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When the Andromeda galaxy crashes into the Milky Way,


Village Elder
... this is what it could look like from Earth

The Andromeda galaxy is our largest galactic neighbor. It's just 2.5 million light-years away. Andromeda and the Milky Way are creeping closer together every minute.

Source: NASA
NASA's Hubble Space Telescope discovered in 2012 that Andromeda was on track to collide with the Milky Way in 3.9 billion years.

Source: NASA
Measurements from the European Space Agency's Gaia space telescope suggest a different timeline, though. That data indicates the collision might just be a glancing blow 4.5 billion years from now.

Even if that is the case, the galaxies will merge over time, first in a chaotic mess of stars altering each others' orbits. Eventually, they will settle into one stable mega-galaxy. Here's what NASA expects that to look like from Earth.

Source: National Geographic, NASA
Today the Milky Way looks like a band across the night sky. Andromeda is a mere light in the distance — the only other galaxy we can see without the help of a telescope.

In 2 billion years, Andromeda will loom much larger in the night sky.

In 3.75 billion years, Andromeda will fill the sky, and its tidal pull will begin to distort the Milky Way, according to Hubble's measurements.

About 3.9 billion years from now (by NASA's estimate), the galaxies will make their first close pass. Because galaxies are mostly empty space, they will pass through each other with very few collisions between stars, if any.

The close approach will, however, compress gas in that interstellar space. The sky will glow bright and colorful as the compressed gas and dust collapse to form new stars.

In 4 billion years, both galaxies will be stretched out and warped as they briefly drift apart again.

As gravity brings the galaxies back together 5.1 billion years from now, their cores — where ancient, swollen stars live — will be visible in the night sky. The period of rapid star formation will be over.

In about 7 billion years, the cores of the two spiral-shaped galaxies will merge, forming one oval-shaped galaxy. The night sky will be overwhelmed by light from the cluster of the two galaxies' largest, oldest stars.

But by the time Andromeda approaches, "our sun will have become a white dwarf and bye bye life on Earth," Didier Queloz, a physicist and Nobel laureate, said on Twitter.

That's because the sun, in its slow process of dying, will have swollen past Venus's orbit and roasted the Earth.
"But don't worry, other life on other stars will have flourished by then," Queloz added.


Village Elder
Earth will be dead by then since sun will destroy life on it less than a billion years.

In about one billion years, the solar luminosity will be 10% higher than at present. This will cause the atmosphere to become a "moist greenhouse", resulting in a runaway evaporation of the oceans.. no water no life.

The only hope for long term survival is if humans manage to become a multi planetary species which means they move to Mars for a few million years as earth becomes unbearably hot and then later to the moons of jupyter. This is actually very likely given the incredible work Elon musk and NASA are doing.

Beyond a few billion years, humans will have to look for planets outside the solar system since the sun itself will run out fuel and grow into a giant ball that engulfs most of the solar system. there will be tons of options for humans to work with and they will have all the time "on earth " to figure it out, so good luck to the future generations