May 29, 2013
Predicting The End Of Stars Relies On How Much Sodium They Have
Lee Rannals for redOrbit.com — Your Universe OnlineAn international team of astronomers published a paper today in the journal Nature detailing the role sodium plays in determining how stars evolve and how they end their lives.
Astronomers believe that stars like the Sun will blow off much of their atmospheres into space near the ends of their lives, which is known as asymptotic giant branch (AGB). However, new observations of a huge star cluster showed that they did not get to this stage in their lives at all.
Detailed computer models have predicted that stars of a similar mass to the Sun would have a period toward the end of their lives 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 ejection goes on to form the next generation of stars and provides the material required for the formation of planets.
However, Australian stellar theory expert Simon Campbell of the Monash University Centre for Astrophysics (MoCA) found that some stars just do not follow the rules.
“For a stellar modeling scientist this suggestion was crazy! 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 said in a statement.
The researchers used the European Space Observatory's Very Large Telescope (VLT) to study the light coming from stars in the globular cluster NGC 6752 in the southern constellation of Pavo. The two generations can be distinguished by the amount of sodium they contain.
“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. And it allowed us to observe a large part of the globular cluster in one go,” added Campbell.
They found that all of the AGB stars in the study were first generation stars with low levels of sodium and that none of the higher-sodium second generation stars had become AGB stars. According to the results, up to 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!” concluded Campbell.
Scientists recently discovered a few silica grains from meteorites that most likely originated from either red giant or AGB stars. They wrote in the Astrophysical Journal Letters in May about how two tiny grains of silica originated from an ancient supernova, offering up a great example of how these mass ejections shape the chemistry in our universe.