The sodium content of sunlike stars determines if they skip a phase of death.
Image courtesy ESA/NASA
The way that stars spend their last years is largely shaped by their sodium "diet," according to a surprising new study published on Wednesday.
The study could upend current theories about how some stars that are similar to our sun die and become the basic building blocks for the next generation of stars and planets.
According to existing stellar evolution models, sunlike stars—those that are similar in size and chemical composition to our sun—swell to become so-called red giants in their final stage of life, before losing their atmospheres in a spectacular bubble of gas and dust.
This fate awaits our own sun in 4 to 5 billion years, scientists say.
The final period in a sunlike star's life, when stars make their greatest contribution to the universe, is known as the asymptotic giant branch (AGB).
"They puff off all their outer layers of gas and dust, enriching and polluting the surrounding space," said Simon Campbell, an astronomer at Monash University in Australia and the co-author of the new study published in the journal Nature.
"This gas and dust gets recycled and goes into the formation of the next generation of stars, planets—and possibly even life."
But now astronomers have found that not all sunlike stars follow the same rules when it comes to the end of their life cycles, and that some can skip the AGB phase altogether.
Campbell and his team studied a giant ball of stars known as NGC 6752, one of the sky's brightest globular clusters. The collection of about one million stars sits 13,000 light-years from Earth in the southern constellation Pavo.
Globular clusters are considered the perfect cosmic laboratories for studying stars and testing stellar computer models because they have so many stars, in all phases of life.
Campbell's team used the European Southern Observatory's Very Large Telescope in the Atacama Desert of Chile. The giant telescope was equipped with a spectrograph—a prism used to break starlight into its colors—which allowed the researchers to obtain the chemical fingerprints of 130 cluster stars at once.
The team immediately noticed that the cluster is home to both a first generation of stars, at least 10 billion years old, and a second generation that is billions of years younger—and that the two groups appeared to contain different amounts of sodium.
"It's a bit like using sodium as a chemical 'tag' to follow each population of stars," said Campbell.
By tracking the sodium levels, the researchers were able to identify which stars would undergo the AGB phase at the end of their lives.
"We suspected sodium might be a good 'tracer' because it cannot be altered by the stars themselves because they are too low mass, and not hot enough to create or burn sodium," Campbell said.
His team quickly realized that some members of the cluster didn't appear to follow accepted theories when it came to undergoing the final burn stages, or AGB, when they were dying. Some skipped this final burst of nuclear burning entirely.
All the AGB stars in the study were first-generation stars with low levels of sodium, while none of the higher-sodium second-generation stars had become AGB stars.
It turns out that up to 70 percent of the stars in the NGC 6752 cluster were not undergoing the final "nuclear burning and mass-loss phase so indicative of sunlike stars," said Campbell.
Exactly why there are two broad groups of stars with starkly different sodium levels in globular clusters is still unknown, Campbell said, and is a hot topic in stellar research.
How the newfound breed of sodium-rich, sunlike stars ultimately die is still something of a mystery. Campbell suspects they may directly evolve into small-Earth-size white dwarf stars that gradually cool over many billions of years.
But he said that one thing's for sure: Existing computer models of how sunlike stars die will need to be adjusted.
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