Imagine what goes through the mind of a baseball batter as he stares down a speeding pitch. The baseball is hurtling toward him, but difference between a strike and a ball depends on precisely gauging the amount of sideways motion. He’ll want to swing for the fences if it’s out over the plate, or jump back out the box if it’s headed for his ear.
Astronomers have been pondering a similar situation for about a hundred years. We can easily measure whether the neighboring Andromeda galaxy is moving toward or away from us (we call this “radial” motion). The result, known to Edwin Hubble in the 1920s, is that Andromeda is approaching our Milky Way galaxy at the tremendous speed of more than 250,000 miles per hour.
However, it is extremely difficult to measure the sideways (or “tangential”) motion of a galaxy. We have been left wondering: what is Andromeda’s full trajectory? Will the two galaxies simply pass by each other like cars on opposite sides of an interstate highway, or will there be a colossal pileup involving hundreds of billions stars in each galaxy?
It has taken Hubble’s namesake telescope, with its exquisite resolution, and some innovative imaging and computational measurement techniques to finally answer this question. By measuring minute shifts in the stars of Andromeda over the course of about a decade of observations, we now know that Andromeda has very little sideways motion. It is heading straight for the Milky Way, and, unlike the baseball batter, our galaxy can’t get out of the way.
VISUALIZATION: NASA, ESA, and F. Summers (STScI)
SIMULATION: NASA, ESA, G. Besla (Columbia University), and R. van der Marel (STScI)
The two galaxies have been destined by gravity to crash together in about four billion years. Their thin and beautiful spiral disks will become warped, stretched, and distorted beyond recognition. The centers of the galaxies will smash through one another not just once, but several times as the galaxies merge. Eventually, about six billion years from now, the two spirals will become one elliptical galaxy, mixed together for all eternity.
Our visualization of this galactic gravitational dance is the first scientific look into the far-flung future of our galaxy. Our news release features artistic visions of what the view might be from within the Milky Way during the collision. The stars and the band of the Milky Way across the night sky seem static and constant on human time scales. But taking an astronomer’s perspective, and looking forward billions of years, they have quite a dynamic future ahead of them.