A mega-merger is on the cosmic cards. In roughly four billion years, our home galaxy, the Milky Way, will collide, and join forces, with its nearest large galactic neighbour, Andromeda, scientists say.
Like the Milky Way, Andromeda is a spiral galaxy – although it's bigger than our creamy coterie of suns and contains as many as a trillion or so stars.
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Andromeda and the Milky Way collide
Simulation shows what will happen when the Milky Way and Andromeda collide and come together to merge into an even bigger galaxy. Credit: ICRAR-UWA.
Viewed almost edge-on from the northern hemisphere, Andromeda appears as a fuzzy, oval patch of starlight that can just be made out on a clear night. Right now, it is 2.4 million light years away – and closing.
Gravity is pulling the two galaxies together at about 400,000 kilometres an hour. By the time you finish reading this sentence, Andromeda will be something like 200 kilometres closer to us.
The collision will mean the end of the two galaxies as separate collections of stars. But the inevitable merger into one enormous galaxy – dubbed "Milkdromeda" – need not destroy stars in either galaxy.
This is because interstellar distances are enormous: most individual stars lie more than four light years apart and so they will seldom, if ever, crash into one another. But many will be flung into other orbits around the new galactic centre.
"There will be a kind of messy sloshing motion as the galaxies fly back and forth - with the final result being known as an elliptical galaxy," said Swinburne University astrophysicist Dr Alan Duffy.
"As the two galaxies approach, the hydrogen gas of both will likely be partially stripped, forming large tails," said CSIRO astrophysicist Dr Baerbel Koribalski. "Even a few new galaxies, called tidal dwarf galaxies, may result from the event. Since both the Milky Way and Andromeda have a black hole at their centre, these would also be expected to merge." This, in turn, would produce a powerful burst of gravitational waves.
The merger will be preceded by an era of interactions. "Simulations show that the two galaxies will approach each other in orbits that get smaller and smaller until finally merging into one new, probably gas-poor galaxy," Dr Koribalski said.
"Since this 'cosmic dance' very much depends on the total mass and speed of both galaxies, the merger will also provide information on the dark matter content and distribution in both galaxies."
When the galaxies merge, the night sky will become brighter. "This is because of the extra stars from Andromeda along with the formation of new stars," said astrophysicist Dr Luke Davies of the University of Western Australia.
As happened with our sun, stars are born deep within dark clouds of interstellar gas and dust. When these clouds get squeezed together, and stirred up as massive galaxies collide, more stars are created.
As the two galaxies approach, the hydrogen gas of both will likely be partially stripped, forming large tails.Dr Baerbel Koribalski, CSIRO
"This is what'll happen when the Milky Way and Andromeda get closer than about 150,000 light years: their gravitational interactions will set off bursts of new star formation," Dr Davies said.
At the moment, stars form at the rate of more than one solar mass (the mass of our sun) a year in the Milky Way – and a little more slowly in Andromeda.
When the galaxies close in, tens of stars will be forming every year. And by the time they finally merge, there will be billions of new stars – especially massive young ones that shine more brightly than their elderly counterparts.
The Milky Way, meanwhile, is on the move. Rotating regally like a huge Catherine wheel, it's whizzing through intergalactic space at more than 200 kilometres a second towards the Virgo Cluster of more than 2000 galaxies, roughly 60 million light years away.
Our galaxy is not doing this alone. The Milky Way is orbited by dozens of dwarf galaxies, the biggest being the Small and Large Magellanic clouds, about 150,000 light years away. Together, they comprise what's known as the Local Group.
The Local Group and Virgo Cluster, in turn, are all hauled along at more than two million kilometres an hour towards the Great Attractor – an agglomeration of thousands of galaxies, about 250 million light years away, with a gravitational force equivalent to a million billion stars like our sun.
ICRAR-Curtin University scientist Dr Rob Soria said there is a small chance that, well before the merger, another star world, the Triangulum spiral galaxy, could collide with the Milky Way – or get ejected from the Local Group, of which it's the third-largest member.
"But the likeliest scenario is that, approximately 10 billion years after Andromeda and the Milky Way combine, Triangulum will unite with Milkdromeda," Dr Soria said. "Sometime after this, the swarm of dwarf galaxies will follow suit – to form a single super-galaxy."
Some of the mathematics used to produce galactic simulations is explained in Cambridge Mathematical Methods, Unit 1 and 2 (2016)
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