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In the simulation all the positions of the stars are calculated as a function of the previous positions of all the stars and when that is done the display is updated. That means when you observe the display at each update you get an instantaneous diplay of the present position of all the stars.
In reality this is not possible. Suppose you are hanging in a helicopter straight above the center (Black Hole) of the galaxy. The most current view at that position is the center of the galaxy. All the other stars you observe in the past. This delay is a function of distance and the speed of light.
The left picture shows (almost) the initial position of all the stars. The structure is like the spokes of a wheel in straight lines. In each simulation that means there are 30 spokes.
In reality when you are hovering above the center when you start the simulation what you see is first the star of the most inner ring. This is "star 1". Next the star in ring two. This is "star 2" etc etc and finally star 30. All these 30 stars become visible in a straight line. However when "star 30" becomes visible "star 1" is not any more at its initial position but has moved forward, counter clockwise.
The reason why this exercise is important is because when you consider to perform a simulation based on real situation i.e. actual stars. Suppose you are capable to measure instantaneous the position of all the stars. In that case, even if you can measure all the positions simultaneous, what you have measured are not the positions of all the stars at the same moment but at different moments in the past. This time difference in the past is a function of the distance and the speed of light. To calculate the positions at the same moment you need at least the speed of all the stars.
The same problem arises when you want to test at the end of the simulation the predicted results in the future with actual observations: again you have to take the time delays into account.
The important lesson is here that only when actual observations are involved, that means at the beginning to calculate the initial positions and at the end to test the results, the speed of light is important, not during the actual simulation i.e. calculation. The simulation is a mathematical model that describe the processes involved, in this case the movements of the stars in a galaxy. These processes are almost completely independent of the speed of light, photons neutrinos and single atomic particles.
(*) When you observe the simulations sooner or later chaos starts and all structures disappear. The reason does not lie in the physical realm, but is purely caused by computer limitations i.e. accuracy.
As mentions above the simulation only involves 600 stars. In fact each of these stars is a collection of thousands of stars. By combining the stars you make the simulation possible. In Picture B you see an image of a spiral arm. In reality when you perform a simulation with more stars you will not get spiral arms, but a structure which is only rotational symmetric like an elliptical galaxy. To observe spiral arms you have to inject huge "gas clouds" which will move radial towards the center of the galaxy.
A similar problem exists when you study the spiral arms in a spiral galaxy. Spiral arms are parts of a galaxy disc which contains more visible mass (stars) than in the space in between. That means it does not make sense to proclaim that there is more baryonic matter in this "in between" space than in the arms. The issue is how do you explain this apparent discrepancy, the fact that the supposed involved dark matter is not every where available in the same amounts. With explain meaning the processes involved i.e. galaxy evolution. A much more logical assumption is that throughout the disc there is baryonic dust in order to explain the missing matter issue.
When you are in a train at rest at a platform you can calculate the speed of an approaching train by performing different observations. For an observer at rest not only the platform, but also the horizon and the earth are at rest.
From the point of view of an observer on the moving train, the moving train is at rest. For this observer the train at the platform is moving, so is the platform, so is the horizon and so is the earth.
Not so long ago the Earth was considered the center of the universe. Than along came Nicolaus Copernicus who unveiled that this is not correct, but that the Sun is at the center of the solar system. The result is that the Sun is more or less at rest and the planets are moving. For the Earth Moon system a very similar solution exists: The Earth is at rest and the Moon is moving.
The same for any subject on the surface of the Earth: The Earth is at rest and all the objects (including humans) are considered moving.
For a galaxy the same logic applies: The center of the Milky Way galaxy is at rest and all the stars are moving.
But when more galaxies are involved also that is true.
This type of reasoning should be considered when you want to simulate a galaxy
A rather similar question is: Suppose you have two galaxies (at large distance) which move around/towards each other. In each galaxy there is a supernova. The tricky part is you can only have one answer which should equally apply to both. The points where the supernovae happen are P and Q. For example: it can not be that point P is at the center of his circle but point Q not. They should be either stay both at the center or both not.
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