The universe has changed considerably in the 14 billion years since it came into being. It started out dustier and lacked all the chemical elements we have today. Stars have formed as the universe evolved, and astronomers divide them into 3 groups or populations. They classify the youngest, most metal-rich stars, like the Sun, as Population I. They classify older, metal-poor stars as Population II.
Following this pattern, astronomers would classify the most ancient stars without any metals as Population III or Pop. III. To this day, no astronomer has discovered a Pop. III star. The problem is that these stars would, in theory, be so old that they would predate the Milky Way and all of the surrounding galaxies. So, to find them, astronomers would have to search at distances that hardly any telescopes can reach.
An international team of scientists recently suggested a new way to look for Pop. III stars. Specifically, they hypothesized that if astronomers expanded their search to look not just for active Pop. III stars, but also the explosions produced as they die, known as supernovae, then the odds of finding these ancient stars would shift in favor of the researchers.
The team focused on a type of supernova that occurs when the remains of an exploded star, known as a white dwarf, are reignited by an infusion of material. The following giant flare-up is known as a Type Ia supernova. Astronomers have observed instances of Type 1a supernovae for decades to measure cosmic-scale distances and estimate the age and speed of expansion of the universe.
To test their hypothesis, the astronomers ran detailed simulations of how 2 stars evolve and move around each other, using the code Modules for Experiments in Stellar Astrophysics or MESA. In 1,000 simulations, the team tracked what happened as 2 Pop. III stars, one of which was a white dwarf, orbited each other. Astronomers previously doubted that Type Ia supernovae would happen in Pop. III stars because they have a different chemical makeup than the other 2 star populations. However, this research team found that the Pop. III stars exchanged mass, consistently leading to a Type Ia supernova. Additionally, the team assumed that the merger of 2 white dwarf Pop. III stars could form a Type Ia supernova based on previous studies.
Once the astronomers showed that Type Ia supernovae could form from Pop. III stars, they calculated how frequently these supernovae might happen in a region of the cosmos scientists could observe. They used a series of equations that built on each other, starting with how often stars form. Scientists calculate star formation rates differently depending on when in the universe’s history the stars formed. The stars these researchers were interested in are so old that cosmological factors that don’t impact how stars form today, like dark matter and dark energy, were relevant to their calculations.
From there, they evaluated the statistical chances that 2 stars of the right sizes would form a binary system at the right distance apart to form a supernova without merging too early and forming 1 large star. Then, they estimated how long the merger process would take based on their simulations.
Finally, they used typical observation times for telescope missions and the scope of how much sky these missions cover to estimate the number of Type Ia supernovae that scientists can actually expect to find. They concluded that for a 3-year mission covering 0.002% of the sky, scientists can expect to find at most 2 of these Pop. III Type Ia supernovae. They also expressed the caveat that the JWST is likely the only telescope capable of seeing out to the necessary distances of 24+ billion lightyears (1 with 23 zeroes behind it miles or 2 with 23 zeroes behind it kilometers).
They conceded that their findings rely on assumptions about the physics of stars that have never been seen before, but should hopefully point observational astronomers in the right direction for future studies. The team predicted that the majority of supernovae at such great distances should come from these ancient stars, and if so, there may one day be images published of a light show almost 14 billion years in the making.
