Scientists from an underground laboratory discovered the secrets of the first stars

Scientists have been able to look into the oldest stars in the universe by conducting nuclear fusion experiments in an underground laboratory located 2.4 km below China’s Jinping Mountains, writes Vice.

One of the greatest tasks in astronomy is observing the first stars ever to shine in space, known as Population III. Scientists believe that this initial generation of stars ignited somewhere between 100 and 250 million years after the Big Bang, then quickly burned out and exploded as huge supernovae.

People have never seen Population III stars, but scientists have seen stars born from their elements. One such star, called SMSS0313-6708, has been shining for 13.6 billion years, making it one of the oldest stars ever observed by humans. Located just 6,000 light-years from Earth, the ancient star has puzzled scientists by containing a higher concentration of the element calcium than expected for a star from the early universe.

Now, scientists led by Beijing Normal University researcher Liyong Zhang have recreated an important nuclear reaction that explains the production of heavy elements such as calcium in ancient stars. The team conducted the experiment at the China Jinping Underground Laboratory (CJPL), an underground tunnel located under 2,400 meters of vertical rock, which is the deepest operating laboratory for particle physics and nuclear physics experiments in the world.

Zhang and his colleagues found that a particular reaction that produces a version of the element neon may be 7.4 times more common in Population III than previously estimated. The finding explains the high calcium content of SMSS0313-6708 and provides an updated measurement of this crucial reaction previously unavailable in terrestrial laboratories, according to the study, published by the Nature journal.

“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.”

In other words, each new generation of stars is enriched with heavy metals produced by their ancestors, and then turns the cycle forward, seeding the universe with a new batch of complex heavy elements. Population III stars were composed almost entirely of the light elements hydrogen and helium, but their deaths created the heavier elements that made up 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.”

Scientists faced the task of taking advantage of the underground location of the laboratory, which protects it from cosmic radiation that reaches the Earth and interferes with the operation of precision instruments, to study nuclear fusion reactions. Experiments conducted deep underground have revealed the mechanisms that control the production of elements deep inside stars — including Population III.