Ever since the universe was created almost fourteen billion years ago, everythings been on the move, the planets, thestars and even galaxies. Some galaxies have moved so far away, we can't see light from them because light from them has not reached us. Take ULAS J1342+0928 which at the time of writing was the furthest quasar ever spotted.
Whilst most are moving away which we can tell by the redshift of the light, there are some that are moving towards us. The Andromeda Galaxy is one such galaxy that is moving towards us at a speed of 300km/s or 402,000 kmph. This might seem an extremely fast speed but you need to take into account the distance between the two galaxies, the galaxies are around 2,500 million light years apart. The two galaxies will one day collide.
The Andromeda Galaxy is not the nearest galaxy to ours, the nearest is the Canis Major Dwarf Galaxy so called because it is located in the Canis Major constellation. The Canis Major Dwarf Galaxy will merge with ours first but compared to the Milky Way and Andromeda, it is much smaller than either of the two main galaxies. The Canis Major Dwarf Galaxy is closer to the sun than Sagittarius A*, the centre of our galaxy.
All the other nearby galaxies are dwarf galaxies, meaning they are smaller than ours. The Andromeda Galaxy is double the size of our galaxy and over twice the size of ours. Another thing that is different to ours is that Andromeda might have two rather than one nucleus at the centre. The evidence for two nucleus is a photo from Hubble space telescope that used the Wide Field and Planetary Camera (WFPC) (Lauer et al. (1993)) Look Upwards
Before you go off having nightmares that another star will smash into your house, don't worry, it won't. The collision will not happen for another four billion years so there's no need to start having End of the World plans.
We've known that this would happen since the early 1900s when Vesto Slipher measured the Radial Velocity of the Andromeda Galaxy. Radial Velocity is the speed at which something is moving. Vesto Slipher was an American astronomer who first calculated the speeds of galaxies. He calculated the distance by looking at the light spectrum, the more red it is, the object is moving away with blue meaning its coming near you. National Geographic
Its not the first time that a collision has occurred, the milky way has been involved in other galaxy collisions in the past. For the galaxy to have grown over time, it would have had to merge with other galaxies. Stack Exchange
The largest galaxy that we currently know of, IC 1101 on the borders of Virgo and the Serpens is the largest galaxy. IC 1101 has grown consuming other galaxies. It is a large spherical galaxy compared to our spiral galaxy.
Methuselah Star, located in the constellation of Libra, in addition to being possibly the oldest known star is also believed to be one that has come into our galaxy as the result of a merge between us and another galaxy.
When the galaxies do merge, the distance between stars is is so vast hat most will feel nothing more than a wobbly in their journey if anything. Some stars will collide, some will obliterate each other or one cannabalising one another such as what has happened to BP Piscium, which is believed to have eaten another star.
At the time of the merger, our Sun will have grown into its red giant star phrase meaning Earth will no longer be hospitable. Should humanity have survived that long and still be around, they would have found refuge in another star system or moved to a planet further out from the Sun, Pluto anyone?. Our Sun is believed to escape destruction.
"From Life Comes Death, From Death Comes Life"
I don't know where the origins of the above phrase is, I heard it from somewhere and have not been able to track it down. Immaterial of the source, it applies to galactic collisions. The death of the two separate galaxies will create a new galaxy. Also happening at the time of merger is a raft of new star creation.
Whilst there are no photos of what will happen, there are a few best guesses out there on the Internet. We can look at a collision that has taken place to theorise what will happen. The galaxy chosen for this is N.G.C. 2623 in the constellation of Cancer, the crab because it best illustrates what will happen when the collision occurs.
Stars that are on the outskirts of the galaxy will be flung out and become know as rogue stars, these are stars that no longer orbit a galaxy but rather float on their own in space. Instead of being an oval shape as you would see when you down on it, you will see large tails with thousands and millions of stars being flung out.
The most catastrophic thing that will happen is when the two black holes at the centres of each galaxy merge. Nearby stars will probably be destroyed or even consumed into the black hole.
In addition to the NGC 2623 galaxy collision, the Large Magellanic Cloud and Small Magellanic Cloud are believed to have collided recently, recent being in the past few hundred million years. The team of scientists that discovered this are basing the evidence on stars in the south eastern region are all moving in the same direction at the same speed.Cosmos Magazine,
The Large and Magellanic Clouds are two dwarf galaxies that are amongst our nearest neighbours. The Large Cloud is located in the constellation on the borders of Dorado. The Small Clouds is located in the constellation of Tucana.
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