Kitt Peak image of Arp 78, also known as NGC 772.
One of NGC 772’s spiral arms is particularly prominent. This is likely due to gravitational interactions with its companion galaxy NGC 770, the small elliptical galaxy in the center near the top of the frame. The interactions left NGC 772's bottom arm elongated and asymmetrical.
Credit: KPNO, NOIRLab, NSF, AURA, A. Block
Source: 
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NGC 772
NGC 772