TheAndromeda–Milky Way collision is agalactic collision that may occur in about 4.5 billion years between the two largestgalaxies in theLocal Group—theMilky Way (which contains theSolar System andEarth) and theAndromeda Galaxy.[1][2][3][4][5]
The stars involved are sufficiently spaced that it is improbable that any of them would individually collide,[6] though some starsmay be ejected.[7][self-published source]
 | This section needs to beupdated. The reason given is: The "certainty" that was claimed in 2012 is cast into question by recently published studies. Please help update this article to reflect recent events or newly available information.(June 2025) |
The Andromeda Galaxy is approaching the Milky Way at about 110 kilometres per second (68.4 mi/s)[2][8] as indicated byblueshift. However, the lateral speed (measured asproper 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.[9] Researchers then used theHubble Space Telescope to measure the positions of stars in Andromeda in 2002 and 2010, relative to hundreds of distant background galaxies. By averaging over thousands of stars, they were able to obtain the average proper motion with sub-pixel accuracy. The conclusion was that Andromeda is moving southeast in the sky at less than 0.1milliarc-seconds per year, corresponding to a speed relative to the Sun of less than 200 km/s towards the south and towards the east. Taking also into account the Sun's motion, Andromeda's tangential or sideways velocity with respect to the Milky Way was found to be much smaller than the speed of approach (consistent with zero given the uncertainty) and therefore it would eventually merge with the Milky Way in around five billion years.[1][2][10]
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,[11] and severaldwarf galaxies such asSgr dSph are currently colliding with the Milky Way and being merged into it.
Studies from 2012 also suggest that M33, theTriangulum 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.[9]
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 theLarge Magellanic Cloud and theTriangulum Galaxy are taken into account, something that is only possible with data from theGaia 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.[12][13]
While the Andromeda Galaxy contains about 1 trillion (1012)stars and the Milky Way about 300 billion (3×1011), the chance of even two stars colliding is negligible because of the huge distances between the stars. For example, the nearest star to theEarth after theSun isProxima Centauri, about 4.2 light-years (4.0×1013 km; 2.5×1013 mi) or 30 million (3×107) solar diameters away.
To visualize that scale, if the Sun were aping-pong ball, Proxima Centauri would be a pea about 1,100 km (680 mi) away, and the Milky Way would be about 30 million km (19 million mi) wide. Although stars are more common near the centers of each galaxy, the average distance between stars is still 160 billion (1.6×1011) km (100 billion mi, 1075AU). That is analogous to one ping-pong ball every 3.2 km (2 mi). Thus, it is considered extremely unlikely that any two stars from the merging galaxies would collide.[6]
The Milky Way and Andromeda galaxies each contain a centralsupermassive black hole (SMBH), these beingSagittarius A* (c.3.6×106 M☉) and an object within theP2 concentration of Andromeda's nucleus (1–2×108 M☉). 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 asdynamical friction: as the SMBHs move relative to the surrounding cloud of much less massive stars, gravitational interactions lead to a net transfer oforbital 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 emitgravitational waves that would radiate further orbital energy until they merge completely. Gas taken up by the combined black hole could create a luminousquasar or anactive galactic nucleus, releasing as much energy as 100 millionsupernova explosions.[14][15] 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.[16] 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.[17]
Based on data available in 2007, two scientists with theHarvard–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.[3] They also predict a 12% chance that the Solar System will be ejected from the new galaxy sometime during the collision.[18][19] 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.[18][19]
Excludingplanetary 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 increasingluminosity of the Sun; by the time of the collision, the Sun's luminosity will have risen by 35–40%, likely initiating arunaway greenhouse effect on the planet by this time.[20][21]
When two spiral galaxies collide, thehydrogen present on their disks is compressed, producingstrong star formation as can be seen on interacting systems like theAntennae 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 aquasar.[19]
The hypothetical galaxy product of the collision has been namedMilkomeda orMilkdromeda.[22] According to simulations, this object would probably be a giantelliptical galaxy, but with a centre showing less stellar density than current elliptical galaxies.[19] It is, however, possible the resulting object would be a largelenticular or super spiral galaxy, depending on the amount of remaining gas in the Milky Way and Andromeda.[23][24]
Over the course of the next 150 billion years, the remaining galaxies of theLocal Group will coalesce into this object, effectively completing its evolution.[25]
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| Location | Andromeda Galaxy → Andromeda subgroup →Local Group →Local Sheet →Virgo Supercluster →Laniakea Supercluster →Local Hole →Observable universe →Universe Each → may be read as "within" or "part of". | .jpg) |
|---|---|---|
| Satellite galaxies |
*It is uncertain whether these are companion galaxies of the Andromeda Galaxy | |
| Catalogued stars | ||
| Other | ||
