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How is a galaxy formed? I know that the center of our galaxy is considered to be Sagittarius A* and it's surrounded by a lot of stars (also a lot of neutron stars).

But what makes this happen? I mean, why are all those stars in that right formation, like other galaxies, rather than just being random clouds of stars in the universe?

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Well this is not a question that can be answered in a few sentences!

In short


We still do not know, how exactly the galaxies formed and have the shapes they possess. The large-scale structure (LSS) in the Universe as we see them today are a consequence of tiny primordial density fluctuations that arose right after the Big Bang. The reason for these tiny density fluctuations is believed to be quantum in nature.

So, these tiny fluctuations in the early past lead to agglomeration of gas and dust clouds, leading to certain areas becoming denser. These denser areas slowed down the expansion of the Universe, allowing the gas to accumalate into small protogalactic clouds. Gravity in these clouds casued the gas and dust to collapse, and in turn form stars. These stars burned quickly and became globular clusters (while gravity was still collapsing the dust and gas).

Also, according to the $\Lambda$CDM model, the structures form in a "bottom-up" fashion, i.e. small structures forming first (stars and galaxies) followed by large structures (galaxy clusters). This is exactly what we are observing today thanks to surveys that are probing high redshift ranges.

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You are closer to the answer than you think with this question. That revolves around the connection between the SMBH central object and the mass of the Galaxy. For instance; all stars rotate around this thing, the Galactic Plane is about its 'equator'(if it ever associates to jetting - thats at right angles to the Galactic plane), a connection in mass and also the density of stars increases, across the galaxy, up to this object. That connection is not really explained by merger, which is unlikely to result in stars orbiting v close to the centre. Matter is 'burping' or spewing from this object in outflow that forms stars locally and that is their origin. Just like in Dwarf Galaxies and Globular Clusters, which seem to have a correlation in general characteristics and morphology, show signs of star forming in the centre with, sometimes, large numbers of young stars in the middle.

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