Astronomers Say They Have Spotted the Universe’s First Stars | Quanta Magazine

Astronomers Say They Have Spotted the Universe’s First Stars | Quanta Magazine

A group of astronomers poring over data from the James Webb Space Telescope (JWST) has glimpsed light from a rare isotope of helium in a distant galaxy, which could indicate the presence of the universe’s very first generation of stars.

These long-sought, inaptly named “Population III” stars would have been ginormous balls of hydrogen and helium sculpted from the universe’s primordial gas. Theorists started imagining these first fireballs in the 1970s, hypothesizing that, after short lifetimes, they exploded as supernovas, forging heavier elements and spewing them into the cosmos. That star stuff later gave rise to Population II stars more abundant in heavy elements, then even richer Population I stars like our sun, as well as planets, asteroids, comets and eventually life itself.

“We exist, therefore we know there must have been a first generation of stars,” said Rebecca Bowler, an astronomer at the University of Manchester in the United Kingdom.

Now Xin Wang, an astronomer at the Chinese Academy of Sciences in Beijing, and his colleagues think they’ve found them. “It’s really surreal,” Wang said. Confirmation is still needed; the team’s paper, posted on the preprint server on December 8, is awaiting peer review at Nature.

Even if the researchers are wrong, a more convincing detection of the first stars may not be far off. JWST, which is transforming vast swaths of astronomy, is thought capable of peering far enough away in space and time to see them. Already, the gigantic floating telescope has detected distant galaxies whose unusual brightness suggests they may contain Population III stars. And other research groups vying to discover the stars with JWST are analyzing their own data now. “This is absolutely one of the hottest questions going,” said Mike Norman, a physicist at the University of California, San Diego who studies the stars in computer simulations.

A definitive discovery would allow astronomers to start probing the stars’ size and appearance, when they existed, and how, in the primordial darkness, they suddenly lit up.

“It’s really one of the most fundamental changes in the history of the universe,” Bowler said.

Population III

About 400,000 years after the Big Bang, electrons, protons and neutrons settled down enough to combine into hydrogen and helium atoms. As the temperature kept dropping, dark matter gradually clumped up, pulling the atoms with it. Inside the clumps, hydrogen and helium were squashed by gravity, condensing into enormous balls of gas until, once the balls were dense enough, nuclear fusion suddenly ignited in their centers. The first stars were born.

The German astronomer Walter Baade categorized the stars in our galaxy into types I and II in 1944. The former includes our sun and other metal-rich stars; the latter contains older stars made of lighter elements. The idea of Population III stars entered the literature decades later. In a 1984 paper that raised their profile, the British astrophysicist Bernard Carr described the vital role this original breed of star may have played in the early universe. “Their heat or explosions could have reionized the universe,” Carr and his colleagues wrote, “… and their heavy-element yield could have produced a burst of pregalactic enrichment,” giving rise to later stars richer in heavier elements.

Carr and his co-authors estimated that the stars could have grown to immense sizes, measuring anywhere between a few hundred and 100,000 times more massive than our sun, because of the large volume of hydrogen and helium gas available in the early universe.

Those at the heavier end of the range, so-called supermassive stars, would have been relatively cool, red and bloated, with sizes that could encompass almost our entire solar system. Denser, more modestly sized variants of Population III stars would have shone blue hot, with surface temperatures of some 50,000 degrees Celsius, compared to just 5,500 degrees for our sun.

In 2001, computer simulations led by Norman explained how such large stars could form. In the present universe, clouds of gas fragment into lots of small stars. But the simulations showed that gas clouds in the early universe, being much hotter than modern clouds, couldn’t as easily condense and were therefore less efficient at star formation. Instead, entire clouds would collapse into a single, giant star.

Their immense proportions meant the stars were short-lived, lasting a few million years at most. (More massive stars burn through their available fuel more quickly.) As such, Population III stars wouldn’t have lasted long in the history of the universe — perhaps a few hundred million years as the last pockets of primordial gas dissipated.

There are many uncertainties. How massive did these stars really become? How late into the universe did they exist? And how abundant were they in the early universe? “They’re completely different stars to the stars in our own galaxy,” Bowler said. “They’re just such interesting objects.”

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