🌟 Remnants of a colossal primordial star identified?

Supermassive black holes have been observed when the Universe was still very young, far too early for them to have formed according to classical accretion models.

The James Webb Space Telescope now provides a solid clue to solving this mystery. Its observations have revealed, in a distant galaxy, an exceptional chemical signature. This signal indicates the past presence of stars of colossal mass and size, thousands of times more massive than our Sun, which would have populated the first ages of the cosmos.

Until now only theorized, this observation could confirm the existence of these so-called "Population III" stars.


This discovery focuses on the galaxy GS 3073. By analyzing its composition, scientists have identified a marked imbalance between the quantities of nitrogen and oxygen. This particular chemical profile does not match any type of star known today. The research team, involving the University of Portsmouth and the Center for Astrophysics, sees this as indirect evidence that these now-vanished giants did indeed exist.

Nitrogen plays the role of a true cosmic tracer here. Its unusually high abundance relative to oxygen in GS 3073 forms a unique fingerprint. Only primordial stars of prodigious mass can generate such a ratio.

Computer models help understand how these giant stars could have produced so much nitrogen. In their cores, fusion reactions transform helium into carbon. This carbon is then transported to outer layers where, reacting with hydrogen, it produces nitrogen via a carbon-nitrogen-oxygen cycle (see below). This process, coupled with very active internal convection, disperses the nitrogen within the star before it is ejected into space.


The end of life of these stellar colossi is equally particular. Rather than exploding as a supernova, they collapse directly upon themselves into a black hole. This phenomenon leads to the formation of black holes from the outset with masses of several thousand solar masses. The active black hole at the center of GS 3073 could be the remnant of one of these primordial stars.

This discovery opens a new window onto the first chapters of cosmic history. It shows that the initial stellar population was probably very different from today's, including objects with extreme properties. The James Webb Telescope should allow the identification of other galaxies with a similar excess of nitrogen, consolidating this vision of a young Universe populated by giants.

The carbon-nitrogen-oxygen (CNO) cycle in massive stars

In very massive stars, energy production does not rely solely on the direct fusion of hydrogen into helium. Another mechanism, called the carbon-nitrogen-oxygen cycle, takes over and becomes dominant. This cycle uses carbon, nitrogen, and oxygen as catalysts to transform hydrogen into helium, releasing a tremendous amount of energy in the process.

The process begins with carbon-12 present in the star's core. It captures a hydrogen nucleus (a proton) to transform into nitrogen-13, which is unstable. After radioactive decay, it becomes carbon-13. The latter in turn captures a proton to become nitrogen-14, a stable isotope.

Nitrogen-14 can then capture another proton to become oxygen-15, which is unstable. Upon decaying, it returns to a state of nitrogen-15. Finally, nitrogen-15 captures a fourth proton, but this time, instead of creating a heavier element, it splits into helium-4 and carbon-12. The carbon-12 is thus regenerated, allowing the cycle to start again.

In extremely massive primordial stars, this cycle is particularly efficient and rapid due to the enormous temperatures and pressures. It leads to significant production and mixing of nitrogen inside the star. This nitrogen is then expelled into the interstellar medium, leaving behind this distinctive chemical signature observed today.