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Scientists say they now know the precise age of our galaxy, the Milky Way.

By doing a survey of the stars, astronomers were able to find out that the Milky Way formed about 13 billion years ago.

In a new paper published in the journal Nature, Maosheng Xiang and Hans-Walter Rix of the Max-Planck Institute for Astronomy did a survey of almost 250,000 stars to work out the age of the Milky Way.

Xiang and Rix looked at stars in their subgiant phase, which is when they get larger and cooler after using up the hydrogen at their centres. The brightness and temperature of a subgiant reveals what age that star is.

Stars only spend a few millions of years in the subgiant phase so a huge survey of stars was used to find enough for the study.

The study of a quarter of a million subgiant stars, ranging in age from 1.5 billion to 13.8 billion years old, was then compared to previous studies of the Milky Way's movements.

Until recently, many astronomers believed the Milky Way's halo formed before the disk.

However, the new findings reveal the Milky Way's thick disk was likely the first structure to emerge in the galaxy, already beginning to form about 13 billion years ago, just 800 million years after the universe began.

The inner galactic disk formed about two billion years later.

The researchers of the study say not only does the information they have gathered show the age of our galaxy, but it will also be useful in learning more about how other galaxies form.