Galaxies in Collision
The two galaxies together form an playful image called “The Penguin and the Egg,” owing to the distortion of the once spiral galaxy NGC 2936 and athe elliptical galaxy NGC 2937
The “Penguin” has taken the brunt of the damage during its close encounter with the “Egg.” This image combines visible and infrared data gathered by the Hubble Space Telescope.
I don’t want to scare you, but our own Milky Way is on a collision course with Andromeda, the closest spiral galaxy to our own. At some point during the next few billion years, our galaxy and Andromeda – which also happen to be the two largest galaxies in the Local Group–are going to come together, and with catastrophic consequences.
Stars will be thrown out of the galaxy, others will be destroyed as they crash into the merging supermassive black holes. And the delicate spiral structure of both galaxies will be destroyed as they become a single, giant, elliptical galaxy. But as cataclysmic as this sounds, this sort of process is actually a natural part of galactic evolution.
Astronomers have known about this impending collision for some time. This is based on the direction and speed of our galaxy and Andromeda’s. But more importantly, when astronomers look out into the universe, they see galaxy collisions happening on a regular basis, as shown in the image above.
Galaxies are held together by mutual gravity and orbit around a common center. Interactions between galaxies is quite common, especially between giant and satellite galaxies. This is often the result of a galaxies drifting too close to one another, to the point where the gravity of the satellite galaxy will attract one of the giant galaxy’s primary spiral arms.
In other cases, the path of the satellite galaxy may cause it to intersect with the giant galaxy. Collisions may lead to mergers, assuming that neither galaxy has enough momentum to keep going after the collision has taken place. If one of the colliding galaxies is much larger than the other, it will remain largely intact and retain its shape, while the smaller galaxy will be stripped apart and become part of the larger galaxy.
Such collisions are relatively common, and Andromeda is believed to have collided with at least one other galaxy in the past. Several dwarf galaxies (such as the Sagittarius Dwarf Spheroidal Galaxy) are currently colliding with the Milky Way and merging with it.
However, the word collision is a bit of a misnomer, since the extremely tenuous distribution of matter in galaxies means that actual collisions between stars or planets is extremely unlikely.
In 1929, Edwin Hubble revealed observational evidence which showed that distant galaxies were moving away from the Milky Way. This led him to create Hubble’s Law, which states that a galaxy’s distance and velocity can be determined by measuring its redshift–i.e. a phenomena where an object’s light is shifted toward the red end of the spectrum when it is moving away.
 See Steve Gotlieb, “Galaxies in Collision,” Sky and Telescope (133, 5, May, 2017, pp. 28-33). Read more at: https://phys.org/news/2016-10-galaxies-collide.html#jCp