Geeky Astronomy, Geeky Physics, Spaceheads

When Galaxies Collide


A galaxy of stars is mostly empty. And in the emptiness of space, even the scale of a galaxy is nothing by comparison. And I thought about how unlikely it is for two objects in the universe to ever meet, how all the infinitesimal points that make up a galaxy will mostly pass through each other, but still be affected by gravity. And how gravity acts at a distance, and how little we understand it.

And then I had to go lie down.

Hat tip to toxicavenger1 for introducing me to NASA’s Astronomy Picture Of The Day.

  • The Professor

    Collisions between one or more galaxies is a pretty common occurrence. There are traces around our galaxy that it has collided and merged with a couple of much smaller galaxies, and being the larger gravity well, it won – so far. The Milky Way will collide with the Andromeda galaxy in a few billion years, and that will be quite a messy event. The two spiral galaxies will end up as a larger elliptical galaxy, with a few dwarf galaxies forming from the detritus.
    I'd absolutely love to see it all happen. Just image what the sky would look like…

  • Felis_Concolor

    Galaxies are much closer relative to their size than stars are to each other. I belive the measure, using an aspirin tablet to represent Sol's disc, would place Sirius approximately 8km away. Using the same aspirin to represent the Milky Way galaxy's disc places Andromeda a scant 23cm away. That's 5 orders of magnitude difference, which is why galaxies collide relatively frequently, yet the stars within tend to slip past each other.

    And it would definitely be the single most impressive sight in the sky; as Carl Sagan fantasized in his Cosmos TV series, just imagine the visual treat of not a sunrise, but a galaxyrise.

    • skitter

      That example greatly changes and improves my understanding.

      Edit: But what are the relative speeds of individual stars and galaxies?

      • Felis_Concolor

        Ah, this is when the science starts to squish a bit. It all depends how far away from each other the galaxies are. If the galaxies are within the same cluster, gravity is pulling them together, while they move faster the further away they are from one another. Once the expansion of space counteracts the force of gravity within a distant cluster, galaxies can recede from each other faster than the speed of light, although within a closer group, they're not actually moving faster than light within their own space.

        So while galaxies are closer to each other relative to their size, space is expanding such that most of them are moving away from each other much faster than stars themselves move relative to each other.