In the recent Hubble press release about the collision between our Milky Way galaxy and the Andromeda galaxy, we provided some artistic illustrations of what a future night sky might look like. It shows the fantastic sights as Andromeda approaches, smashes through, and eventually merges with our galaxy.
Although it may seem like splitting hairs, I note that we specifically did not say that those illustrations show what Earth’s night sky would look like. There are three reasons for this.
First, those illustrations were motivated by views from a computer simulation. The viewpoint inside the simulation was held fixed in space at the Sun’s current location relative to the center of the galaxy. Though the Sun continually orbits the core of the Milky Way, a fixed location provided a more easily understandable sequence of images.
Second, the Sun’s orbit in the galaxy will probably change greatly during the collision. Our star could get flung far into the outskirts of the Milky Way, drop deep into the core of the galaxy, or do both on a spirograph-like orbit. Almost all scenarios greatly change our night-sky view.
Third, and most important, while Earth should still orbit the Sun in 4 billion years, it probably won’t be habitable. Our Sun is slowly getting hotter as it ages. According to calculations, in about 3 billion years the Sun will be hot enough that Earth’s oceans will evaporate and its atmosphere will escape. Human life on the surface of our planet will not be feasible without planet-scale engineering efforts.
The illustrations show what a night sky might look like, but the view seen by a future human civilization is guaranteed to be different. Still if our species survives to see it, it will be awesome to have two galaxies stretched across the sky. Although, as an astronomer, I can’t help but imagine how confusing it might be to detangle the star and galaxy motions.