The Milky Way began forming approximately 13 billion years ago, only about 800 million years after the Big Bang. That makes our galaxy nearly as old as the universe itself, which is 13.8 billion years old. But “formed” is a bit misleading, because the Milky Way wasn’t built all at once. It assembled itself over billions of years through waves of star formation, massive collisions with smaller galaxies, and the slow growth of the spiral disk we live in today.
The Earliest Stars and the Thick Disk
The oldest components of the Milky Way date back roughly 13 billion years. A 2022 study published in Nature, using precise age measurements of nearly 250,000 stars, found that the galaxy’s thick disk started forming approximately 13 billion years ago. That places its origin just 800 million years after the Big Bang. Some of the most ancient stars found in the galactic halo have chemical signatures suggesting they formed when the universe was barely 300 to 500 million years old, at a time when the tiny proto-galaxies that would eventually merge into the Milky Way had only just begun producing their first generations of stars.
These earliest stars were extremely metal-poor, meaning they formed from gas clouds that contained almost nothing heavier than hydrogen and helium. The chemical fingerprint of these stars tells astronomers how far along the galaxy’s internal recycling had progressed. By the time the universe was about 3 billion years old, the Milky Way’s precursor galaxies had already built up measurable levels of heavier elements, setting the stage for more complex chemistry and, eventually, rocky planets.
The Collision That Shaped the Galaxy
Around 10 to 11 billion years ago, a smaller galaxy slammed into the early Milky Way. Astronomers call this object Gaia-Enceladus-Sausage, named partly after the ESA’s Gaia space telescope that revealed its traces and partly after the sausage-shaped pattern its stars leave on orbital charts. The merger was significant: the incoming galaxy was roughly one-quarter the mass of the Milky Way at the time.
This collision did two important things. First, it stirred up the existing disk of stars, puffing it outward and contributing to what we now call the thick disk, a broader, older layer of stars that sits above and below the galaxy’s main plane. Second, it dumped enormous amounts of gas into the Milky Way, triggering a burst of new star formation. Most of the stars in the inner galactic halo formed around this period. The merger essentially reshaped the galaxy’s structure and jumpstarted a new phase of growth.
Building the Thin Disk
After the violence of the Gaia-Enceladus merger settled, the Milky Way entered a calmer period. Gas that had been heated and disrupted gradually cooled and flattened into a thinner, more orderly disk. This thin disk is where the Sun and most of the galaxy’s younger stars live today.
The thin disk was long thought to have started forming around 8 to 10 billion years ago, but recent research has pushed that date earlier. Ancient stars found within the thin disk suggest it may have begun taking shape less than a billion years after the Big Bang, roughly 4 to 5 billion years earlier than previous estimates. That doesn’t mean the thin disk looked anything like it does now at that point. It took billions of years of steady star formation, punctuated by bursts of activity, for the disk to fill out into the spiral structure we recognize.
Our Sun, at 4.6 billion years old, is a middle-aged resident of this thin disk. It formed long after the galaxy’s major structural pieces were in place.
Later Mergers and Ongoing Growth
The Milky Way’s construction didn’t stop after the thin disk formed. Around 6 billion years ago, a noticeable dip in star formation occurred, followed by a disruption in the galaxy’s chemical evolution. This event lines up with the first infall of the Sagittarius dwarf galaxy, a smaller galaxy that has been spiraling into the Milky Way and passing through its disk repeatedly ever since. Each close pass appears to have triggered new waves of star formation, like a rock periodically disturbing a pond.
Data from the Gaia space telescope shows that the Milky Way’s star formation has been “bursty” at all ages. Rather than a smooth, steady production of new stars, the galaxy has gone through repeated cycles of activity and quiet. About 4 billion years ago, star formation dipped again before picking back up to a rate that continues today. The galaxy still forms roughly one to two new stars per year, mostly in the spiral arms of the thin disk.
A Timeline of the Milky Way’s Formation
- 13 billion years ago: The thick disk begins forming, just 800 million years after the Big Bang. The earliest proto-galactic fragments are already producing metal-poor stars.
- 11 billion years ago: The inner galactic halo assembles, about 2 billion years after the thick disk started forming.
- 10 to 11 billion years ago: The Gaia-Enceladus-Sausage merger reshapes the galaxy, heating the thick disk and triggering major star formation.
- 8 to 10 billion years ago: The thin disk grows steadily, with star formation proceeding in bursts of varying intensity.
- 6 billion years ago: The Sagittarius dwarf galaxy begins falling into the Milky Way, causing repeated disruptions and star formation episodes.
- 4.6 billion years ago: The Sun forms in the thin disk.
The Milky Way is not a galaxy that was born in a single moment. Its age of roughly 13 billion years marks when the first pieces started coming together, but the galaxy has been assembling, merging, and rebuilding itself across nearly the entire history of the universe.