They find, in our own galaxy, three of the oldest stars in the Universe

A team of astronomers from the Massachusetts Institute of Technology (MIT) has just found three of the oldest stars known in the entire Universe in the halo of our own galaxy. With ages between 12,000 and 13,000 millions of years ago, these three survivors, which are located about 30,000 light years from Earth, already existed when our Milky Way began to exist. And they were each part of ultra-faint dwarf galaxies, among the first to emerge in the early Universe and which ended up being absorbed by our galaxy a long time later.

Similar, intact galaxies are still observed today. They are a treasure trove of scientific data, but they are too far away and faint for astronomers to study in detail. For this reason, these three old stars found in our own ‘home’ are a real treasure, since they constitute a close sample of these very ancient structures, which can be studied without having to chase them over enormous distances in the confines of the Universe. The study has just been published in ‘Monthly Notices of the Royal Astronomical Society’.

A valuable surprise

The Milky Way, our home in space, contains between 200,000 and 400,000 million stars, measures about 100,000 light years from end to end, and contains practically everything we can see with the naked eye. Around it, beyond the dense spiral of stars that give it its shape, there is an extensive halo that completely surrounds it and that, although to a lesser extent, also contains stars. It was there, in the halo, where these three stars, which are not together, but separated by great distances, first caught the attention of astronomers in 2022, who upon verifying their extreme antiquity immediately wondered how they could have gotten there. stop a galaxy like ours.

Therefore, the researchers decided to study the movement of these stars in depth using data from the European Gaia satellite, and discovered that, surprisingly, all three were going ‘in the opposite direction’ to that of most other stars in the Milky Way. Astronomers call this ‘retrograde motion,’ and it’s a clue that these objects didn’t form in the galaxy, but came from somewhere else.

“The only way for stars to go in the opposite direction to the rest of the group,” says Anna Frebel, co-author of the study, “is if you throw them in the wrong direction.”

The chemical composition of the three stars, with very few components, revealed that they were born at a time when many of the elements had not yet been formed, so the stars must have been part of some weak and tenuous galaxy located at most about a billion years after the Big Bang.

Searching for more relics

Curious, the researchers tried to find other stars that also traveled in the opposite direction, and after an exhaustive review of the available scientific literature they found another 65, which also shared the fact of having a limited variety of chemical elements.

«Interestingly – says Frebel – they are all quite fast, and they go in the opposite direction at hundreds of km per second. They seemed to be running away! “We don’t know why that is, but it was the missing piece of the puzzle that I didn’t anticipate when we started the study.”

Researchers want to find more similar stars, and now that they have the ‘recipe’ to achieve this (first look for stars with low chemical abundance and then check if they move in the opposite direction to the rest), they will try to find them without having to leave our own galaxy. In this way, Frebel and his colleagues are convinced that they will find a ‘small but significant’ number of the oldest stars in the Universe without having to look for them 13 billion light years away, close to the Big Bang and where they are extremely difficult to find. study.

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