When galaxies collide, stars almost never hit each other
In about 4.5 billion years, our Milky Way will collide with the neighboring Andromeda galaxy in a titanic cosmic merger. While this sounds catastrophic, individual stars are highly unlikely to collide. The distance between stars is so immense that they will simply slide past one another like ghosts. Gravity will reshape both galaxies into a single elliptical giant, but the vast emptiness of space keeps the actual stars perfectly safe.
A Looming Galactic Encounter
The Milky Way and its largest neighbor, the Andromeda Galaxy (Messier 31), are currently rushing toward each other across intergalactic space. Observations of spectral lines reveal that Andromeda is approaching our galaxy at a speed of approximately 110 kilometers per second. While both systems are gravitationally bound members of the Local Group, their initial separation spans roughly 2.5 million light-years. Astrophysical simulations indicate that over the course of the next four to five billion years, this distance will steadily diminish until the outer envelopes of the two systems begin to overlap.
When people hear the word collision, the intuitive mental picture is one of shattering impacts, planetary destruction, and catastrophic physical smashups. In astronomical parlance, however, a galactic collision describes an intricate gravitational dance rather than a sudden physical smash. The two spiral galaxies will initially sweep past one another, pull out long tidal streams of gas and stars, swing back under their combined gravitational attraction, and ultimately settle into a single, merged remnant over hundreds of millions of years.
The Scale of Stellar Separation
The primary reason stars do not collide during a galactic merger lies in the sheer ratio between stellar diameters and the interstellar distances separating them. A typical star like our Sun has a diameter of roughly 1.4 million kilometers. By contrast, the average distance between neighboring stars in the solar neighborhood is roughly four light-years, which is approximately 38 trillion kilometers. The space between stars is roughly thirty million times larger than the stars themselves.
To visualize this disparity, one can scale the Sun down to the size of a ping-pong ball, roughly four centimeters across. On this proportional scale, the nearest neighboring star would be located more than one thousand kilometers away. If two enormous clouds containing billions of ping-pong balls spaced a thousand kilometers apart were swept through one another, the odds of any two individual balls making direct physical contact would be vanishingly small. Because of this vast emptiness, the stars in Andromeda and the Milky Way will pass cleanly through the opposing galaxy without physical impacts.