Astronomers have discovered that the James Webb Space Telescope (JWST) may have already found the long-sought first generation of stars born shortly after the Big Bang.
These initial stars, referred to as Population III or POP III stars, dwell in a galaxy called LAP1-B which was previously studied by the $10 billion space telescope. The light from this galaxy has been travelling for 13 billion years to reach the JWST, meaning that we see LAP1-B as it was just 800 million years after the Big Bang.
This galaxy is so distant that it was only visible, even to the highly sensitive infrared vision of the JWST, thanks to a phenomenon first predicted by Albert Einstein in his 1915 theory of general relativity. Known as gravitational lensing, this phenomenon describes the magnification of light from a distant object via the warping of space by an intermediate massive body. The gravitational lens that magnified LAP1-B is a massive cluster of galaxies that lies between Earth and LAP1-B at a distance of around 4.3 billion light-years, called MACS J0416.1-2403 (MACS0416).
Identifying the universe's firstborn stars
The JWST sees the galaxy LAP1-B as it was during an age of the universe called "the epoch of reionization," during which ultraviolet light from the first stars and galaxies is thought to be transforming neutral gas of hydrogen and helium into a charged superheated gas called plasma. As such, it marks the end of the "cosmic dark ages."
These POP III stars are thought to have formed before this epoch; coming together around 200 million years after the Big Bang, after the universe had expanded and cooled enough to allow electrons and protons to form the first atoms of hydrogen, the lightest element in the cosmos.
"In the standard model of cosmology, POP III stars form in very small dark matter structures that serve as building blocks for larger galaxies," Visbal said. "Thus, they teach us about the earliest stages of galaxy formation and evolution. They may also constrain the properties of dark matter since alternative dark matter models impact where they first form."
That means astronomers have been keen to identify POP III stars, but this first generation of stellar bodies has thus far proved hard to spot.
"POP III stars have been elusive because they mostly form at early times, so they are very far away and in small clusters," Visbal said. "This makes them very faint."
Because POP III stars were forged at a time when the universe was filled with little more than hydrogen and helium, with just a smattering of heavier elements (which astronomers call "metals"), the first generation of stars should stand out from modern "metal-rich" stars like the sun (a POP I star) due to their low-metallicity.
This low metallicity had another impact on POP III stars, too, allowing them to reach tremendous masses equivalent to 100 times that of the sun and more. Additionally, POP III stars are also thought to cluster in relatively small groups due to their vast masses.
"Simulations indicate that since primordial gas cools less efficiently than gas with heavy elements like carbon and oxygen, there is less gas fragmentation during star formation," Visbal said. "This leads POP III stars to be more massive than metal-enriched stars, possibly with typical masses of 100 times the mass of the sun."
Indeed, the team found that the stars in LAP1-B are surrounded by gas with minimal traces of metals, and that they seem to be in groups of around 1,000 solar masses.
These findings also suggest that gravitational lensing could be an effective way to hunt for more POP III stars at early times, or at high redshifts.
"Until we did the calculation, I thought our model would find that Pop III stars are too rare at a redshift of 6.6 to be found in a strongly magnified part of a gravitational lens. I was pleasantly surprised to find that our calculation showed that they should be common enough to observe behind a cluster like MACSJ0416," Visbal concluded. "Next, we want to perform more detailed hydrodynamical simulations of the transition from Pop III to Pop II stars [the universe's second generation of stars] to see if they are consistent with the spectrum of LAP-1B and similar objects."
The team's research was published in late October in The Astrophysical Journal Letters.
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