How Dying Stars Helped Scientists Finally Understand Our Galaxy's True Age

Astronomers observing a short-lived evolutionary phase of dying stars have concluded that parts of the Milky Way are much older than previously thought.

Using data from the Gaia mission, a team of researchers from the Max-Planck Institute for Astronomy in Heidelberg, Germany, found that parts of the thick disk of our galaxy began forming 13 billion years ago. Not only is this 2 billion years earlier than previous estimates, but it means this region of our galaxy was forming just 800 million years after the Big Bang.

The team reached its findings, discussed in a paper published in the journal Nature, by studying 250,000 stars that had reached the end of their main-sequence lifetimes. That is marked by the exhaustion of hydrogen in the star's core and nuclear fusion and energy generation ceasing in this region.

It leads to a short period called the sub-giant phase in which nuclear fusion is still happening, but only in a thin layer between the core and the star's outer layers. Following this, these outer layers will puff out, increasing the star's radius by as much as 100 times, forming a red giant star.

Our own star, the sun, will go through this process in around 5 billion years, with its radius expanding out to the orbit of Mars consuming the inner planets, including Earth.

Because this phase is so brief, it makes an excellent way for researchers to determine the age of the star undergoing it with great accuracy. Determining the age of stars usually isn't quite so easy, but it can be inferred by the proportion of elements heavier than helium, known as "metallicity," that the star possesses.

Working Out a Star's Age

Because the early Universe was just hydrogen and helium, with each subsequent generation of stars enriching it with heavier elements, the lower the metallicity of a star, the older it is. This metallicity can be measured by astronomers using China's Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). When the metallicity of these stars was combined with their brightness, astronomers could derive their age.

Before Gaia, however, this calculation had a margin of error that ranged between 20 percent and 40 percent. This uncertainty means that the ages of the stars studied could be off by as much as a billion years or more.

Max-Planck Institute for Astronomy researcher Maosheng Xiang said in a press release from the European Space Agency (ESA): "With Gaia's brightness data, we are able to determine the age of a subgiant star to a few percent."

Thus data taken from the third Gaia release and precise ages for a quarter of a million subgiant stars spread throughout the galaxy allowed the team to build a timeline of the Milky Way.

This involved dividing the Milky Way into its components, its surrounding halo—generally considered the oldest part of our galaxy—and its disk, comprising the thin disk and thick disk.

The misty band of light that represents the thin disk contains most of the stars we consider as comprising the Milky Way, while the thick disk is double the height, but has a smaller radius containing a small percentage of our galaxy's stars.

The stellar ages calculated by the team showed that the formation of our galaxy has two distinct phases The first saw the formation of stars in the thick disk, just 800 million years after the Big Bang. While the second phase saw the formation of stars, including the sun, in the thin disk.

During the first phase, the inner parts of the galactic halo may have also begun to form at this time, but this process was accelerated greatly by a collision with a dwarf galaxy (Gaia-Sausage-Enceladus) 2 billion years later. This filled the halo with stars by triggering an intense burst of star formation, with this new research suggesting that this led to the thick disk forming the majority of its stars.

The thin disk continued to form stars, with the metallicity of these stellar bodies increasing by as much as 10 times, until around 6 billion years after the Big Bang, when the gas that forms the building blocks of young stars was finally exhausted.

The team's observations implied that the early Milky Way's disk regions must have been formed from highly turbulent gas that widely dispersed elements heavier than helium throughout the galaxy.

Xiang said: "Since the discovery of the ancient merger with Gaia-Sausage-Enceladus, in 2018, astronomers have suspected that the Milky Way was already there before the halo formed, but we didn't have a clear picture of what that Milky Way looked like.

"Our results provide exquisite details about that part of the Milky Way, such as its birthday, its star-formation rate, and metal enrichment history. Putting together these discoveries using Gaia data is revolutionizing our picture of when and how our galaxy was formed."

Milky Way
A file photo of a slow-exposure of the Milky Way over Earth. By studying a quarter of a million stars in the brief phase before they expand into red giants, astronomers have revolutionized our picture of when and how the Milky Way was formed. OMAR HAJ KADOUR/GETTY