The way in which stars evolve and end their lives was considered to be well understood for many years. Detailed computer models predicted that stars of a similar mass to the Sun would have a period toward the ends of their lives — called the asymptotic giant branch (AGB) — when they undergo a final burst of nuclear burning and puff off a lot of their mass in the form of gas and dust.
This expelled material goes on to form the next generations of stars, and this cycle of mass loss and rebirth is vital to explain the evolving chemistry of the universe. This process is also what provides the material required for the formation of planets — and indeed even the ingredients for organic life.
But when Australian stellar theory expert Simon Campbell of the Monash University Center for Astrophysics in Melbourne scoured old papers, he found tantalizing suggestions that some stars may somehow not follow the rules and might skip the AGB phase entirely. “For a stellar modeling scientist, this suggestion was crazy,” said Campbell. “All stars go through the AGB phase according to our models. I double-checked all the old studies but found that this had not been properly investigated. I decided to investigate myself, despite having little observational experience.”
Campbell and his team used ESO’s VLT to very carefully study the light coming from stars in the globular star cluster NGC 6752 in the southern constellation Pavo the Peacock. This vast ball of ancient stars contains both a first generation of stars and a second that formed somewhat later. The two generations can be distinguished by the amount of sodium they contain — something that the high-quality VLT data can be used to measure.
“FLAMES, the multi-object high-resolution spectrograph on the VLT, was the only instrument that could allow us to get really high-quality data for 130 stars at a time,” said Campbell. “And it allowed us to observe a large part of the globular cluster in one go.”
The results were a surprise. All of the AGB stars in the study were first-generation stars with low levels of sodium, and none of the higher sodium second-generation stars had become AGB stars at all. As many as 70 percent of the stars were not undergoing the final nuclear burning and mass-loss phase.
“It seems stars need to have a low-sodium ‘diet’ to reach the AGB phase in their old age. This observation is important for several reasons. These stars are the brightest stars in globular clusters, so there will be 70 percent fewer of the brightest stars than theory predicts. It also means our computer models of stars are incomplete and must be fixed!” said Campbell.
The team expects that similar results will be found for other star clusters, and further observations are planned.