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==== Shell galaxy ==== [[File:NGC 3923 Elliptical Shell Galaxy.jpg|thumb|[[NGC 3923]] Elliptical Shell Galaxy (Hubble photograph)]] A shell galaxy is a type of elliptical galaxy where the stars in its halo are arranged in concentric shells. About one-tenth of elliptical galaxies have a shell-like structure, which has never been observed in spiral galaxies. These structures are thought to develop when a larger galaxy absorbs a smaller companion galaxy—that as the two galaxy centers approach, they start to oscillate around a center point, and the oscillation creates gravitational ripples forming the shells of stars, similar to ripples spreading on water. For example, galaxy [[NGC 3923]] has over 20 shells.<ref>{{cite web|title = Galactic onion|url = http://www.spacetelescope.org/images/potw1519a/|publisher = [[European Space Agency]]|access-date = 2015-05-11|archive-date = August 6, 2020|archive-url = https://web.archive.org/web/20200806221639/https://www.spacetelescope.org/images/potw1519a/|url-status = live}}</ref>
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