The Milky Way cannibalised a neighbouring galaxy called Gaia-Enceladus 10 billion years ago3 min read


The Milky Way cannibalised a neighbouring galaxy called Gaia–Enceladus 10 billion years ago

  • Comes from a study designed to detect and age stars in the Milky Way 
  • Found the galaxy Gaia-Enceladus was a 25% the size of the current Milky Way 
  • Most stars close to the Sun date back 10 million years  and this  is when the merger is thought to have happened 

The Milky Way cannibalised a galaxy one quarter of its current mass ten billion years ago, according to new research. 

It comes from a study designed to detect and age stars when it found the large galaxy, called Gaia-Enceladus, was engulfed by the Milky Way. 

Smaller galaxies often merge to create bigger ones but when this happened in the Milky Way has long been debated. 

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Carme Gallart, from the Instituto de Astrofisica de Canarias in Spain, and colleagues have built what they say is an accurate image of the age distribution of stars in the current disk and inner halo of the Milky Way.

They find the majority of stars in the halo of the Milky Way closer to the Sun have ages ranging up to 10 billion years old.

Using simulations, the authors identified this as the point when the precursor of the Milky Way merged with one of its then companions, Gaia-Enceladus.

Research suggests the Gaia-Enceladus galaxy had about 30 per cent of the mass of stars in the main originator of the Milky Way.,

However the true ration cold be vastly different and the researchers say the finding comes with significant uncertainty.  

But it would indicate a total mass ratio of around 4:1 between the two galaxies, given the relation between stellar mass and total mass.

The authors pinpointed stars that were present before the merger and those that originated after it, using the knowledge of their exact ages.


One solar mass is equivalent to  2 times 10 to the 30th of a single kilogram. 

The entire galaxy is 1.5 trillion times greater (1.5 multiplied by ten to the power of 12) than this. 

That means the the sun weighs 3 x 10^42 kg.  

This equates to 3 x 10^39 tonnes. 

In non-mathematics, this means the Milky Way’s weight is therefore equal to 3,000 trillion trillion trillion tonnes. 

Stars that are redder in appearance due to their higher metal content, trace the original stars formed in the Galaxy pre-merger.

The researchers say the merger heated up some of the stars formed in the Galactic disk to be part of its halo.

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It also provided the Milky Way with material to create new stars and give it its current appearance.

The authors say accurate distances to individual Milky Way stars now provided by the Gaia spacecraft mission have allowed them to derive the ages.

They add: ‘Because accurate stellar ages were lacking, the time of the merger and its role in our Galaxy’s early evolution remained unclear.

‘Here we show that the stars in both halo sequences share identical age distributions, and are older than most of the thick-disk stars.

‘The sharp halo age distribution cut-off at 10 billion years ago can be identified with the time of accretion of Gaia-Enceladus to the Milky Way.

‘Together with state-of-the-art cosmological simulations of galaxy formation, these robust ages allow us to order the early sequence of events that shaped our Galaxy.’