As telescopes become more powerful, we see further into the past. Explore the early universe with Conor Feehly. This article originally appeared in the Cosmos Print Magazine in December 2024.
At a certain point in the early universe, the lights turned on. The birth of the first stars represented a dramatic shift in the evolution of the universe, where the pristine gas left over from the Big Bang collapsed to form the first structures that would give rise to galaxies, black holes, planets, and eventually us.
What's more, they were spectacularly big and bright. Perhaps 500 times as massive and a million times as luminous as the Sun.
"These first generation stars are thought to be really massive, and we see that the more massive the stars, the more luminous and hotter they are," says Roberto Maiolino. He's an astrophysicist from the University of Cambridge who has spearheaded efforts to observe the first stars in the universe with the James Webb Space Telescope (JWST).
Glimpsing these stars might help astronomers explain several persistent cosmic mysteries. This includes how the first stars bridged the gap between the chemically simple early universe and the chemically complex latter universe. Doing so, though, has proven extremely difficult.
But hope is not lost. Maiolino and his team's recent observations from the outer regions of an ancient galaxy have left a trail of breadcrumbs for astronomers to follow. Astronomers have also devised novel methods for detecting the signatures of these primordial stars. This could help researchers make sense of the role these cosmic giants played in the evolution of the very first galaxies, and the universe as a whole.
The early universe was a strange place, unrecognisable compared to the complex cosmic fabric we see today.
Once the ultra-hot high-energy particle soup left over from the Big Bang had sufficiently expanded and cooled, the universe entered an epoch of darkness, known ominously as the 'cosmic dark ages'. During this time, which astronomers believe may have lasted roughly 100 million years or so, interstellar space was filled with opaque neutral hydrogen - darkness in every direction.
Eventually, though, gravity would reign supreme, acting to collapse dense clouds of hydrogen and helium into the very first stars. Astronomers know this first generation, somewhat confusingly, as Population III (Pop III) stars. Subsequent generations of stars are known as Population II, and the most recent - such as our Sun - as Population I stars.
The major reason, other than age, that astronomers differentiate between these stellar populations, is to do with the chemical evolution of the universe at the time. "When these first stars ended their lives as supernovae, we think they enriched the intergalactic medium with the first elements heavier than helium. Carbon, nitrogen and oxygen, for example," says Maiolino.
Part of why astrophysicists believe Pop III stars were so massive has to do with them forming entirely out of hydrogen and helium.
Since stars are born out of collapsing clouds of gas, the bigger the cloud, the bigger the star. However, these clouds can separate into smaller clouds depending on the density of the materials they contain, and clouds that are enriched with heavier elements will be more likely to differentiate into smaller clouds, and therefore smaller stars.
Since Pop III stars formed from hydrogen and helium, the 2 lightest elements on the periodic table, the clouds they formed from would have been less likely to differentiate - creating stars that, on average, were far larger than those that are born in the universe today.
This means that Pop III stars were crucial players in bridging the chemically simple beginnings of the universe with the more chemically complex later stages of the universe, making it easier for galaxies, stars and planets to form.
Observing Pop III stars would give astronomers fundamental insights into this crucial time in our cosmic history.
There are many reasons why spying on the first galactic furnaces is extremely hard.
Pop III stars were enormous in and of themselves, making them hard to spot. This may seem counterintuitive, but a general rule for stars is that the bigger and brighter you are, the faster you burn through your fuel. "Most of these stars were very short lived, maybe only for a few million years," said Maiolino. That sounds like a lot, but on cosmic timescales, where our Sun is at least 4.5 billion years old, this is really a flash in the cosmic pan. This means the brief window of time where astronomers might be able to spot Pop III stars is very short.
While Pop III stars were extremely large individually, the initial clusters they formed were very low-mass and faint compared to today's big bright galaxies, such as the Milky Way. Such clusters may have only contained a handful of stars. Their overall weak signal is one of the main reasons why observing them has proven difficult for astronomers.
Then, when we look out into the universe as far as we can, we are also looking back in time. Light emitted from the most remote regions of the universe has taken billions of years to reach us, and thanks to the expansion of the universe, this light gets stretched on its vast journey here through a process called redshift. As such, when the light reaches us and our telescopes, it has shifted to the dim, infrared part of the electromagnetic spectrum. "That's why we need facilities and telescopes that can observe light in the infrared such as the JWST," says Maiolino.
Another reason why direct observations of Pop III stars have so far eluded astronomers has to do, again, with their chemistry. By analysing the light spectrum emitted from ancient stars with JWSTs Near Infrared Spectrograph, astronomers get a pretty good idea of what elements that light might have been absorbed by. If the light has only been absorbed by hydrogen and helium - the stuff Pop III stars are made of - they know they may be detecting the direct spectral signature of Pop III stars.
However, if the light shows signs of absorption from heavier elements, such as carbon and oxygen, then they know that they are likely witnessing light from stars that are older, which formed from the leftovers of Pop III stars. Or do they?
In a slightly confusing twist, it's also possible that Pop III stars self-pollute. Since heavier elements are getting produced inside Pop III stars, these elements may rise to the surface and astronomers could detect their presence in the light the stars emit, tricking astronomers into thinking they are stars from a later generation.
