First stars may lurk in our galactic neighborhood

Astronomers dig through 13 billion years of cosmic pollution to find stellar fossils

star

OLDER THAN IT LOOKS  Some very old stars in the Milky Way, such as HE 0107-5240 (arrow), might be first-generation stars in disguise, a new study suggests.

ESO

They’re hiding among us. Some of the first stars to appear in the universe might still be lurking in the Milky Way, masked by nearly 13 billion years of cosmic pollution.

Computer simulations indicate that relatively lightweight first-generation stars might be scattered throughout the galaxy. Observations have yet to turn up any but that’s because exposure to interstellar dust and gas make the few remaining first stars look younger than they are, Jarrett Johnson, an astronomer at Los Alamos National Laboratory in New Mexico, suggests in the Nov. 1 Monthly Notices of the Royal Astronomical Society.

The first stars were made of only hydrogen and helium; they came on the scene before later generations of stars forged nearly all the heavier elements. While astronomers have found stars in the Milky Way with just traces of heavier atoms, pristine samples remain elusive.

Johnson calculated how the slow, steady rain of interstellar detritus might change the makeup of any aboriginal stars. Starlight would push back on some dust grains, he finds, but gas would be undeterred. The polluted star would end up with a relative abundance of gas elements (such as carbon and oxygen) whereas those locked away in dust (such as titanium and iron) would be mostly missing.

While the calculations make several assumptions, such as how interstellar chemistry has changed over the age of the universe, “we have to start somewhere and this is a really good effort,” says MIT astronomer Anna Frebel.

Searching for very old stars “is a numbers game,” Frebel says. There probably aren’t that many left, so astronomers have to spend years combing through the galaxy for that perfect specimen. Recent calculations indicate that researchers need to look at about 20 million stars in the Milky Way before knowing if any primordial stars are still around.

Frebel and others have found several stars that roughly resemble Johnson’s predictions for the blend of elements — though not quite. Observed candidates are relatively abundant in titanium, she notes, whereas the calculations suggest that polluted first-generation stars should have very little. 

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