A NASA simulation of the Milky Way's collision
The Andromeda-Milky Way collision is a future galactic collision between the two largest galaxies in the Local Group, the Milky Way Galaxy and Andromeda Galaxy, predicted to occur in about 4.5 billion years. Because of the great distances between the stars, none are expected to individually collide, though some stars will be ejected.
Certainty[]
This series of photo illustrations shows the predicted merger between the Milky Way galaxy and Andromeda Galaxy.
The Andromeda Galaxy is approaching the Milky Way at around 300 m/s (200 miles/120 kilometers per hour), 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 or not the possible collision was definitely going to happen or not. Researchers used the Hubble 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 0.1 milliarc-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 will eventually merge with the Milky Way in around 5 billion years.
Such collisions between galaxies are quite common in the universe. For example, it is generally believed that Andromeda collided with at least one galaxy in the past, and several dwarf galaxies such as Sgr dSph are currently colliding with the Milky Way and being merged into it.
The studies also suggested that the Triangulum Galaxy will participate in this collision event as well. Its most likely fate is to end up orbiting the merger remnant of the Milky Way and Andromeda galaxies and finally 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.
Stellar collisions[]
Although the Andromeda Galaxy contains 1 trillion stars, and the Milky Way contains 300 billion stars, the chances of even two stars colliding is extremely unlikely because of the huge distances between stars. For example, the distance between the nearest star to the Sun, Proxima Centauri, is 4.2 light years (4.0 x 1013 km; 2.5 x 1013) mi away, or 30 million (3 x 107) solar diameters away. If the Sun were the size of a ping-pong ball, Proxima Centauri would be 1,100 km (680 mi) away, and the Milky Way would be around 30 million km (19 million mi) wide. Although stars are more common in the centers of each galaxy, the average distance between stars is still 160 billion (1.6 x 1011) km (100 billion mi), or one ping-pong ball every 3.2 km (2 mi). Thus, it is extremely unlikely that any stars will collide.
Black hole collisions[]
The Milky Way and Andromeda galaxies both contain a central supermassive black hole (SMBH), those being Sagittarius A* and an object within the P2 concentration of Andromeda's nucleus, respectively. These black holes will converge near the center 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 them 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 each other, they will begin to strongly emit gravitational waves that 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 may be brought near the center of the combined galaxy, potentially coming near one of the black holes before being ejected entirely out of the galaxy. Alternatively, the Sun may 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[]
Two scientists with the Harvard-Smithsonian Center for Astrophysics stated that, when, and even whether, the two galaxies collide will depend on Andromeda's transverse velocity. Based on current calculations, they 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 any planetary engineering, by the time the two galaxies collide, the Earth will be uninhabitable, being far too hot for liquid water to stably exist, which would end all terrestrial life. This is estimated to occur within 0.5 to 1.5 billion years, due to the gradually increasing luminosity of the Sun; by the time of the collision, the Sun's luminosity will have risen by 35-40%, likely creating a runaway greenhouse effect on Earth and creating surface conditions similar to Venus.
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 collided galaxy will be named Milkomeda or Milkdromeda.. According to simulations, the object is likely to be a giant elliptical galaxy, but with a center showing less stellar density than current elliptical galaxies. It is also possible the resulting object will be a large lenticular or super spiral galaxy, depending on the amount of remaining gas in the Milky Way and Andromeda.
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.