*Image credit: JianJun He.
After the Big Bang 13.8 billion years ago, there existed only hydrogen, helium, and lithium. All the heavier elements were made later in stars. We now know that there are 92 naturally occurring elements and 283 variations, called isotopes. But what elements were made first? A new international study co-led by Professor Alexander Heger, from Monash University's School of Physics and Astronomy and published in Nature suggests that calcium may have been the heaviest element made in the first stars. Previous studies have theoretically identified calcium, but this is the first to definitively confirm the production of calcium and how it was made in massive first-generation stars. Professor Heger said the study contributes significantly to improving our understanding of the chemical evolution of the galaxy. "The nature of the first generation of stars – the largest and oldest stars remain a most fascinating open question in astrophysics," Professor Heger said. "Our study describes, for the first time, how most of this calcium was made: during the calm simmering of hydrogen burning rather than during the violent supernova explosion." Once hydrogen burning in the star's core is complete, it continues in a shell while the helium core experiences advanced nuclear burning phases that change abundances. Most material below the helium core disappears in large star models. Ejected material experiences thermodynamic and mixing reactions. Shell hydrogen burning is usually hotter, less dense, and faster than core hydrogen burning. To analyse the complete range of these impacts and how the revised rate influences the conclusion, the researchers undertook full stellar model computations for a 40 M star of primordial composition. They used the KEPLER code, a fully coupled adaptive nuclear reaction network, to follow the evolution and nucleosynthesis in detail. The study shows how important it is to study nuclear reactions in the domain of the (advanced) Carbon Nitrogen Oxygen (CNO) cycle to understand nucleosynthesis in the first generation of stars and to use observational data to draw exclusive conclusions about the nature of the first stars in the early universe. The most iron-poor star known as, SMSS0313-6708, shows a remarkable signature of calcium whereas iron, that usually should be made and ejected by supernovae along with the calcium, remained undetected. "It is thought to be a direct descendant of the first stars in the universe, which formed from the matter left over after the Big Bang," Professor Heger said. "The ultra-metal poor UMP star is very old, and what we can see of its composition is like a time capsule from the time before the first galaxies formed.
"This is a great addition to the exciting observations that will be made by the James Webb telescope in the near future."
