What will happen when the Milky Way smashes into Andromeda?

When our galaxy collides with Andromeda in about 4 billion years, there will be no crying over spilt milk.

Astronomers have long believed the Milky Way would be torn apart and lapped up by its big brother, but new research suggests the two galaxies are actually about the same size.

Australian scientists say the amount of dark matter in Andromeda is far less than previously estimated - about two-thirds less.

"By examining the orbits of high-speed stars… suddenly 50 years of collective understanding of the local group has been turned on its head," astrophysicist Prajwal Kafle said in a statement.

"We had thought there was one biggest galaxy and our own Milky Way was slightly smaller, but that scenario has now completely changed."

With Andromeda and the Milky Way on a collision course, he says new calculations will have to be done to figure out what will happen.

It won't be Andromeda's first mashup, however. Another new study suggests it was formed when two smaller galaxies bumped into each other between only 1.8 billion and 3 billion years ago, when life on Earth was already underway.

Astronomers in France and China ran simulations of the "physical mechanisms of the Andromeda formation… lifting the veil on its origin".

Video of the simulation shows the two galaxies passing each other a few times, ripping stars away, before finally merging.

Their research explains why some of Andromeda's stars orbit the galactic centre erratically, while in the Milky Way, they all go in the same direction.

The Milky Way is estimated to be more than 13 billion years old - there from almost the very beginning of the universe.

Both studies were published in Monthly Notices of the Royal Astronomical Study on Thursday (NZ time).


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