Elusive as Pop III stars might be, astronomers also have reasons to be optimistic about observing them. Maiolino and his team may have already detected their presence in a galaxy that existed only 400 million years after the Big Bang. But wait a second, if Pop III stars are so short lived, and they started forming 100 million years after the Big Bang, how could astronomers possibly detect them in a galaxy that's 400 million years old?
"Clumps of pristine gas left over from the Big Bang could still exist well into the lifespan of galaxies because gases don't mix that well. This means we might not have to look back all the way to the cosmic dawn to spot them," Maiolino says.
And this is what Maiolino and his team detected. A clump of pristine gas observed with the JWST in the outer regions of the galaxy GN-z11, which existed 400 million years after the Big Bang, shows telltale signs that gigantic Pop III stars may be bathing the surrounding interstellar medium with high-energy radiation, the type that should be typical for massive, hot Pop III stars.
Even with the unprecedented sensitivity of Webb, astronomers are still limited to observing clumps, or regions of stars this far away, rather than individual stars. "To have sufficient luminosity, we have to detect clusters of these stars... we are pushing Webb to its limit in terms of sensitivity," says Maiolino.
Maiolino and his team have been approved to spend 40 more hours using Webb to look at this region of the sky to confirm their observations. "It's possible that the signature we are seeing is being created by a direct collapse black hole, but new observations should clarify this," he says. Based on what they know, Maiolino believes they can tentatively constrain the mass distribution of Pop III stars. "We think they can reach up to 500 solar masses," he adds.
While the possible detection of a clump of Pop III stars is certainly exciting, there are also possible ways scientists may be able to get higher resolution observations of them, and one of these methods utilises a quirk in the way massive objects distort light - something called gravitational lensing.
"The way scientists do this, is they aim a telescope through a foreground galaxy cluster which acts as a lens and enhances the light that you get from background objects," explains Erik Zackrisson, an astrophysicist from Uppsala University in Sweden.
Zackrisson was part of a team in 2018 who successfully used the Hubble Space Telescope to observe a gravitationally lensed star at redshift 6.2, which means it existed 900 million years after the Big Bang. While the star didn't have the features of a Pop III star, it at least showed researchers that lensing may be a viable way to 'zoom in' on ancient stellar populations.
"With such distant objects, we have to get lucky," says Zackrisson. He explains that, in principle, if astronomers were lucky enough see an unresolved image of a gravitationally lensed star, then they could measure the spectral signature of that star. If it only shows signs of hydrogen and helium absorption, it will indicate that it is Pop III.
"The problem is that, in reality, the lensing situation is very complicated because there are several types of lensing happening at the same time, which means it can be hard to tell exactly where your signal is coming from," said Zackrisson. Given the strength in sensitivity of our current observatories, Zackrisson still thinks our best bet lies with observing clusters of Pop III stars, rather than individually lensed stars.
"If we want to find the missing link for where these heavier elements came from, Pop III stars are where we have to look," he added.
Another route towards observing these evasive Pop III stars has to do with how they might have interacted with primitive black holes.
When a star gets very close to a black hole, the strong gravitational forces can tear it into pieces. This process is known as a tidal disruption event (TDE). During a TDE, fragments of the ripped up star can get eaten by the black hole, which can result in bursts of high energy electromagnetic radiation, or bright flares.
In a paper from earlier this year, astrophysicists from the University of Hong Kong, led by Rudrani Kar Chowdhury explained how these TDE flares could be a window into understanding Pop III stars. "So what we can observe is this very bright flare, and the information of the star can be decoded from the properties of the flare. This flare can even outshine all the stars in the galaxy hosting the massive black hole," says Chowdhury.
The bright flashes of light that Pop III stars generate as they get ripped apart by black holes may be the lighthouses that astronomers need to spot these stellar outcrops on the distant cosmic shore. "Our research on Pop III stars will serve as a pathfinder to detect them by probing their tidal disruption signature. We believe that with our proposed technique and the identifying properties of Pop III stars, observers will be able to detect them with the current and upcoming powerful telescopes," Chowdhury says.
Speaking of black holes, another driving motivation for astrophysicists to unravel the properties of Pop III stars has to do with the presence of supermassive black holes in galaxies that, according to our best models, are just too young.
Recently, observations of early galaxies by Maiolino and his collaborators found that early galaxies were at a more advanced stage of galactic evolution than what was expected. Namely, they were housing supermassive black holes that shouldn't have had the time to consume the materials needed to bloat to their size.
Both Chowdhury and Maiolino think it is possible that the first Pop III stars may have been the origin of these early supermassive black holes. "Astronomers believe that a fraction of these first stars end up being heavy black holes in the early universe. These seed black holes eventually grow in mass to become super massive black holes which are present at the centres of most of the galaxies in the local universe, including our very own Milky Way," says Chowdhury.
With a more robust understanding of Pop III stars and their properties, 2 important gaps in our understanding of galactic evolution could be filled: the chemical evolution of the early universe, and the origin of supermassive black holes that dominate almost every galaxy in the universe today.
The story of the universe and where we came from will not be complete until we learn from the first stars in the cosmos.