One Spiral Arm, Puffed Up by a Neighbouring Galaxy

Arp 78 is a spiral galaxy about 100 million light-years away and roughly 200,000 light-years across, according to NASA's Astronomy Picture of the Day entry for September 30. What makes it peculiar is a single overdeveloped outer spiral arm, traced by sweeping dust lanes and lined with young blue star clusters, that appears pumped up by galactic-scale gravitational tides rather than by anything internal to the galaxy itself. APOD attributes the effect, likely, to interactions with Arp 78's brightest companion — the more compact elliptical NGC 770, which sits just below the larger spiral in the image. The pairing is a neat illustration of how much of a galaxy's shape can be borrowed from its neighbours: the same gravitational tugs that can strip a small galaxy can also yank a big one's arm out of symmetry.