ESA’s Gaia mission has collected invaluable data about the Milky Way galaxy, and astronomers discovered that a ‘thick disc’ began forming 13 billion years ago, approximately 2 billion years earlier than expected, and just 0.8 billion years after the Big Bang. More specifically, astronomers took brightness as well as positional data from Gaia’s Early Data Release 3 (EDR3) dataset and then combined it with measurements of the stars’ chemical compositions for around 250,000 stars to derive their ages. Read more for another picture and additional information.
The Milky Way galaxy is composed of different components, split into the halo and disc. The former is the spherical region surrounding the disc, and has traditionally been thought to be the oldest component of the galaxy. The disc is basically composed of two parts: the thin disc and the thick disc. The thin disc consists of the stars that we see as the misty band of light in the night sky that we call the Milky Way, while the thick disc is more than double the height of the former, but smaller in radius, containing far fewer stars in the solar neighborhood.
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The stellar ages clearly revealed that the formation of the Milky Way fell into two distinct phases. In the first phase, starting just 0.8 billion years after the Big Bang, the thick disc began forming stars. The inner parts of the halo may also have begun to come together at this stage, but the process rapidly accelerated to completion about two billion years later when a dwarf galaxy known as Gaia-Sausage-Enceladus merged with the Milky Way,” said the ESA.