ESA | Space Science | Science & Technology | Gaia | 2018 Oct 31
ESA’s Gaia mission has made a major breakthrough in unravelling the formation history of the Milky Way.
Instead of forming alone, our Galaxy merged with another large galaxy early in its life, around 10 billion years ago. The evidence is littered across the sky all around us, but it has taken Gaia and its extraordinary precision to show us what has been hiding in plain sight all along.
Gaia measures the position, movement and brightness of stars to unprecedented levels of accuracy.
Using the first 22 months of observations, a team of astronomers led by Amina Helmi, University of Groningen, The Netherlands, looked at seven million stars – those for which the full 3D positions and velocities are available – and found that some 30,000 of them were part of an ‘odd collection’ moving through the Milky Way. The observed stars in particular are currently passing by our solar neighbourhood.
We are so deeply embedded in this collection that its stars surround us almost completely, and so can be seen across most of the sky.
Even though they are interspersed with other stars, the stars in the collection stood out in the Gaia data because they all move along elongated trajectories in the opposite direction to the majority of the Galaxy’s other hundred billion stars, including the Sun.
They also stood out in the so-called Hertzprung-Russell diagram – which is used to compare the colour and brightness of stars – indicating that they belong to a clearly distinct stellar population. ...
The merger that led to the formation of the Milky Way's inner stellar halo and thick disk ~ Amina Helmi et al