In 1877, French astronomer Édouard Stephan turned a telescope to a spot in the constellation Pegasus and discovered this cozy collection of five large galaxies. Stephan's Quintet, as the group is now known, includes four distant galaxies that are connected to each other through gravity and one galaxy that is much closer to us but just happens to lie in the same direction in the sky. The imposter is easy to pick out in this Hubble image, because it looks different than the other galaxies. The bluer galaxy, called NGC 7320, is only about 40 million light-years from Earth, while the other, redder galaxies are about 290 million light-years away.
The four allied galaxies are involved in a gravitational relationship that's transforming some of their shapes and sparking new waves of star birth both inside and outside of the galaxies. In the twisted galaxy at top right, called NGC 7319, each blue fleck in the upper spiral arm and red blotch to the right of the galaxy's core is a cluster of many thousands of stars. Near the center of the image, two galaxies, NGC 7318A and NGC 7318B, are so close together they at first seem to be one. Wrapped around the pair are sparkling garlands of bright blue star clusters and pinkish gas clouds where new stars and star clusters are developing. The smooth, sedate galaxy in the lower left, NGC 7317, seems strangely unaffected by the gravitational commotion happening nearby but claims some orange-colored star clusters of its own.
Hubble was the first telescope to distinguish individual star clusters in the galaxies of Stephan's Quintet. Astronomers used Hubble, first in the late 1990s with the Wide Field Planetary Camera 2 and then again in 2009 with the Wide Field Camera 3, to examine those star clusters. Both studies used Hubble to analyze the clusters' colors, which indicate the clusters' ages. As a star cluster matures, its hot, blue stars die off, leaving the longer-lived red stars behind. So, the redder a cluster, the older it is.
The most recent Hubble study of Stephan's Quintet, led by Konstantin Fedotov of the University of Western Ontario, identified hundreds of star clusters with varying ages, some within the galaxies themselves and others in long streamers of material stretching away from the galaxies. Because gravitational encounters between galaxies can trigger star formation, Fedotov's team was able to use the ages of the star clusters to reconstruct a history of the galaxies' interactions.
Distance: 290 million light-years (90 million parsecs)
Instrument: Wide Field Camera 3/IR and /UVIS
Image Filters: WFC3/IR: F140W (J-H); WFC3/UVIS: F438W (B), F606W (V), F657N (H-alpha + [N II]), F665N (H-alpha + [N II]), F814W (I)