How Old Is The Milky Way?

The Milky Way is a vast spiral galaxy some 100,000 light years across and is home to over 100-billion stars. It has been known for some time that the Milky Way is a very ancient galaxy, with most estimates placing its age at 10-billion years. However, thanks to a telescope called Gaia, astronomers now have the most accurate measurement of the Milky Way’s age. As it turns out, it is far older than previously thought. Data now suggests that the Milky Way began to form around 13-billion years ago, or about 800-million years after the Big Bang

Determining The Age Of A Galaxy

Milky Way illustration
Computer generated image of what the Milky Way might look like from the outside

Finding the age of the Milky Way may seem like an easy task. After all, can’t you just determine the age of the oldest stars and go from there? Although the oldest stars may offer some clues about the age of our galaxy, there are some factors to consider. For example, the oldest stars may predate the formation of the galaxy itself, or they may have formed outside our galaxy and simply been drawn in by gravity. Or perhaps they were part of another galaxy that merged with the Milky Way billions of years ago. Furthermore, knowing the precise age of the stars is difficult, and it is not always possible to determine the age of some stars. These factors have made it difficult to determine the true age of the Milky Way as they simply give any measurement margins of error that are far too high to be precise. However, there is a more accurate way to determine the Milky Way’s age.

Astronomers looked for stars that are nearing the end of their lifecycle, called subgiants. Subgiant stars contain a much higher abundance of elements whose ages can be more precisely determined, and thus the age of subgiant stars can be calculated far more accurately than most stars. However, subgiant stars are somewhat rare in the Milky Way, and so astronomers used the Gaia telescope to find as many as they could. Researchers ended up finding over 200,000 subgiant stars whose ages could be measured with a high amount of accuracy. Observations of all these subgiant stars revealed that the oldest sections of the Milky Way are around 13-billion years old. 

Evolution Of The Milky Way

Hubble deep field
The Hubble deep field shows some of the oldest known galaxies in the universe. Some of the galaxies in this image are 12-billion years old. Image credit: NASA/ESA

Gaia was able to not only determine when the Milky Way began to form, it also mapped the evolution of our galaxy by determining the age of different regions. It found that the youngest region of the galaxy, called the thin disk, formed around 6-billion years ago. Meanwhile, the oldest region, called the thick disk, formed around 13-billion years ago. These observations have revealed our galaxy to be far older than previous estimates.