There are more than one hundred billion galaxies in the observable universe. Some are dwarfs with just ten million stars, and some are giants with one trillion stars. So, how are these massive galaxies created? The answer may be revealed sooner than scientists thought…
Four giant galaxies are crashing into each other in one of the largest cosmic collisions ever observed, eventually forming a galaxy ten times bigger than our Milky Way!
All of you know what galaxies are – massive, gravitationally bound systems made of stars, interstellar medium of gas and dust, and dark matter, orbitting a common centre of mass. Our solar system is part of the Milky Way galaxy. Being members of the family of Space, galaxies often interact with each other, somewhat like us.
Galaxies share the same Space, and gravity makes some close-by galaxies affect each other. This plays an important role in their evolution. When they interact, three different things are can happen:
Near misses: Near misses between galaxies cause distortions, and exchange of gas and dust.
Collisions: Collisions occur when two galaxies pass directly through each other and have enough relative momentum not to merge. The stars within these interacting galaxies pass without colliding, while the gas and dust interact. These may create new stars, and severely distort the shape of one or both galaxies, forming bars, rings, or tail-like structures.
Galactic mergers: Mergers happen when the relative momentum of two galaxies is insufficient to allow them to pass through each other. Instead, they gradually merge together to form a single galaxy. For example, Milky Way will unite with the Andromeda galaxy in five billion years. Mergers between giant galaxies are either gas-rich or gas-poor. In gas-rich mergers, the galaxies are soaked with gas that ignites to form new stars. No new stars are formed in gas-poor mergers. These interactions are fairly common. So why is the collision of these four galaxies so important?
Galactic interactions are common, but most of them take place between pairs of galaxies of similar sizes. No major merger between multiple giant galaxies has been seen till now. Here, three galaxies are as big as our Milky Way, and the fourth is three times as big. And the outcome will be one of the biggest galaxies in the universe - ten times bigger than the Milky Way!
During galaxy mergers, when one of the galaxies is much bigger than the other, it is called cannibalism. The larger galaxy remains relatively undisturbed by the merger, while the smaller one is shattered. For example, our Milky Way galaxy is in the process of cannibalising the Sagittarius Dwarf Elliptical Galaxy and the Canis Major Dwarf Galaxy.
The merger will be gas-poor. So no new star will be formed. Some stars will be tossed out, and live in isolated areas outside the borders of the galaxies. And these stars may have planets! The night skies of these planets would have fewer stars and more visible galaxies.
Well, all we need to do to witness this event is to survive for a hundred million years when these galaxies would collide...