Another Galaxy Was Absorbed Into Ours After Colliding With The Milky Way 10 Billion Years Ago

by : Hannah Smith on :
Danny Horta-Darrington (Liverpool John Moores University), ESA/Gaia, and the SDSS

There’s so much about the universe that we still don’t know, and recently scientists have discovered another one of its secrets lying right on our doorstep.

A ‘fossil galaxy’ has been found hidden inside the Milky Way, and it could change our understanding of the history of our galaxy.


The long-dead galaxy, known as Heracles, is thought to have been absorbed by the Milky Way after the two collided 10 billion years ago.

Scientists at John Moores University in Liverpool made the discovery after they observed a number of stars in our galaxy with vastly different chemical compositions and velocities. The team studied tens of thousands of stars, and found a few hundred whose properties meant that they ‘could only have come from another galaxy.’


The remnants of the fossil galaxy make up a significant proportion of the Milky Way’s halo, but scientists say they only caught sight of its existence thanks to more detailed information collected about the galaxy’s stars as part of an ongoing project that has mapped roughly half a million stars across the Milky Way.


Dr Ricardo Schiavon, who led the research with the Apache Point Observatory Galactic Evolution Experiment – or Apogee – project said:

To find a fossil galaxy like this one, we had to look at the detailed chemical make-up and motions of tens of thousands of stars.

That is especially hard to do for stars in the centre of the Milky Way, because they are hidden from view by clouds of interstellar dust.

Apogee lets us pierce through that dust and see deeper into the heart of the Milky Way than ever before.


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Apogee’s findings were recently published in the journal Monthly Notices of the Royal Astronomical Society. Heracles was named by the astronomers after the Greek hero who was given the gift of immortality after the Milky Way was created.

PA Images

After studying the composition of these unusual stars, the team was able to trace the precise history and location of the fossil galaxy, which was absorbed in the early days of Milky Way. Researchers say that, while smaller galaxies often merge with one another, for such a collision to occur between two large galaxies is highly unusual, adding that it was likely a ‘major event’ in the history of our home system that sets it further apart from other galaxies.

In total, Dr Schiavon says that roughly 100 million stars are leftover from the fossil galaxy, accounting for about half of the Milky Way’s mass. That seems like a lot, but with the ESA estimating that the Milky Way alone contains roughly 100 thousand million stars, this discovery shows just how much about our galaxy is still a mystery.

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Topics: Science, Milky Way, Now, Space


The Independent
  1. The Independent

    'Fossil Galaxy' found hidden in the Milky Way