All lifeforms on Earth, including humans, are made out of elements forged by extreme reactions deep in the bellies of stars. Phosphorus is among the core group of elements that make life possible, but scientists aren’t sure exactly what stellar processes create this important biological ingredient.
Now, a team of scientists has deepened the mystery of the element’s origins by discovering 15 “chemically peculiar stars” in the Milky Way that show “very high phosphorus abundances” that cannot be explained by any existing theories, according to a study published Tuesday in Nature Communications.
“Phosphorus is particularly interesting because there are only five elements that we know life on Earth depends on: Carbon, nitrogen, oxygen, phosphorus, and sulfur,” said lead author Thomas Masseron, an astronomer at Instituto de Astrofísica de Canarias in Tenerife, Spain, in a call.
“Phosphorus is a less abundant one,” he added, “so there are big uncertainties in general about where it is from.”
Masseron and his colleagues outlined several models to explain these phosphorus-rich (P-rich) stars, which may be similar to the long-dead stars that seeded our own solar system with this life-giving element.
The most likely origin story is that the stars were born from the ashes of a previous generation of massive stars that, for some reason, overproduced phosphorus in their lifetimes. This would require the progenitor stars to go through “a quite specific and peculiar nucleosynthesis,” the team said, referring to the process that goes on in the interior of stars to create new elements.
These massive stars would have spewed their phosphorus reserves across space when they exploded into supernova, thereby giving rise to a new brood of P-rich stars. However, computer models of this scenario suggest that it should also lead to carbon, sodium, and sulfur enrichment in the next generation of stars. The P-rich stars observed by the team, in contrast, do not show this pattern.
It’s also possible that the P-rich stars stole phosphorus from a companion star orbiting close to them, or that they fully merged with another star. Collisions between neutron stars, a class of dead, collapsed stars, provide another potential way to create phosphorus-enriched clouds of gas and debris. However, all of these explanations are contradicted in one way or another by the actual observations.
“The paper is all about exploring all possibilities and ruling out all of them,” said Masseron. “Basically, the answer is we don’t know.”
For this reason, the team is planning to study the stars in different wavelengths of light to learn more about their chemical fingerprints. They also hope to spot more examples of these outlier stars in order to see if they belong to a broader family of stars with equally inexplicable histories.
“Beyond the additional observations we are going to make, there will be another important aspect regarding collaboration with stellar interior and evolution experts, who now need to develop new models and possibly implement new physics to try to reproduce what we have observed, and thus attempt to find the origin of those stars,” Masseron concluded.