Researchers claim to have found the oldest stellar disk in the Milky Way galaxy

A team of astronomers and astrophysicists at the Chinese Academy of Sciences and the University of Toronto has found what they believe is the oldest stellar disk in the Milky Way galaxy. In their study, reported in the journal Nature Astronomy, the group used high-α stars with substantial orbital angular momentum to conduct age determinations across a wide range of stars in the galaxy.

Prior research has shown that the Milky Way started its life as a single entity. Over time, it has drawn in multiple other galaxies, making it much bigger—remnants of such galaxies have been found throughout the Milky Way. Such additions have made it difficult for astronomers to determine the original structure of the Milky Way.

In this new effort, the research team claims that by dating a large number of stars in different parts of the galaxy, they were able to narrow their focus to a particular group that they believe represents the original galaxy disk. The team named the disk PanGu.

By dating the stars in the galaxy, the researchers also found evidence that they have a combined mass of approximately 3.7 billion times that of the sun. These findings challenge prior theories because the first stars likely came to exist approximately 400 billion years after the Big Bang, which translates to 13.4 billion years ago; they could not have belonged to structured galaxies until approximately 12.5 billion years ago.

This suggests the Milky Way would not have had time to gain such mass. But the research team is insistent, suggesting further that a steady rate of star production could have resulted in the mass they calculated—the rest of the stars in the early Milky Way would have expired as supernovas.

That, the team claims, leaves the current disk with 2.2 billion solar masses—a figure that represents just 0.2% of the current mass of the galaxy, demonstrating just how much of the Milky Way is made up of material from other galaxies.

The team also suggests their evidence shows that the Milky Way reached its maximum size approximately 11 billion years ago.

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