We live near a fusion reactor in space that provides all our heat and light. That reactor is also responsible for the creation of various elements heavier than hydrogen, and that's true of all stars. So, how do we know that stars are element generators? Many clues lie hidden in stellar spectra, since they contain fingerprints of various elements cooked up by the stars.
Hints about the creation of carbon and oxygen in particular lay hidden for years in a data set taken in a search for planets around nearby stars. Astronomers have suggested that such stars could be places to look for exoplanets. Thanks to a brainstorm by astronomer Darío González Picos of Leiden University in the Netherlands, he and a research team examined high-resolution spectra of nearby stars to look for rare isotopes of the two elements.
The team studied 32 M dwarf-type stars, which are among the most common in the Galaxy. They live for a long time on the main sequence, which is the period when a star is fusing elements in its core. The atmospheres of the stars preserve the signatures of their chemical evolution from birth to their current status. The stars in the study showed rare isotopes of carbon and oxygen, which tells something about their evolution. The result of the team's work represents a step forward in understanding the creation of elements and how they get dispersed as part of stellar evolution.
Stellar Seeding of Elements
Carbon and oxygen are highly abundant in the Universe. We are carbon-based life forms, as is all of life on our planet, which itself has carbon in its makeup. We breathe oxygen, which is generated by other life forms on the planet. So, it's natural to wonder how these two came about in the process of stellar evolution. That means we also need to comprehend the complexities of the element-making process in stars.
“Nuclear fusion in stars is a complex process and is just the starting point of chemical evolution," said Leiden's Darío González Picos, who headed the research. The process is calleds stellar nucleosynthesis and all stars do it. Our Sun, for example, fuses hydrogen to make helium and it will do that for another few billion years. Then, there will come a time when it runs out of hydrogen in the core and begins to fuse helium to heavier elements, such as isotopes of carbon and oxygen. At that point, it will become a reddish star blowing its elements to space through a strong wind. Stars much more massive than the Sun do the same thing, but they make even heavier elements when they explode as supernovae.
Essentially, stars are part of a huge cosmic recycling project that enriches their galaxies with material to make new stars, and planets. Their light carries the history of all they experienced via the chemical fingerprints left by the creation of new elements.
Finding the Rare Fingerprints
González Picos worked with Ignas Snellen, and Sam de Regt to detect and read the chemical fingerprints in starlight by using isotopes of carbon and oxygen. These are different varieties of those elements and they differ by the numbers of neutrons in their atoms. On Earth, 99% of carbon atoms have 6 neutrons, for example, but a small fraction has 7. The team successfully measured these isotope ratios for both carbon and oxygen in 32 neighboring stars with unprecedented precision. The way they did it was to sift through the data archives from the Canada France Hawai'i Telescope on Mauna Kea in Hawai'i. The data included stars with effective temperatures between 3000 and 3900 K and exhibited strong signals for heavier elements (that is, they had strong metallicity in their atmospheres).
“We now see that stars that are less chemically enriched than the Sun have fewer of these minor isotopes”, said de Regt. “This finding confirms what some models of galactic chemical evolution have predicted and now provides a new tool to rewind the chemical clock of the cosmos.”
"The observations were originally made for a completely different reason than the one we are using them for now", said Snellen. "It was entirely Darío's idea to use the high-resolution spectra, which were actually intended for the discovery of planets, for this isotope research – with impressive results."
The result, as González Picos points out, is another way to use stellar chemistry to trace other types of evolution in the Universe. “This cosmic detective story is ultimately about our own origins, helping us to understand our place in the long chain of astrophysical events, and why our world looks the way it does," he said.
For More Information
Rare Isotopes in Our Neighboring Stars Provide New Insights in the Origin of Carbon and Oxygen
Chemical Evolution Imprints in the Rare Isotopes of Nearby M Dwarfs
Chemical Evolution Imprints in the Rare Isotopes of Nearby M Dwarfs (arXiv preprint)