The night sky, a canvas of seemingly fixed constellations and the faint, silvery band of the Milky Way, presents an image of eternal peace. Beneath this calm appearance, a dramatic, slow-motion cosmic event is unfolding between our home galaxy and its largest neighbor, the Andromeda galaxy. These two colossal spiral systems are locked in a gravitational embrace that will ultimately lead to a spectacular merger. The resulting transformation will erase the familiar features of both galaxies, creating a new stellar structure that will dominate the night sky billions of years from now.
Setting the Stage: Distance and Velocity
The Andromeda galaxy, also known as Messier 31 or M31, is the nearest major galaxy to the Milky Way, currently sitting approximately 2.5 million light-years away from us. This vast distance is constantly shrinking as Andromeda hurtles toward the Milky Way at a speed of about 110 kilometers per second. The light from Andromeda is slightly shifted toward the blue end of the spectrum, a phenomenon called blueshift, confirming this direct approach.
The mutual gravitational pull between the two massive galaxies is strong enough to overcome the general expansion of the universe within our local galactic neighborhood, the Local Group. Although the approach speed seems immense, the sheer scale of the cosmos means it will take billions of years for the collision to begin.
The Collision Timeline and First Gravitational Contact
The first major gravitational contact is predicted to occur in about 4.5 billion years, but the event will not be a sudden, catastrophic smash. Due to the tremendous distances separating individual stars, the probability of any two stars physically striking one another is incredibly low. Instead, the collision will be a drawn-out gravitational disruption of the two galactic structures.
From a future Earth, the first sign of the impending event will be the gradual expansion of the Andromeda galaxy in the night sky. In the period leading up to the initial pass-through, Andromeda will grow to span a field of view much wider than the full moon, dominating the constellation of Andromeda. As the outer edges of the two galaxies begin to interact, the gravitational forces will stretch and pull the disks, causing long, luminous streamers of stars and gas, known as tidal tails, to be flung out into space.
The initial pass-through will dramatically distort the Milky Way’s spiral shape, warping the disk and creating vast new regions of star formation. This initial phase marks the end of the two distinct spiral galaxies as they are known today.
The Galactic Dance: Restructuring the Disks
Following the first encounter, the two galaxies will slingshot away from each other, only to be pulled back together by gravity for a second, and then a third pass-through over the next few billion years. This series of close encounters is the “galactic dance” that fundamentally restructures the disks. During these chaotic close approaches, the massive gravitational perturbation will violently compress the hydrogen gas clouds within the galaxies.
The compression of gas clouds will trigger intense bursts of star formation, called starbursts, lighting up the newly forming structure with bright blue, young stars. As the galaxies repeatedly pass through one another, the stars that make up the spiral arms of both the Milky Way and Andromeda will be thrown out of their neat, circular orbits. Many stars will be scattered into a vast, randomly oriented halo around the new core.
Computer simulations indicate that the Sun will likely be gravitationally flung into a much wider, more distant orbit around the new galactic center. There is also a small chance, perhaps around 12 percent, that the solar system could be entirely ejected from the newly formed galaxy into intergalactic space.
The Final Appearance: A New Elliptical Galaxy
The entire merger process, from first contact to final stability, is expected to take around 7 billion years to complete. At this point, the scattered stars and gas clouds will have settled into a single, massive, and stable structure. The result will be a giant elliptical galaxy, a type of galaxy characterized by its smooth, featureless, and spheroidal shape.
This final galaxy has been nicknamed “Milkomeda” and will look vastly different from its spiral predecessors. The stars will no longer be confined to a rotating disk but will orbit the core in highly eccentric, random directions, giving the new galaxy its characteristic oblong shape. The intense starbursts will have used up most of the available gas, leading to a much lower rate of new star formation.
The light from Milkomeda will be dominated by older, redder stars, reflecting the lack of new, blue-hot stellar nurseries. For any observer within this new galaxy, the view will be a diffuse, glowing sphere of light, replacing the distinct band of the Milky Way that has defined our night sky for billions of years. The central supermassive black holes from both original galaxies will eventually spiral inward and merge, a final cosmic convergence within the heart of the new structure.