What are galaxies? “A galaxy is an enormous collection of a few million to trillions of stars, gas and dust held together by gravity. They can be several thousand to hundreds of thousands of light-years across.” 1 All the stars that we see, observe, or use to guide us through direction, they all belong to one galaxy, and that galaxy is our own Milkyway. At some dark night, you could observe a fuzzy, cloudy strings of very dense stars those are the “disk and bulge”.

The result of gravitational force:
As mentioned earlier in the definition, a galaxy is held together by gravitational force, which means that every galaxy has its own gravitational force acting on it. When galaxies come together or move at a short distance, there are two possible results, one is the gravitational force will push them apart, or they will collide. When two objects or in this case galaxies, come together they would exert some of their gravitational force on each other, as an effect both galaxies can change shape. “Both crashes and near misses between galaxies are referred to as "interactions”. You can see two galaxies interacting. You can see they are being distorted by the gravitational interaction between them.” 2 The second effect is collision, as galaxies become fairly close together, its might merge together and collide. This merging/collision is sometimes known as a pair, or as a companion of one another. Sometimes smaller galaxies can just merge with larger ones; this is called the ripple effect collision. “The Cartwheel galaxy is an example of this type of collision. The outer ring of blue stars in this galaxy indicates a ripple of star formation resulting from the collision.” 1
The reasons for the collision is based on many factors, for example the number of galaxies engaged, their size, mass, type, the distance between them, their gravitational force, and so on. As we just saw how galaxies collision, a common question will be could stars also collide?, well logical people will say yes, cause what’s a galaxy it’s a collection of millions and trillions of stars, and we proved that galaxies collide so that basically means stars do as well. In fact stars are very far away that the collision between them are very rare. “In fact, the Milky Way Galaxy is colliding with the Sagittarius Dwarf Galaxy right now” 2 In additional to our milky way collision, the Antennae galaxies which is made up of two spiral galaxies, are also in the progression of collision, and as mention the process take hundreds of millions of years.
“When two galaxies interact, clouds of gas inside each galaxy may become compressed.” 2 Compressing clouds could lead to collapse of galaxy under their own gravity, as a result transform into stars “This process can lead to a burst of star formation in interacting galaxies, leaving a new generation of stars in a galaxy where normal star formation may have ceased long ago.” 2
The history of galaxies collision:
The topic of “Galaxies colliding” in the 1970s was so strange that people never believed it; they considered it a myth, until François Schweizer came in the late 1970s, and presented his theory of the newborn star clusters as the outcome or result of those collisions. This made him the first discoverer of galaxies collision. Some astronomers argue, that these star clusters were there in the first place (when the universe began), and that it haven’t established into something significant yet. Then comes the Hubble Telescope, which was launch in 1990, and after being in use it has proven Schweizer’s theory. During that time, “Schweizer has proved with the birth of 1000 new stars in the colliding Antennae galaxies, that stars were actually being formed from the collision.” 3 Astronomers weren’t very conceived with that fact back in the days, but now most of them agree and proved its existence, and the fact that galaxies can do all sort of acts example merge, change shape and so on.

1- http://amazing-space.stsci.edu/resources/explorations/galaxies-galore/teacher/scientificbackground.html

2- http://cas.sdss.org/dr3/en/proj/basic/galaxies/collisions.asp

3- http://www.phenomenalwomen.com/showcase/science/april-dean.htm

4- Struck, Curtis, 100,101 (2011)

5- http://www.oglethorpe.edu/faculty/~m_rulison/astronomy/group/fall%2099/galaxy_collisions.htm

6- http://www.galaxyzoo.org/galaxy_morphological_types