NGC 7217 is a gas-poor system[2] whose main features are the presence of several rings of stars concentric to its nucleus: three main ones –the outermost one being of the most prominent and the one that features most of the gas and star formation of this galaxy –[2] plus several others inside the innermost one discovered with the help of the Hubble Space Telescope; a feature that suggests NGC 7217's central regions have suffered several starbursts.[3] There is also a very large and massive spheroid that extends beyond its disk.[4]
Other noteworthy features this galaxy has are the presence of a number of stars rotating in the opposite direction around the galaxy's center to most of them[5] and two distinct stellar populations: one of intermediate age on its innermost regions and a younger, metal-poor version on its outermost ones.[6]
It has been suggested these features were caused by a merger with another galaxy[7] and, in fact, computer simulations show that NGC 7217 could have been a large lenticular galaxy that merged with one or two smaller gas-rich ones of late Hubble type becoming the spiral galaxy we see today;[6] however right now this galaxy is isolated in space, with no nearby major companions.[6] More recent research presents a somewhat different scenario in which NGC 7217's massive bulge and halo would have been formed in a merger and the disk formed later (and is still growing) either accreting gas from the intergalactic medium or smaller gas-rich galaxies, or most likely from a previously existing reserve.[8]
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Verdes-Montenegro, L.; Bosma, A. (1995). "The three rings of the isolated galaxy NGC 7217". Astronomy and Astrophysics. 300: 65. Bibcode:1995A&A...300...65V.
