Ancient stars born during the Cosmic Dawn have been detected in the centre of the Milky Way.

As part of a quest to uncover some of the oldest known stars in the Universe, scientists have conducted an extensive search for these ancient but elusive stars and found that the extent to which they orbit around the galactic centre is relatively calm, despite the chaos around them.

The findings, led by astronomer Anke Arentsen of the University of Cambridge in the UK, were presented at the annual National Astronomy Meeting in the UK.

We can tell how old a star is by how much metal it contains. When the first stars in the Universe formed, they had to make themselves out of the basic material available at the time - mostly hydrogen and helium. But nuclear furnaces burning in their cores began to convert hydrogen atoms into heavier things, from helium to iron.

Then, when they exploded in diffuse supernovae, they seeded these heavier elements into space, along with even heavier elements forged in energetic instantaneous supernova processes. Subsequent stars therefore began their lives with a greater proportion of heavier material. The younger a star is, the more metal it is likely to have.

Conversely, the older a star is, the less metal it is likely to have. We have found quite a few of these "pristine" stars floating around the Milky Way, but mostly on the outskirts and in the galactic halo. Astronomers think the oldest stars should be in the galactic centre, but they are harder to find because the region is predominantly metal-rich and there is a lot of dust blocking our view.

Anke and his colleagues have started a project called the Pristine Inner Galaxy Survey (PIGS) to try to find them. By analysing the spectrum of light emitted from a particular star, astronomers can find wavelengths that are amplified or dampened by the presence of certain elements. They looked for an elemental signature consistent with stars with a very low metal content and identified about 8,000 candidates.

Follow-up observations confirmed the chemical composition of the metal-poor stars, resulting in a sample of about 1,300 old stars in the galactic centre. Because they found so many stars, the researchers were then able to carry out population studies.

They used data from the Gaia observatory, an ongoing project to map the three-dimensional positions and motions of stars in the Milky Way. This allowed the researchers to determine the galactic orbits of their old stars.

Arentsen and colleagues found that the orbits of old stars around the galactic centre are relatively slow.

In addition, old stars have more chaotic orbits, but they still have an average orbit around the galactic centre.

Finally, the orbits of stars are mostly located entirely within the galactic centre. Even stars in elliptical orbits tend to stay mostly in the central region of the Milky Way.

"It is exciting to think that we are seeing stars that formed in the earliest stages of the Milky Way and were previously largely inaccessible. These stars probably formed less than a billion years after the Big Bang, so they are relics from the early Universe," says Arentsen.

"The data available for these ancient objects is growing rapidly. I'm excited to see what we will learn about these first stars that populated our Galaxy in the next few years!"

 

Source: https://www.sciencealert.com/