SUMMARY: This photograph, taken by the Spitzer space telescope and a ground-based telescope in Spain, shows the Stephan's Quintet galaxy cluster, with one of the largest shockwaves ever seen in the Universe. The green arc in the photograph is the point which two galaxies are colliding. There are actually 5 galaxies in this photograph, but two have been so beaten up, all that's left are their bright centers. The galaxies are located 300 million light-years away in the Pegasus constellation.

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Stephan's Quintet is an interesting object. I have enjoyed seeing new observations about this collision over the decades. This Spitzer view of the shockwave is a nice addition.

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How far can a shockwave of this magnitude travel? Can it be a threat to us?

Hi Antxmage, welcome to the BAUT forum.

This shock wave will never reach us, but even if the Sun were right in the middle of it, we would be unaffected. The bigger impact is on stars and planets that haven't formed yet.

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I'm having trouble interpreting this picture. The article says 5 galaxies are to be seen. On the right hand side of the green arc I see what look likes two nuclei in one galaxy(I would assume these are the two that are "so beaten up"). On the left hand side I see two large galaxies, one at the top and one at the bottem. Is it the one at the top that is involved in the collision?

Obviously I've got this picture all wrong. Any help?

Stephan's Quintet was seen as five galaxies in the ninetheenth century by Stephan, but in the modern view it is one very removed galaxy superimposed on three other galaxies. The three are interacting, and a debris cloud from the interaction has become a star forming region, and looks like a fifth galaxy.

Do a Google search to easily find more information about this famous grouping. I thought the Sky and Telescope article from about a year ago was really clear.

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The two nuclei you see belong to two galaxies, NGC7318A on the right, and NGC7318B on the left. The elliptical galaxy at lower right is NGC7317. The deformed spiral galaxy on the left has a tidal tail of stars that extend off to the left, joining a deep red clump at the left edge. That one is NGC7319. Those four galaxies are an interacting group. NGC7318B is colliding with the other galaxies, moving approximately towards us as seen in this picture. That collision is responsible for generating the shockwave that shows up in the image as green. The pink oval galaxy which extends towards the lower left corner is NGC7320, and it is the foreground object. The 4 cluster galaxies are about 270,000,000 light years away, whereas NGC7320 is only about 35,000,000 light years away.

The tidal tail that NGC7319 displays was created in a collison with NGC7320C, which is out of the picture, off to the left. NGC7320C collided with NGC7319 "a few hundred million years ago".

The shockwave is not news, it was seen clearly by Chandra several years ago. It is the presence of so much molecular hydrogen that is news, because H₂ does not normally form in a gaseous reaction, but requires a surface (dust grains, for instance) on which to form. It appears that the rapid cooling behind the shock allows for gas phase formation of H₂, and that is scientific news.

• Spitzer image of Stephan's Quintet
• Labeled image of Stephan's Quintet
• HST image of Stephan's Quintet
• Chandra image of Stephan's Quintet
• Jean Marie Edouard Stephan (1837-1923), discovered Stephan's Quintet in 1876 or 1877, depending on the source.
• Foucault 80-cm (31.5-inch) reflecting telescope, used by Stephan to make the discovery. Foucault invented the knife-edge test commonly used by amateurs today, while making this mirror (see the telescope notes page).

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