The Gaia space observatory showed us where the stars are – and some aren’t from here Data: ESA/Gaia/DPAC, A. Khalatyan(AIP) & StarHorse team; Galaxy map: NASA/JPL-Caltech/R. Hurt (SSC/Caltech)

Early in the Milky Way’s history, it devoured another, smaller galaxy and made that galaxy’s stars its own. Now, astronomers have pinpointed the timing of this monstrous meal and identified which stars make up the grisly remains.

Carme Gallart at the Institute of Astrophysics of the Canary Islands in Spain and her colleagues used data from the Gaia space observatory to determine the ages of nearly 600,000 stars in our galaxy. Some of those stars are in the “thick disk”, just above or below the main disk of the galaxy, and some are in the halo, a spherical structure that extends beyond the disk.

The halo sample contained two types of stars, one with more blue stars than the other. Previous work showed that the blue stars were originally part of a smaller galaxy called Gaia-Enceladus, also known as the “sausage galaxy”, which was absorbed by the Milky Way early in its history.


Gallart and her team found that these two groups of stars had similar ages. They were probably about three billion years old when the Milky Way devoured Gaia-Enceladus about 10 billion years ago. Some of the disk stars had similar properties to the halo stars, leading the researchers to infer that they had the same origin and the merger tossed some stars from the disk into the halo.

The gas from Gaia-Enceladus probably fed stellar nurseries, the birthplace of stars, as it fell into the Milky Way – half a billion years after the merger, there was a peak in star formation. This was likely the largest and most recent galactic merger in the Milky Way’s history, Gallart says.

“It’s fascinating that we can identify the first stars that were here in the progenitor of the Milky Way and how they were modified by this merger,” says Gallart. “We’re uncovering the story of our own galaxy and how things happened at the beginning.” The Milky Way is seen as a fairly normal spiral galaxy, and it’s the only one we can study in this level of detail, so it may help us understand the formation of galaxies more generally.

Journal reference: Nature Astronomy, DOI: 10.1038/s41550-019-0829-5

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