When they breathe their last, stars scatter their ashes in the universe, giving birth to splendid nebulae. These ashes are rich in chemical elements, including carbon. Researchers have finally identified the main source of this element essential to life: white dwarfs at least 1.5 times the mass of our Sun.
Remember that almost 90% of stars end up in the form of white dwarfs. These take billions of years to cool and darken. But, before collapsing, they spread their ashes in the universe. Ashes, some of them at least, rich in carbon.
A white dwarf is an extremely dense end-of-life star. One cubic centimeter of its material weighs… a ton! It is essentially what remains of a rather modest star after it has exhausted its nuclear carbon and ejected its upper layers in the form of a planetary nebula.
It was by observing a mass anomaly that the researchers came to this conclusion. The white dwarfs observed in ancient clusters of stars in our Milky Way appear indeed more massive than the theory of stellar evolution predicts. Rather on the order of 0.7 to 0.75 solar mass than the 0.6 to 0.65 predicted. This is the signature, according to astronomers, of a carbon synthesis for white dwarfs of a certain type.
The open star cluster known as the Caroline Rose — or NGC 7789 — is about 8,000 light years from Earth in the constellation Cassiopeia. Researchers at the University of California, Santa Cruz, USA, found unusually massive white dwarf stars there that likely played a key role in the dispersion of carbon in the universe.
When they are at least 1.5 times more massive than the Sun, stars produce in their core, in the last phases of their life, carbon atoms. Then they transport them to their surface. These carbon atoms are ultimately propagated in the interstellar medium by gentle stellar winds. Models constructed by astronomers indicate that the phenomenon occurs slowly enough to allow the central nuclei of these stars, the future white dwarfs, to grow substantially in mass.
Contrary to what astronomers thought, the relationship between the initial mass and the final mass of a star that has become a white dwarf is not linear. And that could have even more important implications.
“This work also has an impact on the age of known white dwarfs. Now these are posed as cosmic probes essential for understanding the history of the formation of the Milky Way. The relationship between initial mass and final mass is also what defines the lower mass limit for supernovae, these gigantic explosions seen at great distances and which are really important for understanding the nature of the universe, “says Pier-Emmanuel Tremblay , researcher at the University of Warwick, in a statement from the University of California.
Finally, astronomers imagine that most of the light coming to us from very distant galaxies comes from bright, carbon-rich stars, similar to those studied by researchers at the University of California. And a reliable interpretation of this light depends of course on their understanding of the synthesis of carbon in stars.