c. 13.4 billion years ago Β· The Milky Way galaxy
The Milky Way's earliest assembly
The oldest known Milky Way stars are around 13.5 billion years old; our galaxy's core began assembling within the universe's first half-billion years.
The Milky Way did not appear in a single moment. It grew slowly, as small gas clouds, early star clusters and lesser dwarf galaxies merged under gravity. Observations from Hubble and the Very Large Telescope have identified stars in our galaxy's central regions older than 13 billion years β implying that the core of the Milky Way already existed only a few hundred million years after the Big Bang.
Over the billions of years that followed, the galaxy kept building itself. The "stellar streams" still visible in the halo are debris from ancient mergers; ESA's Gaia mission has traced fossil motions left by the Milky Way's merger with a dwarf galaxy known as Gaia-Enceladus around 10 billion years ago. The spiral disc, the barred bulge and the surrounding halo we see today are the long, layered product of these collisions.
The Solar System will be a relatively late arrival to this structure β kindled in an interstellar cloud along one of the spiral arms only some 4.6 billion years ago.
Sources
- Milky Way's central bulge formed early and rapidly β European Southern Observatory
- Gaia uncovers a major event in the formation of the Milky Way β European Space Agency
- Milky Way Galaxy β Encyclopaedia Britannica