If the Universe is expanding and everything separates from everything else, why do galaxies collide with each other?
“Because without collision forces, there is no movement and there is no reality”
Heraclitus of Ephesus (535-475) BC
A century ago, in 1922, the Russian mathematician Aleksandr Fridman published an article where he presented an expanding Universe, contrary to the static Universe published by Albert Einstein.
Einstein warned that Fridman was wrong, but the Russian mathematician, displaying great chivalry, wrote to him showing him his calculations and asking him to allow their publication if they were correct. Einsteinafter studying the letter, accepted his mistake: the Universe is in motion and expanding.
By then, Albert Einstein was already the great renowned scientist we know today, winner of the Nobel Prize (1921), but he had the strong enough to admit his mistake.
The demonstration came in 1927 by Georges Lemaitre and then in 1929 by Edwin Hubble, who observed the galaxies in free flight, the Universe in expansion.
The best example to understand the expansion of the Universe is to paint several dots on a globe with a marker, each dot represents a galaxy. Then we inflate the balloon and the dots will separate more and more with each blow.
And while this is true, it is also true that galaxies collide with each other. How can it be if they are separating?
On our globe, we paint the galaxies as little dots separated from other little dots, but the Universe is not like that, in reality the galaxies are not isolated, but rather forming groups, supergroups and galactic filaments, of tens, hundreds or thousands of galaxies.
In our balloon we should paint groups of dots and many of them joined by lines.
It is within the galactic groups, supergroups and filaments that the gravity of the galaxies themselves acts and leads them to attract and collide.
We should not be afraid of making mistakes, even the planets collide and from chaos the stars are bornCharles Chaplin
Our Sun is a star, one of the 200 billion to 400 billion that make up our galaxy, the Milky Way.
Our galaxy belongs to a group of galaxies called the Local Group. Dominated by three large galaxies: the Andromeda, the Milky Way and the Triangulum, plus about 50 small galaxies.
The gravity of the galaxies causes the Milky Way and the Andromeda Way to move towards colliding, at a speed of between 100 and 300 km/s, no, it is not the speed of light, that is 300 thousand km/s.
The collision will start in 4 billion years and will end 2 billion years later. Galactic collisions don’t happen in seconds like between cars.
For now, the distance between the two galaxies is enormous, 2.5 million light years. That is, the light from the Andromeda galaxy left it 2.5 million years ago, traveled through space at the speed of light, and is barely reaching us.
When two galaxies collide, it is unlikely that the stars and planets will collide with each other, as there is a lot of space between the stars. The collision would rather be a crossing of galaxies. But the gravity of stars and galactic nuclei would warp galaxies, throwing stars and their planets everywhere. Nebulae would collide, and those pressures would lead to massive star birth. The collision would end millions of years later, when both galaxies merge into one, forming a huge new galaxy, with old and new stars or in formation.
Many stars and planets expelled from the galaxies would be left wandering adrift. It is obvious that if there is life on a planet, being torn from its star, life would not prosper.
Thanks to large telescopes like Hubble, we have snapshots of galactic collisions that are happening or happened millions of years ago.
The Dolphin or Hummingbird Galaxy (NGC 2936) has been ripped apart by the galaxy below (NGC 2937). It is notorious how in the head and trunk of the dolphin there is a burst of star birth. Both galaxies look like a penguin protecting its egg. They are located 300 million light years from us, towards the constellation of Hydra.
In the first image: 230 million light-years away in the constellation of Hercules, the Hubble Telescope photographed two colliding galaxies, one embedded on top of the other, NGC 6052.
In the second image: The galaxy NGC 2207 collides with IC 2163, 80 million light years away, in the constellation Canis Major. We look at what happened 80 million years ago.
In the third image: NGC 2623, two apparently merging galaxies, show elongated and shattered star arms. They are located 250 million light years away, in the constellation of the Crab (Cancer).
In the fourth image: The Whirlpool galaxy M51, 31 million light years away, in the constellation of The Hunting Dogs. The small galaxy M51B on the right rips an arm out of the spiral galaxy.
Galactic collisions are a common event in the Universe, they shape the next generations of stars and galaxies. In the photographs we admire collisions frozen in time.
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Star Wars or why galaxies collide