Scientists have opened an unprecedented window into the universe’s very first stars by conducting nuclear fusion experiments in a subterranean laboratory located 1.5 miles under China’s Jinping Mountains, reports a new study.
The results resolve a longstanding mystery about one of the oldest stars ever discovered, while also shedding new light on the murky reactions that powered the ancestors of all modern stars.
One of the biggest quests in astronomy is to directly observe the first stars that ever shone in the cosmos, known as “population III.” Scientists think this initial generation of stars burst into existence somewhere around 100 to 250 million years after the Big Bang, before quickly burning out and exploding as enormous supernovae.
Population III stars have never been seen by humans, but scientists have spotted stars that were born from the ashes of these stellar elders. One such star, called SMSS0313-6708, has been shining for an astonishing 13.6 billion years, making it one the oldest stars ever spotted. Located just 6,000 light years from Earth, the ancient star has puzzled scientists because it contains a higher concentration of the element calcium than expected for a star from the early universe.
Now, scientists led by Liyong Zhang, a researcher at Beijing Normal University, have recreated an important nuclear reaction that facilitates the production of heavier elements, such as calcium, in ancient stars. The team conducted the experiment inside the China Jinping Underground Laboratory (CJPL), a subterranean tunnel located under 2,400 meters of vertical rock, which is the deepest operational laboratory for particle and nuclear physics experiments in the world.
Zhang and colleagues discovered that a particular reaction, which produces a version of the element neon, could be 7.4 times more common in population III stars compared to previous estimates. The finding explains the high calcium content of SMSS0313-6708 and provides an updated measurement of this “crucial reaction” that has been “previously inaccessible in aboveground laboratories,” according to a study published on Wednesday in Nature.
“Stars are the nuclear forges of the cosmos, responsible for the creation of most elements heavier than helium in the Universe,” said Zhang’s team in the study. “Some of these elements are created in the hearts of stars over the course of billions of years, whereas others are formed in just a few seconds during the explosive deaths of massive stars.”
“These heavy elements have an important role in the Universe, enabling the formation of complex molecules and dust, which facilitate the cooling and condensation of molecular clouds, aiding the formation of new stars like our Sun,” the researchers continued. “The first generation of stars, called population III (pop III) stars or primordial stars, formed from the pristine matter left by the Big Bang, thus play a special part in seeding the Universe with the first heavy elements and creating suitable conditions for future generations of stars and galaxies.”
In other words, each new generation of star is enriched by the heavy metals produced by its ancestors, then pays the cycle forward by seeding the universe with a new batch of complex heavy elements. Population III stars were almost entirely composed of the light elements hydrogen and helium, but their explosive deaths created heavier elements that were incorporated into stars like SMSS0313-6708.
“SMSS0313-6708 is an ultra-metal-poor star that is speculated to be a direct descendant of the first generation of stars in the Universe that formed after the Big Bang,” Zhang’s team noted. “The observable composition of an ultra-metal-poor star is a time capsule to the environment before the first galaxies formed—complementing the exciting upcoming observations of the James Webb Space Telescope, which is now aiming to give a first look at the earliest stars and galaxies.”
The task at hand for the scientists was to take advantage of the lab’s subterranean location—which shields it from cosmic radiation that reaches Earth and messes with precise instruments—to probe nuclear fusion reactions.
Previous studies have identified fluorine-19, which is an isotope (or version) of the light element fluorine, as an important player in the interiors of ancient stars. When fluorine-19 is struck by a proton, a type of subatomic particle, it can undergo two types of reactions that have very different impacts on the production of chemicals inside these stellar forges. One reaction produces an oxygen isotope, while another makes the isotope neon-20 and a gamma ray. The first reaction essentially cycles the production back to making lighter elements, whereas the reaction that makes neon-20 causes a “breakout” mechanism that enables stars to forge heavier elements.
Most studies have suggested that the breakout reaction is about 4,000 times weaker than the oxygen reaction with regard to the production of elements in stars, which is a process called nucleosynthesis. Zhang’s team was able experimentally test this idea out in the unique conditions of the CJPL, by shooting protons at fluorine-19 without pesky disruptions from natural radiation. The results showed that the breakout reactions were much stronger than expected, and could account for the calcium content seen in SMSS0313-67086.
“Our stellar models show a stronger breakout during stellar hydrogen burning than previously thought, and may reveal the nature of calcium production in population III stars imprinted on the oldest known ultra-iron-poor star, SMSS0313-67086,” the team said. “Our rate showcases the effect that faint population III star supernovae can have on the nucleosynthesis observed in the oldest known stars and first galaxies, which are key mission targets of the James Webb Space Telescope.”
“We find that all our nucleosynthesis models can reproduce the observed calcium production,” the researched added.
In this way, experiments that were conducted deep inside Earth have exposed the murky mechanisms that govern the production of elements deep inside of stars—including the mysterious population III generation. As Zhang’s team notes, sophisticated observatories, including the James Webb Space Telescope, will add more detail to this emerging portrait of stellar interiors—and perhaps reveal the first starlight that lit the heavens in the early universe.