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Here's the Surprising Way the Milky Way Galaxy Got So Massive

New research suggests that more than half of all the matter in our home galaxy — and in our bodies — may have come from distant galaxies.

by David Freeman /
This image shows a pair of galaxies where "intergalactic transfer" may be occurring. Gas ejected by supernova explosions in M82 (upper left) can travel through space and eventually add to the growth of M81 (bottom right).Fred Herrmann/Northwestern University

The Milky Way galaxy we call home is a pretty colossal place up to 580 billion times more massive than the sun, according to one recent estimate.

Just where did all this matter come from? Surprising new research suggests that half of it originated in distant galaxies a finding that challenges prevailing theories about how galaxies form and that has scientists rethinking the origins of the atoms in our own bodies.

“Given how much of the matter out of which we formed may have come from other galaxies, we could consider ourselves space travelers or extragalactic immigrants,” Dr. Daniel Anglés-Alcázar, a postdoctoral fellow in astrophysics at Northwestern University and leader of the team of astrophysicists who conducted the research, said in a written statement.

The finding stems from a series of supercomputer simulations showing that exploding stars known as supernovas spew out vast amounts of gas and that the resulting “galactic winds” blast atoms across intergalactic space from one galaxy to another. The winds speed along at hundreds of kilometers per second but galaxies are so far apart that even at such stupendous velocities it would have taken billions of years for extragalactic matter to make its way to our galaxy.

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 Animation illustrating the transfer of gas from small satellite galaxies onto a central Milky Way-like galaxy. The background color scale represents the distribution of stars in the central galaxy. The green circles represent the gas transfer. Daniel Anglés-Alcázar/Northwestern University

Previous research suggested that about 10 percent of the mass in galaxies like the Milky Way came about when one galaxy merged with another, Anglés-Alcázar told NBC News MACH in an email. Finding that the amount of matter contributed by intergalactic winds was so much greater, he added, was “very surprising.”

Dr. Frank van den Bosch, a Yale University astronomer with expertise in galaxy evolution who was not involved in the new research, agreed that the magnitude of the contribution of matter from intergalactic winds was greater than previously thought.

“That is quite extreme,” he said of the finding in an email to MACH. It suggests that galaxies like the Milky Way are closely tied to their “large-scale environment” and thus should not be seen as “island universes,” he added.

Which galaxies in particular sent all this mass our way? Anglés-Alcázar said that while there was no way to know for sure, it was probably a couple of dwarf galaxies known as the Large Magellanic Cloud and the Small Magellanic Cloud. The former is located about 163,000 light-years from our galaxy, the latter about 200,000 light-years away.

In any case, the Guardian reported, the Milky Way continues to absorb about one sun’s worth of extragalactic matter every year.

The researchers plan to test the predictions of their simulations with observations made by the Hubble Space Telescope and ground-based observatories.

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