It took 13.8 billion years of cosmic evolution to get us here. Generations of stars had to live and die to create heavy elements; tiny protogalaxies had to merge to form the Milky Way; clouds of interstellar gas would collapse and form new stars with solid planets; complex inorganic and organic chemistry had to make friends on one of these new worlds; biological evolution - and natural disasters - had to follow one of the winding paths for humans to eventually emerge just a few thousand years ago.
Over the past 12,000 years or so, we have created the agriculture, science, countries and all modern civilization that we know today. It is an exciting journey that has transformed our world and, thanks to humanity's space program, is transforming our solar system.
But the world we enjoy today, no matter what we do, will not last forever. A number of earthly events should change the state of affairs in our world and make the Earth completely unrecognizable for everyone who lives today. In about 60,000 years, the sun and stars will move and our present constellations will disappear from the sky. In another 100,000 years, we are likely to enter a new ice age for reasons that have little to do with human activity. And before another million years pass, Earth's volcanoes will forever change the Earth's landscape.
But all this is a trifle compared to what the Universe is preparing for us.
In a little less than four billion years, the Andromeda galaxy (and possibly the Triangle galaxy) will merge with our Milky Way galaxy, dramatically altering the galaxy's structure and the appearance of the night sky. Now it is 2.5 million light years from us and is moving at a speed of 43 km / s, which means the first collision will occur in 3.8 billion years, and in 5.5 billion years the merger will be complete. Gravity will cause the entire local group of galaxies to merge with ours into one giant elliptical galaxy Milkomed. On a large cosmic scale, all other galaxies will continue to move away from us until they completely disappear from our field of view - after about 100 billion years.
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All this time, our solar system will remain in perfect order, unless it looks different. The sun will continue to heat up as it ages until it ends life on Earth in 1–2 billion years, boiling our planet's oceans. After another 5-7 billion, the core of the Sun will run out of nuclear fuel, and our home star will become a red giant, absorbing Mercury and Venus in the process. Due to its special stellar evolution, the Earth-Moon system is likely to be pushed away and lucky enough to escape the fiery fate of our inner neighbors.
After the Sun burns out the remaining nuclear fuel - mostly helium - its outer layers will swell into a planetary nebula, and the core will shrink until it becomes a white dwarf. This is the ultimate fate of almost all stars in our universe. But the planets will still be here, orbiting our cool, dim remnant of a star for another 9.5 billion years (counting from now).
All this time, the Earth will continue to revolve around the Sun, and the Moon will exert gravitational pull on it, which will cause a torque. Therefore, the Moon will move further from the Earth, while slowing down the Earth's rotation. This slowdown will be almost imperceptible; the Earth's rotation will slow down by some 1.4 milliseconds over a hundred years. But after 50 billion years, the Moon's orbital period will be 47 days (now 27.3 days), and our 24-hour day will have to slow down to match this: the day will become 47 times longer in 50 billion years. By that time, the Earth and the Moon will become tidally locked, meaning the Moon will always appear in the same place in the sky.
As star formation continues, dying stars will dump their fuel into interstellar space and failed stars will merge together. In this case, the amount of material for making stars will be limited. Even the longest-lived stars will last for some 100 trillion years (1014), and after a quadrillion years (1015) of star formation, they will dry out completely. Only occasional collisions or mergers between failed stars or their remnants will illuminate our galaxy; otherwise, the process will plunge her into cold and darkness. Finally, white dwarf stars will turn black as they cool down and release their energy. Yes, it will take a long time (about 1016 years), a million times the current age of the universe. The atoms will still be there, but their temperature will be slightly above absolute zero. It is then that the night sky will be really dark and black, without any visible light, since all the stars will cease to exist. At least in our local group of galaxies.
How long would it take for our black dwarf (which was once our sun) to meet another, merge with him and bring him to life? Between us, Andromeda, and the rest of the local group, there are about a trillion stars and stellar remains. In this chaotic system, an ordinary system of stars can collide with nothing and no one for a long, long time, but we have time. After 1021 years, a black dwarf in the center of our solar system will randomly collide with another black dwarf, initiate a Type Ia supernova explosion and destroy what remains of our solar system.
This will be the final fate of many stars of our local group, but not all, and even, probably, not ours. There is another process that will be more effective, and therefore more likely for us: gravitational pushing out of the local group due to the process of forced relaxation. When there are several bodies in a gravitationally chaotic orbit, one of them is ejected once, leaving the others more closely related. This occurs in globular clusters over time and explains why they are so compact and also why there are so many coalesced old stars in the cores of these ancient relics.
A gravitational ejection occurs about 100 times more often than an accidental merger, which means that our star and the rest of the associated planets will probably be thrown into the abyss of already empty space in about 1019 years. But nothing lasts forever, not even space. Every orbit - even the gravitational orbits in general relativity - slowly decay over time. It can take a very long time, perhaps 10,150 years, but eventually the Earth's orbits will fall apart and spiral towards the central mass of our solar system. This will be our fate if we are thrown out.
But if we remain in the giant galaxy that Milkomed will become, we are not destined to end up in the black hole in the center of the galaxy. It will take 10,200 years for this to happen, but black holes don't last that long. They slowly evaporate as Hawking radiation. Thanks to this decay, even the most massive black holes in the Universe will live no more than 10100 years, and a black hole of solar mass - some 1067 years.
After the collapse of the black hole, only dark matter will remain, which means that the Earth will rush to the black dwarf, which was once our Sun. Regardless of how many times our world may have been and ended up on fire, our ultimate destiny is to freeze in a cold, empty universe. All will pass. And that too.
ILYA KHEL