Various schemes have been devised to categorize galaxies (see galaxy classification).
Most galaxy formation is thought to have taken place in the early universe, beginning less than half a billion years after the Big Bang.
Galaxies may be solitary, or in small groups like our Local Group, or in larger clusters of galaxies. In rich clusters of galaxies, the brightest systems tend to be ellipticals and lenticulars, with spirals making up only 5 to 10% of the population. However, the proportion of spirals in these clusters was probably higher in the past, galaxy cannibalism and galaxy mergers having turned them into the more amorphous types. In low-density environments, spirals account for about 80% of the bright galaxy count.
Some galaxies display unusual activity in their cores associated with active galactic nuclei.
Related category GALAXIES
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