NGC 7318A/NGC 7318B (Stephan's Quintet)

NGC 7318A and NGC 7318B are two interacting galaxies that are part of Stephan's Quintet, a famous group of five galaxies located about 280 million light-years away in the constellation Pegasus. Stephan's Quintet is one of the most well-known compact galaxy groups and provides valuable insights into galaxy interactions and evolution.

NGC 7318A is an elliptical galaxy, while NGC 7318B is a barred spiral galaxy. These two galaxies are currently in the process of merging, creating a dramatic scene of gravitational interactions. The collision between NGC 7318A and NGC 7318B has produced a large intergalactic shock wave, which is visible as an arc of heated gas. This shock wave is caused by NGC 7318B colliding with the group at a speed of over 900 km/s, heating the hydrogen atoms in the cluster's gas and producing a green glow.

The interaction between these galaxies has also triggered massive bursts of star formation, drawing out long tails of stars from each galaxy. The molecular hydrogen visible in the collision is one of the most turbulent forms known, providing a glimpse into the processes that occurred in the early universe.

Stephan's Quintet was discovered by French astronomer Édouard Stephan in 1877 and has been a subject of extensive study ever since. The group is cataloged as Arp 319 in Halton Arp's Atlas of Peculiar Galaxies and is also known as Hickson Compact Group 92 (HCG 92).