Arp 78 is a galaxy being remodeled by its neighbor, and this image is the architectural rendering. Located about 100 million light-years away in the constellation Aries, the spiral — also catalogued as NGC 772 — spans roughly 200,000 light-years, making it about twice the diameter of our Milky Way. Its defining feature is an overdeveloped outer spiral arm, pumped up not by internal dynamics but by gravitational tides from the compact elliptical galaxy NGC 770 sitting just below it. Robert Eder's portrait earns its keep through depth. The faint star streams surrounding NGC 770 are the forensic evidence of the interaction — material pulled and stretched by gravitational forces over hundreds of millions of years. These streams are notoriously hard to capture, requiring long exposures and careful processing to pull signal from noise. Their presence here transforms the image from pretty picture to scientific document. The contrast between the two galaxies is the real subject. NGC 772 is all structure — sweeping dust lanes, blue star clusters tracing the spiral arm like lights on a runway. NGC 770 is a fuzzy elliptical blob, structurally simple but gravitationally consequential. The small galaxy is literally reshaping the large one, a reminder that in galactic dynamics, mass matters more than size. Arp 78 earns its 'peculiar' designation from Halton Arp's 1966 Atlas of Peculiar Galaxies, a catalog that was itself a deliberate provocation. Arp compiled galaxies that didn't fit the neat Hubble classification scheme, arguing that their oddities pointed to physical processes mainstream astronomy was ignoring. The atlas was controversial in its time — Arp's later claims about quasar redshifts were rejected — but the catalog endures as one of the great observational resources in extragalactic astronomy. The young blue star clusters lining the overdeveloped arm are a direct consequence of the tidal interaction. Gravitational compression of gas and dust triggers new star formation, so the arm is not just stretched — it's lit up. This is galaxy-scale cause and effect, visible across 100 million light-years. The spiky foreground stars are Milky Way objects, thousands to hundreds of light-years away, photobombing a scene 100 million light-years deep. The diffraction spikes come from the telescope's support struts, a mechanical artifact that serves as an accidental depth cue — everything with spikes is local, everything without is cosmically distant. APOD continues to function as one of the most durable public science communication projects in existence, running daily since June 1995. The editorial team — Jerry Bonnell, Cecilia Chirenti, Robert Nemiroff, and Keighley Rockcliffe — curates a mix of professional observatory images and amateur deep-sky work like Eder's, maintaining a quality bar that has held for over three decades.
