Image Credit: NASA, ESA, and the Hubble SM4 ERO Team
Stephan’s Quintet (also known as Hickson Compact Group 92 and Arp 319) is a group of five galaxies (NGC 7317, 7318A, 7318B, 7319 and 7320) located in the constellation Pegasus. The name, however, is a bit of a misnomer because NGC 7320 (the blue one) is actually a foreground galaxy, located about 39 million light-years away, while the other members of the quintet reside 210-340 million light-years away.
The four yellowish galaxies are gravitationally interacting with each other and will eventually merge into a single big galaxy. Three of the galaxies have distorted shapes, elongated spiral arms, and long, gaseous tidal tails containing myriad star clusters, proof of their close encounters. These interactions have sparked a frenzy of star birth in the central pair of galaxies.
NGC 7320, the blue one at top right, is a dwarf galaxy, that is shown to have extensive H II regions, identified as red and blue dots, where bursts of star formation are occurring.
NGC 7319, at bottom right, is a barred spiral with a type 2 Seyfert nucleus and distinct spiral arms that follow nearly 180 degrees back to the bar. The blue specks in the spiral arm and the red dots are clusters of many thousands of stars.
Continuing clockwise, the next galaxy appears to have two cores, but it is actually two galaxies, NGC 7318A and NGC 7318B. Encircling the galaxies are young, bright blue star clusters and pinkish clouds of glowing hydrogen where infant stars are being born. These stars are less than 10 million years old and have not yet blown away their natal cloud.
NGC 7317, at top left, is a normal-looking elliptical galaxy that is less affected by the interactions.
These farther members are markedly redder than the foreground galaxy, suggesting that older stars reside in their cores. The stars’ light also may be further reddened by dust stirred up in the encounters.
Spied by Edouard M. Stephan in 1877, Stephan’s Quintet is the first compact group ever discovered.