• Question: How come spiral galaxy's have arms and they are in that swirly shape, what formes them like that?

    Asked by Matthew to Miranda, James on 18 Jun 2015.
    • Photo: James Gilbert

      James Gilbert answered on 18 Jun 2015:

      We don’t exactly know, it’s still a bit of a mystery! Not only is how they form not fully understood, but how the arms stay there is puzzling too: a spinning galaxy with spiral arms should quite quickly ‘wind itself up’ so the arms disappear, yet we see far too many spiral galaxies in the universe for this to be the case…

    • Photo: Miranda Jackson

      Miranda Jackson answered on 22 Jun 2015:

      It is thought that the spiral arms are the sites of star formation in galaxies. Stars form where gas has cooled and condensed into molecular form, and this is due to pressure waves and other effects which travel up and down through the plane of the galaxy. Younger stars are brighter, on average, which is why spiral arms appear brighter than the space between them. There are many models and theories of galaxy formation and evolution which try to explain the shape of spiral arms.

Comments

  • Photo: Herp Derp

    Herp Derp commented on :

    Earth doesn’t crash into the sun because of it’s orbit. Maybe the solar sytems that make up the arms do not crash into the center because of there or bit. The reason the arms aren’t straight is because of that same reason, they’re orbiting and spinning. I just made that theory up now so… don’t trust it

  • Photo: Herp Derp

    Herp Derp commented on :

    their*

  • Photo: Miranda

    Miranda commented on :

    You are completely correct about the orbits. That is actually one theory of galactic structure. The whole galaxy is rotating (orbiting around its centre), but the stars, gas, etc. at different distances from the centre rotate at different rates (this is called differential rotation). The linear speed of the rotating material is actually pretty constant, but this means that the stuff that is farther out takes longer to complete a rotation, because it has farther to travel. It is also not necessarily the case that all the material in the galaxy stays in the same place relative to the rest of the galaxy. If we took a snapshot of the galaxy now, and then again in a few million years, the individual stars will have moved, but the spiral arms will stay almost in the same place. It makes sense to conclude that the spiral arms are constantly changing, but very slowly, so we can’t see the change in our lifetimes.

    If you are really interested in galactic structure, you could view the page here: http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit4/spirals.html
    It gives some information about spiral arms and why they look the way they do. That page is for university students, so if you have any questions please ask again. You could also look at
    https://en.wikipedia.org/wiki/Density_wave_theory
    The animations are very illuminating. Animation 3 shows the way scientists think galaxies rotate.

  • Photo: Herp Derp

    Herp Derp commented on :

    i guessed it right 😀