Every second, a star dies in the universe. But these stellar beings don’t just completely vanish, stars always leave something behind.
Some stars explode in a supernova, turning into a black hole or a neutron star, while the majority of stars become white dwarfs, a core of the star it once used to be. However, a new study reveals that these white dwarfs contribute more to life in the cosmos than previously believed.
The study, published Monday in the journal Nature Astronomy, suggests that white dwarf stars are the main source of carbon atoms in the Milky Way, a chemical element known to be crucial to all life.
When stars like our own Sun, a yellow dwarf star, run out of fuel, they turn into a white dwarf. In fact, 90 percent of all stars in the universe end up as white dwarf stars.
White dwarfs are hot, dense stellar remains with temperatures that reach 100,000 Kelvin. Over time, billions of years, these stars cool and eventually dim as they shed their outer material. However, right before they collapse, their remains are transported through space by winds that originate from their bodies.
These stellar ashes contain chemical elements such as carbon.
Carbon is the fourth most abundant chemical in the universe and is a key element in the formation of life as it is the basic building block to most cells.
All of the carbon in the universe originated from stars, therefore the phrase that we are made of stars is not only poetic but rather accurate. However, astronomers could not agree on which type of star is responsible for spreading the most amount of carbon across the cosmos.
The scientists behind the new study used observations of white dwarfs in open star clusters, groups of a few thousand stars formed around the same time, in the Milky Way by the W. M. Keck Observatory in Hawaii in 2018.
They measured the stars’ initial-final mass relation, which is the relationship between the stars’ masses when they first formed and their masses as white dwarfs.
Usually, the larger the star was, the more massive a white dwarf will be. However, the study found that the stars’ masses as white dwarfs were larger than the scientists had anticipated considering their initial mass when they first formed.
“Our study interprets this kink in the initial-final mass relationship as the signature of the synthesis of carbon made by low-mass stars in the Milky Way,” Paola Marigo, a researcher at the University of Padua in Italy, and lead author of the study, said in a statement.
The team of scientists concluded that stars bigger than 2 solar masses also contributed to the galactic enrichment of carbon, while stars of less than 1.5 solar masses did not.
“Now we know that the carbon came from stars with a birth mass of not less than roughly 1.5 solar masses,” Marigo said.
The new study suggests that carbon was essentially trapped in the raw material that formed the Solar System 4.6 billion years ago.
Abstract: The initial–final mass relation (IFMR) links the birth mass of a star to the mass of the compact remnant left at its death. While the relevance of the IFMR across astrophysics is universally acknowledged, not all of its fine details have yet been resolved. A new analysis of a few carbon–oxygen white dwarfs in old open clusters of the Milky Way led us to identify a kink in the IFMR, located over a range of initial masses, 1.65 ≲Mi/M⊙≲ 2.10. The kink’s peak in white dwarf mass of about 0.70−0.75 M⊙ is produced by stars with Mi ≈ 1.8−1.9 M⊙, corresponding to ages of about 1.8−1.7 Gyr. Interestingly, this peak coincides with the initial mass limit between low-mass stars that develop a degenerate helium core after central hydrogen exhaustion, and intermediate-mass stars that avoid electron degeneracy. We interpret the IFMR kink as the signature of carbon star formation in the Milky Way. This finding is critical to constraining the evolution and chemical enrichment of low-mass stars, and their impact on the spectrophotometric properties of galaxies.