When galaxies strip for each other

By Phil Plait | October 19, 2012 7:00 am

It’s been a while since I’ve posted a gorgeous shot of a weird and beautiful galaxy, and I have one that fits the bill perfectly: NGC 660, what’s called a polar ring galaxy:

How awesome is that? [Click to galactenate.]

This picture, taken using the Gemini 8-meter telescope and put together by my old pal Travis Rector, is really pretty, and really pretty weird. Reading about it, in fact, I learned something! Learn it with me:

Ring galaxies are odd. I’ve always thought they were the result of galactic collisions, literally where two galaxies collide. If one is smaller, traveling rapidly, and pierces right through the heart of the other, the weird gravitational effects wind up creating a gigantic ring of material, and you get something that looks like a fried egg. The best and coolest example I know of this is Hoag’s Object, seen here in a Hubble image (put together by another friend of mine, Tiffany Davis).

In general with these kinds of objects, there are tell-tale signs of the collision. The gas in the galaxies gets slammed around a lot, which means it can collapse and form lots of stars, for example. Usually, the ring is perpendicular to the central galaxy, too – that’s why they’re called polar ring galaxies.

But NGC 660 is an oddity. It shows the ring – which is 40,000 light years across, a little less than half the size of our Milky Way – and it has lots of star formation: the ring is studded with red gas clouds, stellar factories, furiously churning out stars. But the ring isn’t aligned right; it’s tilted at a funny angle. It’s shape is off-kilter, too. Plus, although you can’t see it in this image, other observations show that the central galaxy is creating lots of stars in its core too. That’s not easy to explain in the head-on collision scenario, and is weird all by itself, since usually the cores of galaxies stopped forming stars billions of years ago.

But here’s where I found out something new to me: some polar ring galaxies are formed when two galaxies pass near each other. The gravity from one galaxy can strip the gas from the other, and if the geometry is right that gas can form a huge, long arm which wraps around the first galaxy. That can form a ring of material that cranks out stars… just like in NGC 660.

Also, a near pass like that tends to be slower than a head-on collision. That means there’s more time for the gravity of the two galaxies to shake each other up. That dumps gas into the center of the galaxies, where it can form stars. So this explains why we see newborn stars in NGC 660, too.

Well, that was new to me! I’ve written about galaxy collisions before, and the distorted but elegant and lovely shapes they take on (see Related Posts below). And it’s obvious to me now that the disturbed material can form a ring, but it’s just something I’d never run across before. So I learned something.

And now you did too.

So there you go. A picture of a galaxy with a big telescope is high art, beautiful and graceful. But there’s a whole lot of science behind it, too. So, as usual, the appreciation you can have for something like NGC 660 only gets deeper and better when you understand what’s actually going on. You’re seeing two entire galaxies colliding, hundreds of billions of stars, octillions of megatons of gas being tossed around, new stars being born… and all because they are held sway under the invisible grasp of gravity.

See? Isn’t that cooler than just seeing a pretty picture?

Image credit: Gemini Observatory/AURA

Related Posts:

The start of a long, long dance
The delicate aftermath of cosmic violence
Gorgeous galaxies celebrate Hubble’s 21st birthday
Awesome Antennae!
Felicia Day collides galaxies!

CATEGORIZED UNDER: Astronomy, Pretty pictures

Comments (11)

  1. Chris

    I kind of feel a little dirty, like I’m spying on two loving galaxies caught in an intimate moment.

  2. Tara Li

    That really is pretty cool. What I always find most fascinating about Hoag’s Object is that inside of the ring of the ring galaxy, you can see another way off in the distance! And considering how few of these ring galaxies are known so far, that seems an amazing coincidence! (Unless it’s a case of gravitational lensing so severe, we’re seeing the back side of Hoag’s Object through it…)

  3. Leonidas

    It’s articles like this Phil, that remind me why I love your blog so much!

  4. Wzrd1

    While looking at these images, I was pondering the profound effects of galactic mergers and galaxies colliding and still continuing on their course with some effects of the collision on their momentum.
    Then, I pondered what the effect would be if the galactic core singularities combined while the two galaxies were colliding. In the scenario of two passing through each other, but their cores merge.
    Nightmarish pales in that thought, as angular momentum of both cores would be TOTALLY different than the main galaxy bodies, the energies involved unimaginable in the extreme, under those conditions, one can easily, if barely, consider the term shattered galaxies!
    Or sundered galaxies.
    All dependent upon the masses of the galactic singularities involved, their relative velocity vectors in relation to each other, etc.
    The only term that comes to mind is, mind numbing.

