Stephan’s Quintet

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Stephan’s Quintet is a group consisting of four interacting galaxies – NGC 7317, NGC 7318A, NGC 7318B and NGC 7319 – and a visual companion, the spiral galaxy NGC 7320. The galaxies are located in Pegasus constellation.

The four interacting galaxies are members of the first compact galaxy group ever discovered, the Hickson Compact Group 92. Compact groups are typically groups of about five galaxies in close proximity and isolated from other galaxies.

colliding galaxies,compact galaxy group

Stephan’s Quintet, a compact group of galaxies discovered about 130 years ago and located about 280 million light years from Earth, provides a rare opportunity to observe a galaxy group in the process of evolving from an X-ray faint system dominated by spiral galaxies to a more developed system dominated by elliptical galaxies and bright X-ray emission. Being able to witness the dramatic effect of collisions in causing this evolution is important for increasing our understanding of the origins of the hot, X-ray bright halos of gas in groups of galaxies. Image: NASA

The interacting galaxies in Stephan’s Quintet lie at an approximate distance of 300 million light years and will eventually merge into a single large elliptical galaxy. The member of Stephan’s Quintet that is not physically associated with the compact group, NGC 7320, is considerably closer to us, at a distance of 39 million light years.

The galaxies in the middle of the group, NGC 7318A and NGC 7318B (mag. 14.4 and 13.9), have already started to merge with each other. The collision has drawn out long tails of stars from each galaxy and triggered massive bursts of star forming activity.

NGC 7318B is falling into the centre of the group at a speed of several millions of kilometres per hour, causing an enormous intergalactic shock wave, one bigger than our galaxy. This was first detected in radio observations in the 1970s, when a filament of emission between the galaxies was discovered, but its nature was uncertain at the time. In visible light, the same region shows a green arc, a faint glow of ionized atomic hydrogen.

Stephan’s Quintet, image: Adam Block/Mount Lemmon SkyCenter/University of Arizona

Infrared observations with the NASA Spitzer Space Telescope have revealed a strong molecular hydrogen emission resulting from the shock wave. It is one of the most powerful formations of molecular hydrogen ever detected. The strongest emission emanates from an area close to the centre of the green arc seen in visible light images.

Stephan’s Quintet, image: NASA, J. English (U. Manitoba), S. Hunsberger (PSU), Z. Levay (STScI), S. Gallagher, and J. Charlton (PSU)

All five members of Stephan’s Quartet are redshifted, i.e. they are moving away from us. However, NGC 7320 has a considerably smaller redshift – 790 km/s – than the other four galaxies, which are moving away at almost 6,600 km/s.


NGC 7317 has a slightly oval disk and is classified as an elliptical galaxy. It has an apparent magnitude of 14.57 and an apparent size of 0.4’ by 0.4’. The galaxy spans less than 0.5 arc minutes across.

NGC 7317, image: NASA, ESA, and the Hubble SM4 ERO Team

NGC 7318 was believed to be a single galaxy at the time of discovery, but is now known to consist of two separate interacting objects, NGC 7318A and NGC 7318B. NGC 7318A is an elliptical galaxy with an apparent magnitude of 14.4 and an apparent size of 0.9’ by 0.9’. NGC 7318B is a barred spiral galaxy occupying an area 1.9’ by 1.2’ in size. It has an apparent magnitude of 13.9.

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NGC 7318A (bottom) and NGC 7318B (top), image: NASA, ESA, and the Hubble SM4 ERO Team

NGC 7319 is a barred spiral galaxy distorted by the interaction with other galaxies in the group. It has an apparent magnitude of 14.4 and occupies 1.7’ by 1.3’ of the apparent sky. The galaxy has a type 2 Seyfert nucleus and the characteristic bright core.

NGC 7319, image: NASA, ESA, and the Hubble SM4 ERO Team

NGC 7320, the foreground galaxy that is not part of the compact galaxy group, is a barred spiral galaxy with an apparent magnitude of 13.2. It has an apparent size of 2.2 by 1.2’. The galaxy contains vast H II regions where new stars are being formed. The galaxy’s redshift is similar to that of the nearby NGC 7331, which indicates that the two galaxies may be gravitationally associated.

NGC 7320, image: NASA, ESA, and the Hubble SM4 ERO Team

A sixth galaxy, NGC 7320c, can be been a little further away from the visual grouping. It is not a member of Stephan’s Quintet, but likely belongs to the Hickson Compact Group based on its redshift. The galaxy also exhibits a tidal tail that appears to connect it with NGC 7319. NGC 7320c has an apparent magnitude of 16.7 and occupies an area of 0.7’ by 0.6’ in size.

Stephan’s Quintet was named after Édouard Stephan, French astronomer and director of the Marseille Observatory, who discovered the visual grouping in 1877.

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A clash among members of a famous galaxy quintet reveals an assortment of stars across a wide color range, from young, blue stars to aging, red stars. This portrait of Stephan’s Quintet, also known as Hickson Compact Group 92, was taken by the new Wide Field Camera 3 (WFC3) aboard NASA’s Hubble Space Telescope. Stephan’s Quintet, as the name implies, is a group of five galaxies. The name, however, is a bit of a misnomer. Studies have shown that group member NGC 7320, at upper left, is actually a foreground galaxy about seven times closer to Earth than the rest of the group. 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. This drama is being played out against a rich backdrop of faraway galaxies. The image, taken in visible and near-infrared light, showcases WFC3’s broad wavelength range. The colors trace the ages of the stellar populations, showing that star birth occurred at different epochs, stretching over hundreds of millions of years. The camera’s infrared vision also peers through curtains of dust to see groupings of stars that cannot be seen in visible light. NGC 7319, at top right, is a barred spiral with distinct spiral arms that follow nearly 180 degrees back to the bar. The blue specks in the spiral arm at the top of NGC 7319 and the red dots just above and to the right of the core are clusters of many thousands of stars. Most of the quintet is too far away even for Hubble to resolve individual 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. Far away from the galaxies, at right, is a patch of intergalactic space where many star clusters are forming. NGC 7317, at bottom left, is a normal-looking elliptical galaxy that is less affected by the interactions. Sharply contrasting with these galaxies is the dwarf galaxy NGC 7320 at upper left. Bursts of star formation are occurring in the galaxy’s disk, as seen by the blue and pink dots. In this galaxy, Hubble can resolve individual stars, evidence that NGC 7320 is closer to Earth. NGC 7320 is 40 million light-years from Earth. The other members of the quintet reside 290 million light-years away in the constellation Pegasus. 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. Image: NASA, ESA, and the Hubble SM4 ERO Team