The first generation of stars changed the course of cosmic history. Now, thanks to the James Webb Space Telescope, we have a real chance of spotting them.
As turning points in cosmic history go, the birth of the first stars is hard to beat. When they flickered into existence between 200 and 400 million years after the big bang, the energy pouring from them ripped apart the atoms of gas that had been cooling the universe, reheating them in a process called re-ionisation. Then, as they burned and died, they created a cocktail of chemical elements that primed the universe to generate galaxies, planets and, ultimately, life itself.
 |
| akinbostanci/Getty Images |
No wonder astronomers have been itching to glimpse this first stellar generation. They were spectacular, for starters. Huge and ferociously bright, they are thought to have been up to 300 times as massive as our sun and 10 times hotter. But observing them could also tell us a lot about the mysterious early phase of the universe, not least how it became potted with supermassive black holes in what seems like an impossibly short time.
Now, we might finally be on the cusp. Earlier this year, astronomers reported that the James Webb Space Telescope (JWST), by fixing its superior vision on the outer reaches of a very distant galaxy, may have already seen evidence of the first stars. “The observations we can do now are really pushing our knowledge,” says Hannah Übler at the University of Cambridge.
That signal may turn out to be a false alarm. But what’s exciting right now is that others are homing in on different signatures of the universe’s early light. There is even some suggestion that the first stars might be hiding closer to home than we ever thought possible.
To understand the allure of the first stars and how we might identify them, it helps to know that stars in general are intimately connected to the chemical composition of the universe. The big bang created only the two lightest elements, hydrogen and helium (plus traces of lithium and beryllium). All the heavier elements – like the oxygen we breathe, the iron we mine, the silver we prize – were made in the hearts of stars, where nuclear fusion glued subatomic particles together to form larger atoms. As stars died and exploded, they spread these elements across the universe, only for them to be incorporated into new stars, which made heavier and heavier elements. It is true what they say: we are made of stardust.
Primordial stars
Astronomers split the stars we have detected so far into two types. Population I stars, such as our sun, are the youngest and contain the highest proportion of heavy elements. Population II stars are older and contain fewer heavy elements. But it stands to reason that there must be a third type: the very oldest stars that are made exclusively from hydrogen and helium. These are the population III stars.
Today, if a star grows to be more than say 100 solar masses, it will produce so much internal energy that the pressure will lift off its outer layers and blow them into space. This places an upper limit on the mass a modern star can attain. Not so with population III stars. The lack of heavier elements means that outward pressure doesn’t kick in so strongly. “If a population III star forms massive, it stays massive,” says Simon Glover at the University of Heidelberg in Germany. Based on these assumptions, astrophysicists have long thought the majority of the first stars would have been giants, typically hundreds of times heavier than our sun.
 |
| If modern stars like our sun got too big, their atmosphere would start to blow away J Marshall – Tribaleye Images/Alamy |
The lack of heavy elements also means the first stars would have had to become denser and hotter to generate the energy necessary to remain stable by preventing collapse. The hotter and bigger a star, the shorter its life. So astronomers reckon the first stars would have lived fast and died young – lasting perhaps only 5 million years. If that’s true, then the first stars must all be long dead. Still, in astronomy, looking at extremely distant objects equates to peering back in time, because of how long it takes their light to reach us. So although seeing one of the first stars is fiendishly difficult, it isn’t out of the question.
One reason scientists are so keen to do it involves the puzzle of supermassive black holes, which have masses spanning from a million to billions of times that of the sun. We know that one of these monsters lies at the centre of almost every galaxy. How do they get there? Well, when the first stars exploded and died, they would have spread their outer layers into space, with the remaining mass collapsing into black holes, each weighing between 10 and 100 solar masses. These baby black holes presumably collided with each other over time and fed off passing stars and gas clouds, gradually growing into the supermassive black holes we see today.
But there is a problem. The further back into cosmic history we have looked, the more astounded we have been to see nigh-on fully formed galaxies and supermassive black holes that shouldn’t exist because there hasn’t been enough time for them to form. “It is one of the big mysteries at the moment: how do you make very massive black holes very early in the universe?” says Glover.
Black hole origins
He thinks it could be telling us something new and extraordinary about the first stars, and he isn’t the only one. A number of teams have been looking again at their computer models and getting a surprise: it is possible that as re-ionisation increased the temperature of the gas in the universe, these hotter clouds collapsed into single stars with extremely high masses. Rather than hundreds of solar masses, we are talking about a few tens of thousands of solar masses. “These may give you a pathway to making some of the first very massive black holes,” says Glover. That’s because they would accumulate so much mass that after their brief stint as a hyperluminous star, they would collapse directly into a black hole. But to prove this idea, we need to find the stars and see what they were really like.
There is a way to do this, in principle. It involves a technique called spectroscopy, which means measuring the intensity of a range of wavelengths of light. Different elements absorb specific wavelengths, so spectroscopy gives us a fingerprint-like reading for a star, showing exactly which elements it contains and how much of them are present. Since the 1950s, scientists have been conducting spectroscopic surveys of the sky, looking for stars with negligible amounts of heavy elements. They have certainly found some very old stars, but none that makes the cut as a bona fide member of population III.
Sadly, not even JWST – which was specifically designed to look as far into the distant universe as possible – can see population III stars directly. The good news, though, is that it might just be capable of spotting them through their effects.
The intense radiation these stars emitted had the power to illuminate surrounding gas clouds and strip all the electrons from the helium atoms found there, creating helium II ions. Some of these electrons then recombine with their parent atoms, creating a glow. Since the glowing clouds of gas are vast compared with the stars, they should be much easier to see. And it is these clouds that Übler and her colleagues think they have detected.
