"This leaves us the rather discomforting conclusion that most of the inner part of the Galaxy including the Sun might not be saved from M31 after all, and the possibility of a merger in another 4 billion years cannot be ruled out."
6. Spiral Metamorphosis (6:18) / 7. Future Sky (6:37)
The harsh reality of the distant universe with all of its violent interactions seems remote from our human existence and all might seem to be quiet and normal in our home the Milky Way. But it seems likely that in a mere 3 billion years, our neighbouring galaxy Andromeda and the Milky Way will fall together and have a close collision. They will likely merge and be reborn as a single giant elliptical galaxy over the course of another billion years or so. How might this metamorphosis play out and what might you see if you looked up at night over the next 4 billion years! The space between stars is so vast compared to their size that during a galaxy collision no individual stars actually collide with one another. So our sun and its family of planets will be taking a passive but exciting ride through the pair of coalescing galaxies and take on a spectacular view of the unfolding disaster in relative safety.
I have set up a model system of colliding galaxies that reflects the current state of our the Milky Way and Andromeda system. There are still some uncertainties about the exact trajectories and masses of the two galaxies but I have set up a plausible case where they fall together and collide almost directly passing within 60000 light years of each other. Also, I only present the view of the naked stars unobscured by the interstellar gas and dust clouds within the galaxy. We get a chance to see it all from 4 perspectives: two fixed positions in space a million light years away (Spiral Metamorphosis) and two sky views (Future Sky) the first that projects the full 360 degrees of the sky onto an oval map and the second a view of one hemispheric dome of the night sky. In the sky views, one particle is identified as the sun within the model of the Milky Way and our view is always from this perpective with our attention directed towards the central bulge of the Galaxy making for a mind boggling spectacle.
The view from far reveals an exquisite ballet of mutual annihilation and transformation into an elliptical galaxy. The Milky Way is seen coming in from the bottom in a face-on and edge-on view. After the interaction, long tidal tails of stars are flung out in open spiral patterns from both galaxies by the strong gravitational tides during the interaction. While separating, the two galaxies develop detailed spiral structure and then fall back for a second collision finally to merge. The mutual annihilation of the two galaxies leads to a big splash showing up as a complicated system of loops and ripples that represent turning points of stellar orbits. The two galaxies finally settle down into a single elliptical galaxy surrounded by remnant debris of their violent interaction.