The Milky Way Galaxy is a dynamic collection of stars, gas, dust, and dark matter moving through space alongside billions of other galaxies. These massive stellar islands are bound by gravity and interact across cosmic distances. The closest major neighbor, Messier 31 (the Andromeda Galaxy), is currently on a direct collision course with the Milky Way.
Identifying the Cosmic Partner
The Andromeda Galaxy (M31) is the largest member of the Local Group, which includes the Milky Way and about 50 other smaller galaxies. It currently resides approximately 2.5 million light-years away from us in the constellation Andromeda. Andromeda is a majestic spiral galaxy, much like our own, but it is physically larger.
Its disk spans an estimated 220,000 light-years in diameter, more than double the size of the Milky Way’s disk. Andromeda contains about one trillion stars compared to the Milky Way’s estimated 200 to 400 billion. Despite its distance, it is visible to the naked eye on a clear night as a faint, fuzzy patch of light.
The two galaxies are approaching each other at a speed of 110 kilometers per second, or about 250,000 miles per hour. This rapid closure is driven by the mutual gravitational attraction between the two masses. Gravity is pulling the galaxies together through intergalactic space. Andromeda’s size and proximity make it the inevitable partner in this cosmic event.
Calculating the Timeline
The prediction of this galactic collision relies on precise measurements of Andromeda’s movement through space. Astronomers can easily determine the galaxy’s radial velocity, its motion directly toward or away from us, by measuring the Doppler shift in its light spectrum. This measurement confirms the galaxy is approaching the Milky Way.
The more challenging measurement is the transverse velocity, the side-to-side motion of Andromeda across our line of sight. If this sideways motion were large enough, the two galaxies might simply miss each other. For decades, the exact path remained uncertain because accurately measuring such a minute change in position requires extremely high precision.
In 2012, the NASA/ESA Hubble Space Telescope provided the necessary data by tracking the subtle movements of stars in Andromeda over a period of five to seven years. These observations confirmed that Andromeda’s transverse velocity is quite small, indicating a near head-on collision rather than a glancing blow. Based on this confirmed trajectory, the first close pass and subsequent merger is predicted to begin in approximately 4.5 billion years.
The Galactic Merger and Aftermath
The term “collision” is misleading, as it suggests a violent, solid-body crash. A galactic collision is actually a slow, gravitational merger unfolding over billions of years. The immense distances between stars mean that direct stellar impacts are highly improbable, similar to two swarms of bees passing through each other.
The average distance between stars in our corner of the galaxy is about four light-years. When the two galaxies merge, stars will be flung into new orbits due to the shifting gravitational fields of the combining masses. Our solar system will likely survive this reshuffling without the Sun or any planets colliding with a foreign star.
The merger process involves multiple passes over billions of years, with gravitational tidal forces stretching the galaxies. The gas and dust within the galaxies will smash together, triggering a massive burst of new star formation. Eventually, the two spiral structures will be pulled into a single, spherical, or football-shaped galaxy.
This final, larger entity is often referred to by astronomers as “Milkomeda” or “Milkdromeda.” Simulations predict that our solar system has a chance of being flung much farther out from the new galactic core, or even being ejected entirely into intergalactic space. Long before the merger is complete, however, the Earth will have already become uninhabitable due to the natural evolution of the Sun, which will swell into a red giant star in about five billion years.