Working with Roberto Maiolino at the University of Cambridge and others, Übler used JWST’s Near-Infrared Spectrograph instrument to study a distant galaxy whose discovery was originally reported in 2016. This galaxy, GN-z11, was at that time the most ancient ever seen, at 13.4 billion years old. That made it a prime candidate to have population III stars nestled in its halo, the outer reaches of the galaxy where primordial gas would gather.
When the researchers got their hands on the data for GN-z11, they found some tentative evidence of helium II close enough to the galaxy for it to have been in the halo. They applied for more time on JWST and got it, resulting in a more careful look at the GN-z11’s environs. Their results, published this year, revealed more widespread indications of helium II. Could they have found the smoking gun of population III?
 |
| Galaxy GN-z11 (inset), where astronomers may have spotted signs of primordial, population III stars NASA, ESA, P. Oesch (YU), G. Brammer (STScI), P. van Dokkum (YU), and G. Illingworth (UC Santa Cruz) |
Maybe. But just because a population III star could produce this signal doesn’t mean that is what we are seeing. There could be other explanations. “There’s a relatively nearby galaxy, I Zwicky-18, which has a lot of very ionised helium, but we don’t think it’s coming from population III stars,” says Glover. Instead, the ionising radiation there is thought to be the result of gas being pulled from a star onto either an incredibly dense neutron star or a black hole. In other words, being sure that the glowing gas is caused by a population III star will hinge on eliminating other explanations.
Maiolino knows this all too well. “That’s why we are still saying that this is a tentative result,” he says. To try to definitively establish the source of the ionising radiation, he and his colleagues have been awarded another 40 hours of coveted time on JWST to spend solely on this source. The observations have been scheduled for May 2025 and are designed to pick up even the faintest traces of heavier elements. If they see them, it would suggest that a population III star isn’t responsible. If not – it could be a slam dunk. “We are really pushing Webb to the limit,” says Maiolino.
Tidal disruption events
But Übler and Maiolino’s method isn’t the only tool at our disposal. Rudrani Kar Chowdhury at the University of Hong Kong and her colleagues recently proposed that we could look for the exceedingly luminous flares emitted by population III stars as they are torn apart by black holes, a process known as a tidal disruption event or TDE. “JWST is discovering massive black holes [in the very distant universe] and also massive galaxies. That made us think about a [population III] star getting captured by the massive black holes,” says Chowdhury.
When a star is pulled apart by a black hole, it can release more mass than the black hole can easily swallow. This sets up an incredibly turbulent environment, in which the star’s gas heats up and glows. When this happens to a population I or II star, the radiation interacts with the heavier elements and drives a stellar wind that cools the remaining gas, allowing it to settle into the black hole. But, in theory, the lack of heavier elements in the population III stars should prevent these cooling flows, and so the gas stays hot and luminous for longer.
This means that the luminosity and duration of a TDE flare, known as its light curve, can be tied to the mass, density and heavy element content of the star being pulled apart. “So, if we can detect some light curves from this kind of massive stellar TDE, then we can extract the properties of the population III stars,” says Chowdhury.
Seeing these events won’t be easy. While they are within the detection limits of JWST – and NASA’s forthcoming Nancy Grace Roman space telescope – the team calculates that such events would be rare and fundamentally unpredictable. This will give Roman the edge because it is designed to survey a larger swathe of sky. “Compared with JWST, Roman has 100 times the sky coverage, so that gives a higher chance of detections,” says Chowdhury’s colleague Janet Chang, also at the University of Hong Kong.
But there is one final twist to this story. While most astronomers continue to focus on the very distant universe in their hunt for population III stars, others have begun to suspect that there may be some survivors from that primordial epoch in our cosmic backyard.
The first models of the initial population III stars, the ones that began the process of re-ionisation and reheating the gases in the universe, suggested that the stars would all weigh in at hundreds of solar masses. Over the past decade, however, advances in computing power have allowed astronomers to execute more sophisticated simulations. These show that the first models might have neglected the effects of rotation on the collapsing gas cloud that became the star, especially when it comes to slightly cooler clouds of gas.
As a relatively cool gas cloud collapses, it naturally flattens into a disc – think of a pizzaiolo twirling dough. This means not all of the cloud’s initial mass ends up in the central star. Instead it fetches up in the surrounding disc, where it fragments into smaller population III stars. “We no longer think that you just form one massive star. You actually probably get a wide range of stellar masses dominated by massive stars,” says Glover.
A few of these stars could have been really tiny, perhaps as small as half the mass of our sun. As we already know, a star’s mass correlates inversely with its lifetime, so we would expect these meagre celestial spheres to eke out their supply of nuclear fuel for tens of billions or even trillions of years. “If population III stars are produced with masses below 0.8 or 0.7 solar masses, then these stars live longer than the age of the universe. So every single one of these stars should still be around,” says Ralf Klessen at the University of Heidelberg.
There is even reason to think that some of these stars could be lurking close at hand. In 2018, we saw the most pristine star yet detected through spectroscopic sky surveys. Called SMSS J1605-1443, it has just one-millionth of the iron present in the sun and is located in the Milky Way’s halo. This can’t be a population III star – it is more likely to be a direct descendent – because it still contains some iron and, strangely, has a relatively large amount of carbon. We don’t fully understand this star’s unusual composition. But it is a sign that very old stars might not look exactly as we expect them to – and they can be found nearby.
On top of that, a recent study of a catalogue of stars compiled by the European Space Agency’s Gaia mission has revealed 200,000 Milky Way stars that appear to be extremely lacking in heavier elements. That gives us an awful lot of targets. One way or another, you can’t help wondering if we will glimpse the universe’s first lights much sooner, and much closer, than anyone previously imagined.
This article has been shared from NewScientist under creative common license. We strongly recommend to read the original article here.