The galactic center as seen through the infrared eye of the Spitzer Space Telescope. (NASA/JPL-Caltech/S. Stolovy)
Ancient stars born during the Cosmic Dawn have been identified in the center of the Milky Way.
As part of a survey to uncover some of the oldest known stars in the Universe, scientists conducted a comprehensive search for these ancient, but elusive, stars, and found that the measures they spin around the galactic center are relatively sedate, in spite of the chaos around them.
Their findings, led by astronomer Anke Arentsen of the University of Cambridge in the UK, have been presented at the UK's annual National Astronomy Meeting.
We can tell how old a star is based on how much metal is in it. When the first stars in the Universe formed, they had to make themselves out of the elemental material available at the time – mostly hydrogen and helium. But the nuclear furnaces burning in their cores started fusing the atoms of hydrogen into heavier things, from helium all the way up to iron.
Then, when they exploded in messy supernovae, they seeded these heavier elements throughout space, along with even heavier elements forged in energetic split-second supernova processes. Subsequent stars therefore started their lives with a greater proportion of heavier material. The younger a star is, the more metal it's likely to have.
Conversely, the older a star is, the less metal it's likely to have. We've found a fair few of these "pristine" stars floating around the Milky Way, but mostly in the outskirts, and the galactic halo. Astronomers think that the oldest of the old stars should be hanging out in the galactic center, but because the region is predominantly rich in metal, and there's a lot of dust blocking our view, they tend to be harder to find.
Anke and her colleagues embarked on a project called the Pristine Inner Galaxy Survey (PIGS) to try and find them. By analyzing the spectrum of light emanating from a certain star, astronomers can find wavelengths amplified or dampened by the presence of certain elements. They looked for an elemental signature consistent with stars that are very low in metals, and identified around 8,000 candidates.
Follow-up observations confirmed the chemical compositions of metal-poor stars, resulting in a sample of around 1,300 ancient stars in the galactic center. Because they found so many, the researchers were then able to conduct population studies.
They used data from the Gaia observatory, which is an ongoing project to map the three-dimensional positions and motions of the stars in the Milky Way. With this, the researchers were able to determine the galactic orbits of their old stars.
Arentsen and her colleagues found that orbits of the ancient stars around the galactic center are relatively slow.
In addition, the older stars have more chaotic orbits, but they still have an average orbit around the galactic center.
Finally, the orbits of the stars are mostly contained entirely within the galactic center. Even those stars on elliptical orbits tend to stay mostly within the central region of the Milky Way.
"It is exciting to think that we are seeing stars that formed in the earliest phases of the Milky Way, previously largely out of reach. These stars likely formed less than a billion years after the Big Bang, so are relics from the early Universe," Arentsen says.
"The available data for these ancient objects is growing rapidly. I'm excited to see what we will learn about these first stars to populate our Galaxy in the next few years!"
You can follow the team's discoveries about these ancient stars on preprint server arXi.
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