Minor merger A merger is minor if one of the galaxies is significantly larger than the other(s). The larger galaxy will often "eat" the smaller, absorbing most of its gas and stars with little other significant effect on the larger galaxy. Our home galaxy, the Milky Way, is thought to be currently absorbing several smaller galaxies in this fashion, such as the Canis Major Dwarf Galaxy, and possibly the Magellanic Clouds. The Virgo Stellar Stream is thought to be the remains of a dwarf galaxy that has been mostly merged with the Milky Way. Major merger A merger of two spiral galaxies that are approximately the same size is major; if they collide at appropriate angles and speeds, they will likely merge in a fashion that drives away much of the dust and gas through a variety of feedback mechanisms that often include a stage in which there are active galactic nuclei. This is thought to be the driving force behind many quasars. The end result is an elliptical galaxy, and many astronomers hypothesize that this is the primary mechanism that creates ellipticals. One study found that large galaxies merged with each other on average once over the past 9 billion years. Small galaxies coalesced with large galaxies more frequently. Note that the Milky Way and the Andromeda Galaxy are predicted to collide in about 4.5 billion years. The expected result of these galaxies merging would be major as they have similar sizes, and will change from two "grand design" spiral galaxies to (probably) a giant elliptical galaxy. By gas richness Mergers can be categorized by the degree to which the gas (if any) carried within and around the merging galaxies interacts: Wet merger A wet merger is between gas-rich galaxies ("blue" galaxies). Wet mergers typically produce a large amount of star formation, transform disc galaxies into elliptical galaxies and trigger quasar activity. Dry merger A merger between gas-poor galaxies ("red" galaxies) is called dry. Dry mergers typically do not greatly change the galaxies' star formation rates, but can play an important role in increasing stellar mass. Damp merger A damp merger occurs between the same two galaxy-types mentioned above ("blue" and "red" galaxies), if there is enough gas to fuel significant star formation but not enough to form globular clusters. Mixed merger A mixed merger occurs when gas-rich and gas-poor galaxies ("blue" and "red" galaxies) merge. Merger history trees