EvolutionWithin a billion years of a galaxy's formation, key structures begin to appear. Globular clusters, the central supermassive black hole, and a galactic bulgeof metal-poor Population II starsform. The creation of a supermassive black hole appears to play a key role in actively regulating the growth of galaxies by limiting the total amount of additional matter added.During this early epoch, galaxies undergo a major burst of star formation.During the following two billion years, the accumulated matter settles into a galactic disc.A galaxy will continue to absorb infalling material from high-velocity cloudsand dwarf galaxiesthroughout its life.This matter is mostly hydrogen and helium. The cycle of stellar birth and death slowly increases the abundance of heavy elements, eventually allowing the formationof planets.The evolution of galaxies can be significantly affected by interactions and collisions. Mergers of
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