Astro-boffinry world rocked to its very core: Shock as Andromeda found to be not much bigger than Milky Way

Everything we knew was a lie

The Andromeda galaxy is actually roughly the same size as the Milky Way, and may not engulf our galaxy when it is expected to collide in about four billion years time, according to new research.

In other words, no, Andromeda is not the vastly larger sprawling galaxy we all thought it was.

A paper published earlier this week in the Monthly Notices of the Royal Astronomical Society calculated the velocity an object would need to travel in order to flee the gravitational grip of Andromeda. By measuring the escape velocity, scientists have recalculated the galaxy’s mass and size.

Prajwal Kafle, lead author of the paper and an astrophysicist at The University of Western Australia, said on Thursday: "When a rocket is launched into space, it is thrown out with a speed of 11 [kilometers per second] to overcome the Earth's gravitational pull.

"Our home galaxy, the Milky Way, is over a trillion times heavier than our tiny planet Earth so to escape its gravitational pull we have to launch with a speed of 550 kilometers per second. We used this technique to tie down the mass of Andromeda."


The escape velocity for Andromeda is about 470 kilometers per second – a lower than expected value, which suggests scientists overestimated the amount of mass it contains.

"By examining the orbits of high speed stars, we discovered that this galaxy has far less dark matter than previously thought, and only a third of that uncovered in previous observations," Kafle said.

Andromeda - also known as M31 - was considered to be about massive spiral galaxy about two or three times bigger than the Milky Way. As the closest neighbouring galaxy to us, light takes about two million years to arrive.

The new measurement estimates that Andromeda holds a mass equivalent to about 800 billion times the mass of our Sun, a figure similar to the Milky Way’s total mass at about 700 billion solar masses.

"It completely transforms our understanding of the local group," Kafle said. The local group is a cluster of more than 50 nearby galaxies, mainly made up dwarf galaxies.

"We had thought there was one biggest galaxy and our own Milky Way was slightly smaller but that scenario has now completely changed. We can put this gravitational arms race to rest," he said.

Andromeda and the Milky Way are zooming closer to one another day by day. They’re expected to collide about four billion years from now. But now that both their masses are similar, it changes the dynamics of the interaction and new computer simulations will be needed to model the impact. ®

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