar.  It seems to not be a density wave itself like the arms are, but I could be wrong about that. They give a reference to a paper on the subject, but it is heavy reading. The site says it is formed by density waves from the center, reshaping orbits, and that effect apparently grows. I know that a lot of the 'orbits' is not stars at all, but just dense gas clouds being reshaped.  A lot of stars are born in there from that gas.
I suspect the galaxies with long bars have been undisturbed for a long time, and those with nonexistent bars are in the process of merging, which disrupts all the symmetry.  If this is true, then we should see galaxies that have recently 'eaten' and are forming new bars and reestablishing the symmetry lost during the merger.

[Dave Lev (OP)]

I don't understand the dynamics of the bar.  It seems to not be a density wave itself like the arms are, but I could be wrong about that. They give a reference to a paper on the subject, but it is heavy reading.

I assume that even the scientist that wrote the article has no clue how the density wave creates the Bar.
If it was clear to him, he could explain it in few words. There is no need for "heavy reading" to explain something which is quite clear.
Please also be aware that he doesn't even try to explain how the ring had been formed due to density wave.

There are some really good illustrations that show what they're talking about.
Here's a gif showing how a wave forms moving at one speed when all the material moves at a different speed.
https://imgur.com/gallery/dtb8WrD
Follow any particular star and it doesn't stay with either arm.

I wonder how can we get any valid information from this illustration.
There is no ring and no bar in the illustration.
We only see stars that are moving in and out in order to set the spiral arms.
If that was correct, than by definition the 400 Billion stars that move in and out should collide with each other..
How many collisions do we see in the galaxy?
In any case, in this illustration there is no ring no bar no bulge - only spiral arms. So how can we use it as a valid illustration?
Spiral galaxy is not just spiral arms. It's time for our scientists to walk up and see what there is in spiral galaxy.
There is no way that density wave can set all of those different shapes - Bar, ring and spiral arms.

Let's look at your example about the duck wake:

:
https://en.wikipedia.org/wiki/Wake#/media/File:Bodensee_at_Lindau_-_DSC06962.JPG
The duck wake appears completely stationary to a camera that is following the duck, and has that overlapping fallen-dominos signature, not just one clean wave that angles out.  The material of the waves does not follow the wave/duck however.  The water is moving in a completely different direction than is the wake.  The wake appears to be a solid object connected to the duck, but it is not connected at all.

We see the wake - But it has a specific shape.
For example - There is no way to get a wake in a ring shape, zig zag, or that moves in front of the duck.
So, if there is a density wave - it can't technically set all the variety shapes that we need (Bar, Ring, spiral arms, Bulge) and keep each shape at a specific radius and aria while the bar ends exactly at the starting point of the main spiral arms and this point cross the ring.
This is too challenging request from a simple idea of - "Density wave"
Therefore, I would assume that there is a severe mistake with that idea