Worlds – I mean Galaxies – in collision

By Phil Plait | October 17, 2006 12:21 pm

Galaxies are vast islands in space, stars, gas, and dust held together by their mutual gravity. Many sport grand shapes: elliptical, spirals, barred spirals, and so on. Some are just a mess, irregular in shape. Others are called peculiar: they appear to have some overall shape, but it’s weird, distorted, like something bad happened.

For the Antennae Galaxies, something very bad indeed happened: they collided. Two massive spiral galaxies slammed into each other, their mutual gravity ripping out long streamers of gas and stars from each other. In general, the stars will pass by each other without a physical collision. Stars are very small compared to the space between them, and it would be like two dust motes inside an otherwise empty football stadium colliding with each other by chance.

But the gas clouds are big, and they do in fact collide. At the centers of the galaxies these clouds ram each other, collapse, and form stars at a furious rate. A new Hubble image of the Antennae shows this very well. The cores of the galaxies are all you can see here. The pearly glow is from old stars, the dark lanes are dust, and the cherry red is from the vast amounts of gas being lit up by newly formed stars.

If you were on a planet orbiting a star anywhere near the center of that mess, you’d be in big trouble. All that star formation means lots of massive stars, which in turn means a flood of ultraviolet radiation coming out of the core. But far worse, those stars won’t last long: they explode, go supernova, and send out all sorts of nasty particles and radiation. If you’re far enough away it’s not a big problem, but there are a lot of stars ready to blow down there. I can’t imagine what it would be like to see dozens, hundreds of supernova all going off in an astronomically short time… and I hope I never do.

The galaxies started colliding about 500 million years ago, and it will be a long, long time before this dance plays out. In the end, there can only be one: the two will merge, forming a larger elliptical galaxy. We see evidence of this everywhere — ellipticals with multiple cores (indicating they formed from mergers), ellipticals with younger stars than expected, and of course many other galaxies caught in flagrante delicto as they collide. Our own Milky Way will one day collide with the monster Andromeda Galaxy… but not for over a billion years. So you can breathe easy for now.

CATEGORIZED UNDER: Astronomy, Cool stuff, NASA, Science

Comments (16)

  1. Jay

    Awesome image. I love it (new desktop!)

  2. Keith Thompson

    I’ve heard that the Milky Way and Andromeda galaxies are due to collide, but how do we know this? I know we can measure Andromeda’s approach velocity accurately using the Doppler shift, but what about the lateral component of its velocity; how do we know it’s not moving on a trajectory that will miss us?

    I’m assuming that the lateral motion (or lack thereof) wouldn’t be directly detectable unless you watch for a very long time; is my assumption wrong?

    Is there enough information from the movements of the other galaxies in the Local Group to allow us to infer a collision course?

  3. Chip

    Fantastic. What a blast! (Blasts actually.) And then there’s this odd one. A victim of a glancing blow?

  4. Naw, chip – someone just sneezed!


  5. Grand Lunar

    The one line in your last paragraph reminds me of this:

    “There can only be one!”

    Galactic collisions are very cool!

    But is Andromeda really a monster galaxy? I thought it wasn’t much bigger than our own?

  6. Kaptain K

    Our own galaxy is also a monster galaxy. M31 and the MW are both very large for spirals. Only giant ellipticals and a few spirals are larger.

  7. james

    ”If you took the long view, something wonderful was going to happen. If you took the short or medium view, something terible was going to happen. It’s the difference between seeing a new star in the sky, or actually being close to a supernova…”

  8. eddie

    We used that image as main art on the front page of our newspaper tonight. I knew BA would already have it posted the minute I saw it on the AP wire.

  9. Bored Huge Krill

    that’s a stunning picture.

    However, I’ve got to say one thing: the various press releases highlight a particular bete noire of mine, something that I see in many of these HST pictures and accompanying press releases.

    Specifically, I can’t find any mention of the technical details relating to this image. I’d like to see some information about where this is, and how big it is. What are its coordinates? What is the image scale? What is the approximate visual magnitude? How long an exposure was this?

    I’d really like to see NASA (or anybody else releasing these kind of images) always attaching some brief summary of such details, or at least writing it up and linking to it. It would make the whole experience much more informative than simply showing a picture and describing it in broad qualitative terms.

    /rant :-)

  10. Navneeth

    Bored Huge Krill,
    Go to this page and click on Fast Facts. A similar feature is also availble with every press release at the Chandra website.

    Btw, has anyone noticed that the HST pictures on ESA’s site has better clarity than the ones on HST’s own site?

  11. Bored Huge Krill

    thanks for the pointer – that’s what was looking for!

  12. icemith

    Interesting to see that link and to open the “fast facts” as directed. Also to wonder why the “compass” overlay in the image again is seemingly reversed. Did someone imagine in space that it doesn’t matter? Or that “E” and “N” stand for something other than East and Nonth?


  13. icemith

    Ooops, can’t tell the difference between my “n”s from my “r”s! I meant “North” of course.

    But then maybe something could be raised to the “nonth” degree.


  14. Scott

    That was the Great Green Arkleseizure.

    # John B. Sandlin Says:
    October 17th, 2006 at 3:27 pm

    Naw, chip – someone just sneezed!


  15. Irishman

    icemith, the directions aren’t wrong. Think about how East and North are defined relative to Earth. Typcially they are denoted by looking down on the Earth. Now lie on your back and look up at the sky. If north is above your head, then east is to your left, not right.

  16. jess tauber

    I’ve seen estimates on how many stars form or go supernova in the MW per year- what would be the figures in a collisional system like the Antennae?

    Jess Tauber


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