The Milky Way and Andromeda galaxies, each containing hundreds of billions of stars, are currently separated by about 2.5 million light-years. Gravitational forces are steadily drawing them closer, setting the stage for a galactic encounter. This event will unfold over immense timescales, transforming the familiar structures of both galaxies.
Forecasting the Collision
Astronomers have studied the movements of the Milky Way and Andromeda to predict their future interaction. A key piece of evidence is Andromeda’s blueshift. Unlike most distant galaxies that show redshift, Andromeda’s blueshift signifies it is approaching us at approximately 110 kilometers per second (about 68 miles per second). This inward motion results from the strong gravitational pull between the two massive galaxies, overcoming the universe’s general expansion at these distances.
The Hubble Space Telescope provided further precision. By observing the proper motion of stars within the Andromeda galaxy, astronomers accurately mapped its trajectory. These measurements confirmed Andromeda is on a direct course with the Milky Way, making a collision highly probable. Current projections indicate this galactic encounter will begin in approximately 4.5 billion years.
The Galactic Embrace
When galaxies merge, the process is unlike a collision between solid objects. Galaxies are mostly empty space, meaning the vast distances between individual stars make direct star-on-star impacts rare. Instead, the collision will be a slow, gravitational dance spanning billions of years.
After this initial passage, the galaxies will be slowed by their mutual gravitational attraction and begin to swing back towards each other, repeating this process multiple times. During these passes, tidal tails of stars and gas will be flung out into intergalactic space. Over several billion years, these gravitational interactions will gradually draw the two galaxies into an increasingly intertwined state. The distinct spiral structures of both the Milky Way and Andromeda will be transformed as they slowly coalesce into a single, larger entity.
A Stellar Shuffle
While the galactic collision is a significant event, the fate of individual stars and planetary systems, including our Solar System, is less dramatic than often imagined. The immense distances between stars mean that direct stellar impacts are highly improbable. For scale, if the Sun were a ping-pong ball, the nearest star would be hundreds of miles away, illustrating the vast emptiness within galaxies.
Instead of direct collisions, stars and their planetary systems will be gravitationally perturbed, meaning their orbits around the galactic center will change significantly. Our Solar System is highly unlikely to be destroyed during this event, though its position within the newly forming galaxy will be drastically altered. It might be swept farther from the galactic core or even ejected into intergalactic space, but the Sun and its planets would remain intact.
The compression of gas clouds during the merger can trigger bursts of new star formation, illuminating regions of the merging galaxies. Earth’s long-term habitability, long before the galactic merger completes, will be affected by the Sun’s natural evolution into a red giant, a separate process expected around the same cosmic timescale.
The Birth of a New Galaxy
The ultimate outcome will be the formation of a single, new galaxy. This merged entity will be significantly larger than either the Milky Way or Andromeda and is predicted to be an elliptical galaxy, rather than a spiral. Unlike flattened, disk-like spirals, an elliptical galaxy has a more spheroidal shape with stars distributed uniformly.
Astronomers have proposed names like “Milkomeda” or “Milkdromeda” for this future galaxy. It will initially experience a surge in star formation due to the compression of gas and dust. However, this activity will subside as gas is used up, leading to a period of relative quiescence. Milkomeda is expected to become the dominant galaxy within our Local Group, potentially absorbing smaller neighboring galaxies like the Triangulum galaxy over longer timescales.