NASA's Astronomy Picture of the Day for September 30 features Arp 78, a peculiar spiral galaxy in the constellation Aries, imaged by astrophotographer Robert Eder.

Also known as NGC 772, the galaxy lies about 100 million light-years away and stretches an enormous 200,000 light-years across. It sports a single overdeveloped outer spiral arm, lined with young blue star clusters and sweeping dust lanes.

Astronomers believe the arm was pumped up by galactic-scale gravitational tides. Interactions with its brightest companion, the compact elliptical galaxy NGC 770 seen just below it, are likely responsible for the distortion.

The deep exposure also reveals faint star streams and spiky foreground Milky Way stars. Arp 78 is a classic example of how galaxies shape each other through gravity across cosmic time.

The image was captured with a large amateur telescope over many hours of exposure. APOD, which NASA has published daily since 1995, reaches millions of space fans around the world. Eder said he chose Arp 78 for its dramatic single arm, a feature that makes it stand out among spiral galaxies.