A heavily-stretched contrast image shows galaxies galore, including NGC 7331 (upper center) and Stephan’s Quintet (below center right). Credit: CFHT, Pierre-Alain Duc (Observatoire de Strasbourg) & Jean-Charles Cuillandre (CEA Saclay/Obs. de Paris).

A quintet of galaxies that's actually a quartet. Except it's a quintet.

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Jun 14, 2018

The sky is complicated.

When you look up you see a lot of stars, of course, and if you're not familiar with the sky it seems confusing. After some time spent outside at night, though, you pick up the patterns, become more confortable with the stellar placement, and get to know your way around.

That's harder with binoculars or telescopes, because there are simply too many stars up there to know them all. What starts as thousands by eye becomes millions by optical aid.

You might also note there's dust out there in space blocking our view of some stars, plus wispy gas that adds to the visual traffic. And then, on top of that, there are other galaxies, cities of stars like our own Milky Way, removed at great distance so they appear as fuzzy blobs. Only a handful are visible by eye, but binoculars reveal hundreds, and even a small telescope can reveal thousands.

So what do you get if you take a big telescope and point it at the sky, taking a decently long exposure with a very sensitive camera.

Chaos. Or more illustratively, this:

A heavily-stretched contrast image shows galaxies galore, including NGC 7331 (upper center) and Stephan’s Quintet (below center right). Credit: CFHT, Pierre-Alain Duc (Observatoire de Strasbourg) & Jean-Charles Cuillandre (CEA Saclay/Obs. de Paris).

A heavily-stretched contrast image shows galaxies galore, including NGC 7331 (upper center) and Stephan’s Quintet (below center right). Credit: CFHT, Pierre-Alain Duc (Observatoire de Strasbourg) & Jean-Charles Cuillandre (CEA Saclay/Obs. de Paris)

Holy chromatic maelstroms! That image highlights the galaxy NGC 7331 (the large nearly edge-on spiral at upper center). It's about 40 million light years away and roughly the same size as the Milky Way (100,000 light years across). You can also see lots of other galaxies — which is actually what I want to focus on, but hang tight a second — stars, and just a diffuse miasma permeating the field.

Now mind you, this image is displayed using a scale that stretches contrast hugely, so you can see bright objects like stars as well as the very diffuse, faint material. If this were displayed normally, the sky would look fairly black. But the point is, that stuff is there, and it's a mess.

The faint stuff is called galactic cirrus: Feathery clouds of dust (silicate grains and long carbon molecules) between the stars, and very, very faintly illuminated by them. And it's a problem.

In many cases, galaxies near each other in space interact with other gravitationally. In these close encounters (and sometimes direct collisions) the gravity of each galaxy can draw out tendrils of material from the other, creating long arcs of stars and gas called tidal tails. Sometimes there's also a bridge of gas strung out between the two galaxies, too. These are pretty good indicators that the two galaxies passed near each other sometime in the past.

This photo was taken to look for just that. Obviously, that's tough work! The cirrus makes finding such bridges and tails pretty hard. There's another galaxy in this image, NGC 7320, that's also about 40 million light years away. Could it be connected with NGC 7331? Let me show you this a smaller section of that bigger image:

A close-up on the group of galaxies called Stephan’s Quintet, with the contrast stretched to show faint diffuse material. Credit: CFHT, Pierre-Alain Duc (Observatoire de Strasbourg) & Jean-Charles Cuillandre (CEA Saclay/Obs. de Paris).

A close-up on the group of galaxies called Stephan’s Quintet, with the contrast stretched to show faint diffuse material. Credit: CFHT, Pierre-Alain Duc (Observatoire de Strasbourg) & Jean-Charles Cuillandre (CEA Saclay/Obs. de Paris)

This shows a small clot of galaxies just below center and to the right of the wide shot. These galaxies are actually collectively called Stephan's Quintet, and they're pretty well-known in astronomical circles. This gets confusing, so let me show you a Hubble image of them which is clearer:

A group of galaxies known as Stephan’s Quintet, though which galaxy is in the group and which isn’t is difficult to determine. Credit: Subaru Telescope (NAOJ), Hubble Legacy Archive, Robert Gendler & Judy Schmidt

A group of galaxies known as Stephan’s Quintet, though which galaxy is in the group and which isn’t is difficult to determine. Credit: Subaru Telescope (NAOJ), Hubble Legacy Archive, Robert Gendler & Judy Schmidt

Ah, that's better. I've labeled the galaxies; for a clean version click here. Are you ready for this? Because confusion is the theme of this article.

It was thought for a long time that the five galaxies — NGC 7319, NGC 7318A, NGC 7318B, NGC 7317, and NGC 7320 — were all physically associated with one another, forming an actual group of galaxies in space. However, while NGC 7320 is 40 million light years away, the other four of the five galaxies are 300 million light years distant! That means NGC 7320 is in the foreground and completely unaffiliated with the others. It even looks different, for that matter, being blue and containing less dust. So Stephan's Quintet is actually a quartet.

So it's a coincidence NGC 7320 is where it is. And it may be a coincidence that it's at the same distance as the bigger NGC 7331, too: No tidal tails or bridges or anything connecting the two was found in the image. Huh.

But not all is lost! The astronomers studying the shot noticed something odd: In Stephan's Quintet, there's an off-center red glow to the upper right of NGC 7317 (the one on the right). Why is that weird?

Look at the other three galaxies. They all have extended tails, but NGC 7317 doesn't. Because of that, it was thought that perhaps 7317 is a newcomer to the group, and hasn't had as much time to interact with the other galaxies.

But that red glow tells another tale. The color indicates it's composed of old stars, probably ripped away from NGC 7317. That implies it has interacted with those other galaxies. It may be interacting gently with NGC 7319 on the other side of the group.

Confused? It gets worse. See the tail of material that looks like it's coming off of the blue foreground galaxy NGC 7320? That too is a coincidence! That long arcing tail of material is likely connecting NGC 7318 A and B in the group to the tight little spiral galaxy off to the left, called NGC 7320c. It turns out that galaxy may have interacted with the others a half billion years ago.

If that's the case, Stephan's Quintet isn't a quartet after all but is still a quintet! But the five galaxies aren't what you'd think.

Still confused? Yeah, I know. Trying to interpret all this is a mess (and the alphanumeric tossed salad of galaxy names doesn't help). Our ideas about the Quintet have changed over and again over the years as better or just different observations were made. We're still teasing out the info.

And there's still one more thing. This image was made using the Canada-France-Hawaii Telescope, a 3.6 meter 'scope on the island of Mauna Kea. Normally this telescope is booked solid to do science, but sometimes the skies aren't cooperative. In this case, the sky glow was unacceptable for doing science observations, so instead this images was taken to help complete a set of observations intended for use in a calendar sold by CFHT in collaboration with an Italian editor called Coelum.

In other words it was taken as a public outreach and fundraising effort… and still wound up yielding some important science!

So yeah, the sky is pretty confusing sometimes, especially when you push your observations to the limit of what you can see. But once you understand what you are seeing that confusion turns to knowledge, and to wonder.

Creating beauty out of chaos. Science! I love this stuff.

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