Although our galaxy survived, it has never been the same. The smash up also funneled new gas toward the galactic center, adding fuel that mixed and mingled with the Milky Way's existing reservoirs to form new generations of stars.
Over time the dwarf galaxy faded away, but the scars from its collision never really disappeared-not that they have been easy to find.
RG: What does this mean for our understanding of the universe 10 billion years ago?
"The other thing we'd like to do is go beyond this 10 billion years to earlier and earlier and see if we can find evidence of mergers that took place early on and what those mergers looked like", Helmi said. Helmi has been engaged in the development of the Gaia mission for over 20 years and was a member of the data validation group at the second data release.
But thanks to the recent release of Gaia data - the most detailed and accurate map of the sky to date - a team of astronomers has been able to make much more detailed observations of the stars in the inner halo.
"The Milky Way is our home, and people like to know their origins; they like to know their own history".
Stars on the outskirts of our galaxy was "aliens". The chemical composition showed that the backward moving stars do not contain the same heavy elements as stars like our own.
Although Gaia-Enceladus is expected to have been about ten times smaller than the current Milky Way, it was still large; large enough to have collections of stars called globular clusters trailing after it.
Helmi: This is hard to say.
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"This paper is suggesting that the stellar halo is dominated by the cannibalisation of at least one fairly large dwarf galaxy", says astrophysicist Gurtina Besla of the University of Arizona. And it is one that relies on a single-yet massive-merger.
Helmi and her colleagues analyzed 50,000 Gaia stars within the Milky Way's halo, finding 33,000 of them share similar amounts of angular momentum and orbit in the opposite direction than they should. But for her, the implications of research like this stretch far beyond the Milky Way, she said. And then you have these stars that have made a decision to move in the opposite sense. Exactly which element is formed depends on the star.
Brown: We discovered the remnants of a significant merger between the Milky Way and another smaller galaxy which happened some 10 billion years ago in the Gaia DR2 data.
The galaxy that researchers believe collided into ours has been named Gaia-Enceladus, after the Greek goddess and her offspring.
There's now more work to be done, trying to locate the stars left over from other collisions - and learning more about the Enceladus event. "And you never know when you see a merger in another galaxy, if it's just anecdotal". The sheer confirmation alone has sent many astronomers into a state of euphoria.
"The youngest stars of Gaia-Encelado are actually younger than the native Milky Way stars in what is now the thick disc region". "At almost three million light-years away, we're never going to Andromeda to populate it or study it in detail", says Kim Venn, an astronomer at the University of Victoria in British Columbia who was not involved in the study. It shook the Milky Way, changing the structure of the galactic disk, setting off bursts of star formation in its wake. This merger probably contributed to the formation of the so-called thick disk of the Milky Way.
Helmi and her colleagues drew on data from a European project called Gaia, which is mapping the location of 1 billion stars in the Milky Way with unprecedented precision. Indeed, Helmi wants to use the Milky Way's evolution "as if it were a Rosetta stone"-to better understand how galaxies across the universe evolve and how they affect the cosmos".
The Milky Way's halo is not like the ones of other spiral galaxies.