Despite years of research, there is still remarkably little known about the processes of galaxy formation and why there is such variety in the structure of different galaxies. Most astronomers suggest that small density fluctuations in the primordial matter led to the formation of many small "pregalactic masses" that were similar to present-day dwarf galaxies, and that collisions and mergers of these galaxies led to the formation of the larger galaxies common in the pres ent universe. As the universe expanded, these merged galaxies grew into the large-scale clusters and voids that now occupy the universe. Much of the evidence for such a history of galaxy formation comes from observations of the most distant objects in the universe, whose light and other electromagnetic radiation that reaches Earth now was generated billions of years ago when the universe was young. It seems that the further back in time one observes, the smaller and less organized individual galaxies appear. Furthermore, there are many examples of galaxies merging, and many stages are observed of irregular galaxies merging to form more complex systems.
Different ideas and models attempt to explain why some galaxies are elliptical, some spiral, and some irregular. This variation may relate to the timing of when the stars in the galaxy formed compared to the galactic formation—if the stars formed early in the galactic evolution, an elliptical galaxy would most likely form, since the gas would be used up, and no central disk would form. In these elliptical galaxies star formation would occur early, and their present state would be dominated by systems with relatively old and cold stars. In contrast, if a lot of gas remained in the galaxy after it formed, then gravity would make this gas tend to collapse into a rotating disk, forming spiral galaxies, with stars able to form throughout the history of these types of galaxies.
The reason why some galaxies may have early star formation and form ellipticals, whereas others have late star formation and form spirals, is still not clearly known. But it is known that spiral galaxies are comparatively rare in parts of the universe that have high galactic density, perhaps because collisions between galaxies are more common in areas of high galactic density, and collisions tend to destroy the spiral arm structures of these galaxies. Observations and computer models suggest that collisions between galaxies tend to leave new galaxies that have elliptical characteristics, and these collisions eject large amounts of gas into intergalactic space. Also observations of deep space show that ellipticals were more common earlier in the universe, and are becoming less common with time, suggesting that collisions may be destroying their spiral structure with time.
Although many galaxies formed early in the history of the universe, many are still forming or being extensively modified through collisions and other interactions that are ongoing in the present-day universe. As galaxies interact, their halos of dark matter first interact and may be transferred from one (relatively smaller) galaxy to another, then the galaxies may spin in toward each other and merge, typically with the larger galaxy absorbing (or cannibalizing) the smaller. other computer models of interactions between galaxies show that it is possible for galaxies to come close but not merge, and one possible outcome of these types of interactions is the formation of spiral arms in one galaxy, where none existed before. other interactions between galaxies produce sudden bursts of new star formation in the affected galaxies, showing that galaxy and stellar formation is an ongoing process in the universe.
See also astronomy; astrophysics; cosmology; dark matter; galaxy clusters; Hubble, Edwin; Kepler, Johannes.
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