When and How our Milky way Galaxy will Collide with Andromeda Galaxy.

The Andromeda and Milky Way galaxy collision Illustration

The Andromeda and Milky Way galaxy collision are predicted to happen in about 4.5 billion years. two largest galaxies in the Local Group—the Milky Way (which includes the whole of the Solar System and Earth) and the Andromeda Galaxy. The stars involved are sufficiently far apart that it is improbable that any of them will individually collide. Some stars will be ejected from the resulting galaxy, often nicknamed Milkomeda or Milkdromeda, a portmanteau of the respective galaxy names.

Our neighbor Andromeda Galaxy is approaching the Milky Way at about 110 k.m.per seconds (68 mi/s) as indicated by blueshift. However, the lateral speed (measured as proper motion) is very difficult to measure with precision to draw reasonable conclusions. Until 2012, it was not known whether the possible collision was going to happen or not. But then researchers reported on using the Hubble Space Telescope to measure the positions of stars in Andromeda in 2002 and 2010, relative to hundreds of background distant galaxies. By averaging over thousands of stars they were able to obtain the average proper motion with accuracy much better than one-pixel accuracy.

The conclusion was that Andromeda is moving southeast in the sky at less than 0.1 milliarcseconds 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 concerning 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 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.

The studies also suggest that M33, the Triangulum Galaxy—the third-largest and third-brightest galaxy of the Local Group—will 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 Andromeda Galaxy contains about 1 trillion stars and the Milky Way contains about 300 billion, the chance of even two stars colliding is negligible because of the huge distances between the stars. For example, the nearest star to the Sun is Proxima 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 a ping-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). That is analogous to one ping-pong ball every 3.2 km (2 mi). Thus, it is extremely unlikely that any two stars from the merging galaxies would collide.