Andromeda–Milky Way collision

Certainty

The Andromeda Galaxy is approaching the Milky Way at about 110 km/s as indicated by blueshift. However, the lateral speed (measured as proper motion) is very difficult to measure with sufficient precision to draw reasonable conclusions. Until 2012, it was not known whether the possible collision was definitely going to happen or not.

Such collisions are relatively common, considering galaxies' long lifespans. Andromeda, for example, is believed to have collided with at least one other galaxy in the past, and several dwarf galaxies such as Sgr dSph are currently colliding with the Milky Way and being merged into it.

Studies from 2012 also suggest that M33, the Triangulum Galaxy—the third-largest and third-brightest galaxy of the Local Group—may participate in the collision event, too. Its most likely fate is to end up orbiting the merger remnant of the Milky Way and Andromeda galaxies and finally to merge with it in an even more distant future. However, a collision with the Milky Way, before it collides with the Andromeda Galaxy, or an ejection from the Local Group cannot be ruled out.

The certainty, as well as the timescale for such a collision, have since been questioned. In 2025, Till Sawala and colleagues found that, when the gravitational pulls from both the Large Magellanic Cloud and the Triangulum Galaxy are taken into account, something that is only possible with data from the Gaia spacecraft and Hubble telescope which was not available in 2012, the chance for a collision is much lower. There is a probability of about 50% for no collision during the next 10 billion years.

Stellar collisions

While the Andromeda Galaxy contains about 1 trillion () stars and the Milky Way about 300 billion (3), the chance of even two stars colliding is negligible because of the huge distances between the stars. For example, the nearest star to the Earth after the Sun is Proxima Centauri, about 4.2 ly or 30 million (3) solar diameters away.

To visualize that scale, if the Sun were a ping-pong ball, Proxima Centauri would be a pea about 1100 km away, and the Milky Way would be about 30 e6km wide. Although stars are more common near the centers of each galaxy, the average distance between stars is still 160 billion (1.6) km (100 billion mi, 1075 AU). That is analogous to one ping-pong ball every 3.2 km. Thus, it is considered extremely unlikely that any two stars from the merging galaxies would collide.

Black hole collisions

The Milky Way and Andromeda galaxies each contain a central supermassive black hole (SMBH), these being Sagittarius A* (c. ) and an object within the P2 concentration of Andromeda's nucleus (). These black holes would converge near the centre of the newly formed galaxy over a period that may take millions of years, due to a process known as dynamical friction: as the SMBHs move relative to the surrounding cloud of much less massive stars, gravitational interactions lead to a net transfer of orbital energy from the SMBHs to the stars, causing the stars to be "slingshotted" into higher-radius orbits, and the SMBHs to "sink" toward the galactic core. When the SMBHs come within one light-year of one another, they would begin to strongly emit gravitational waves that would radiate further orbital energy until they merge completely. Gas taken up by the combined black hole could create a luminous quasar or an active galactic nucleus, releasing as much energy as 100 million supernova explosions. As of 2006, simulations indicated that the Sun might be brought near the centre of the combined galaxy, potentially coming near one of the black holes before being ejected entirely out of the galaxy. Alternatively, the Sun might approach one of the black holes a bit closer and be torn apart by its gravity. Parts of the former Sun would be pulled into the black hole.

Fate of the Solar System

Based on data available in 2007, two scientists with the Harvard–Smithsonian Center for Astrophysics predict a 50% chance that in a merged galaxy, the Solar System will be swept out three times farther from the galactic core than its current distance. They also predict a 12% chance that the Solar System will be ejected from the new galaxy sometime during the collision. Such an event would have no adverse effect on the system and the chances of any sort of disturbance to the Sun or planets themselves may be remote.

Excluding planetary engineering, by the time the two galaxies may collide, the surface of the Earth will have already become far too hot for liquid water to exist, ending all terrestrial life; that is currently estimated to occur in about 0.5 to 1.5 billion years due to gradually increasing luminosity of the Sun; by the time of the collision, the Sun's luminosity will have risen by 35–40%, likely initiating a runaway greenhouse effect on the planet by this time.

Possible triggered stellar events

When two spiral galaxies collide, the hydrogen present on their disks is compressed, producing strong star formation as can be seen on interacting systems like the Antennae Galaxies. In the case of the Andromeda–Milky Way collision, it is believed that there will be little gas remaining in the disks of both galaxies, so the mentioned starburst will be relatively weak, though it still may be enough to form a quasar.

Merger remnant

The hypothetical galaxy product of the collision has been named Milkomeda or Milkdromeda. According to simulations, this object would probably be a giant elliptical galaxy, but with a centre showing less stellar density than current elliptical galaxies.

Over the course of the next 150 billion years, the remaining galaxies of the Local Group will coalesce into this object, effectively completing its evolution.

References

References

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