There are many kinds of structures in the universe. There are, for example, neutron stars, white dwarfs, main sequence stars like the sun, planets, molecular clouds, globular clusters, elliptical galaxies, spiral galaxies, clusters of galaxies, super clusters of galaxies and so on. The size of these structures ranges from 105 cm(neutron stars) to 1026 cm (super clusters) while the density varies from 1015 g/cm 3 to 10−29 g/cm 3. There is a difference of 44 orders of magnitude in density between neutron stars and superclusters of galaxies. Although there are four kinds of fundamental interactions in our universe which are responsible for the formation of these structures, gravity is the most important because the range of the force is long compared with the strong and weak interactions and because there is no shielding distance in contrast to the electromagnetic interaction. These two characteristic of gravity are by far the most important for the formation of the various structures in the universe.
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