  5. Double U

    Very, very cool stuff and I did learn something new about galaxies…not that I even knew a little about them and yes, it is very good to know more than just the beautiful pictures.

    Thank you Travis Rector or Phil Plait, whoever the author is of this article, for all you do!!

  6. mike burkhart

    Is the ring galaxy a new classifacion for galaxys? You know one thing that gets people confused about Astronomy is the many ways objects are classafied look at stars for instance we classify them by: apperent britness (magnatude),absolute britness (luminosity),color (how hot a star is) size,luminosity and color (H R diagram) and when the star formed (population) plus other systems based on insterment readings (u,v,b) it can make your head spin maybe we need to simplefy. Back to the topic you know I don’t think this is love , this is more highway robery ,These galaxys are riping each other off in stars and the bigest will end up with the most. Kind of like the Mafia the larger Galaxy is giving the smaller one “an offer he can’t refuse”.

  7. Dragonchild

    Before you get all excited, these strippers are billions of years old.

  8. Artor

    Alright, the polar ring is pretty cool, but how can you throw out a pic of something like Hoag’s object & not tell us more? It looks like it should have elvish writing on the ring and a slit pupil over the fiery central object. How did that ring get so perfectly symmetrical? What’s the composition of the center, and why is it so yellow compared to the ring? Is that another distant galaxy at 12:30 just inside the ring or is that a planetary nebula in the foreground? How far away is it? How big is it?
    Inquiring minds want to know…

    Okay, the Wikipedia pare on Hoag’s Object is kinda short, but I learned it’s about 600 million light years away, and the object I mentioned inside the ring is ANOTHER ring galaxy behind it. The blue ring is new, hot stars while the yellow center is old, but beyond that, it seems we know a whole lot of nothing about it. I hope Phil can fill us in?

  9. But the ring isn’t aligned right; it’s tilted at a funny angle. It’s shape is off-kilter, too.

    I would’ve expected that pretty much. It actually surprises me that more ring galaxies aren’t more distorted and asymetric with more off-kilter, misaligned tilts and kinks and warps.

    Would’ve thought the chances are that exact “dead centre” symmetrical collisions would be rarer than ones that are exactly through the middle entirely horizontal / vertical axis.

    I’d always guessed that examples such as Hoag’s object – one of my very favourite astrophotos (please relay my thanks and congrats to Tiffany Davis for me BA! :) ). were simply the most spectacular and rare of these and shown more often for that “clean” sublime beauty of theirs.

    Never would’ve guessed a mere close encounter could produce such stunning forms – surely if they’re that close together the galaxies are either gravitationally bound and eventually going to merge or travelling too quickly to merge and passing through other or colliding in glancing blows that distort each one less symmetrically ripping through an edge and drawing out trails more likely than hitting the bullseye and forming ripple ring structures?

  10. @ 8. Artor :

    Hoag’s Object~wise have you seen :


    ‘The Story of Hoag’s Object’ by Ray A. Lucas (STScI) and :


    What is Hoag’s Object? with a few different formation theories listed there?

    Plus :


    which notes merely that :

    Curiously, an object that bears an uncanny resemblance to Hoag’s Object can be seen in the gap at the one o’clock position. The object is probably a background ring galaxy.

    I’d guess that that background micro-Hoag’s galaxy is extremely difficult to study due to its faintness, size and being seen behind Hoag’s Object which would probably makes it very difficult to get detailed spectra and other details from.

    Suppose we could assume the micro-Hoag’s has a certain diameter say equal to Hoags Object or an average ring galaxy and calculate an approximate distance from that based on its apparent size and characteristics. I wonder if anyone has attempted that? There’d be pretty large error bars and uncertainties tho’.

    My net-fu fails to find much else on that mcro-Hoags-inside0-Hoags disappointingly. Perhaps others here can enlighten us further?

  11. @9. D’oh! :-( Make that :

    Would’ve thought the chances are that exact “dead centre” symmetrical collisions would be rarer than ones that are *not* exactly through the middle entirely horizontal / vertical axis.

    For clarity’s sake.

    Y’all probably figured that was what I meant to say but still.

    Final possibly silly thought here – wonder if the micro-inside-Hoags ring galaxy could be the missing impactor galaxy that created Hoag’s Object in the collisional theory – if it passed through behind Hoag’s Object and is either much smaller or moving much faster or some combination thereof and thus appears as it does and its own ring structure results from the same impact somehow?


    “.. galaxies are a bit like people. They may look normal enough at first sight, but when you get to know them better, they’re all a little different and strange in some way!”
    – Vera Rubin, astronomer, as quoted in that first link by Ray A. Lucas.


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