White Dwarfs Help Explain Source of Carbon in Universe

By: Rhea Agrawal

New studies show that white dwarfs, ancient stars near the end of their existence, are a main source of carbon in the Milky Way Galaxy. The origin of carbon, an element essential to all life, has long been debated by astrophysicists.

Just around 90 percent of stars conclude their prolonged lives as white dwarfs. White dwarfs are incredibly dense stars that have burned all their hydrogen that was used as nuclear fuel. Before final termination, these stars spread their remains into the surrounding space. Stars created all carbon atoms existing in the universe through the bonding of three helium nuclei.

On July 6, international astronomers of Nature Astronomy examined white dwarfs to help provide information about the origin of carbon in the Milky Way. The white dwarfs were discovered in open star clusters, groups of a few thousand stars close in age and evolved from the same molecular cloud. Mutual gravitational attraction holds these clusters together.

Previous studies discussed the initial-final mass relationship, the correlation between the initial and final masses of white dwarfs. The direct relationship connects the life cycle of a star; the larger a star is at birth, the larger it will be at its death. However, the newly discovered clusters show that the mass of the dwarfs was remarkably greater than expected.

Paola Marigo, lead author at the University of Padua in Italy, said, “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."

Stars more enormous than our sun create carbon atoms in their hot insides, move them to the surface, and then spread them through stellar winds as the stars approach the end of their life. The outside deteriorates gradually enough to allow the core of the stars to grow generously in mass.

"Now we know that the carbon came from stars with a birth mass of not less than roughly 1.5 solar masses," Marigo explains. Researchers have deduced that 1.5 solar masses, one equivalent to the size of the sun, is the minimum mass required for a star to spread carbon ashes after its demise.

Per-Emmanuel Tremblay, co-author at the University of Warwick said, “One of most exciting aspects of this research is that it impacts the age of known white dwarfs, which are essential cosmic probes to understanding the formation history of the Milky Way. The initial-to-final mass relation is also what sets the lower mass limit for supernovae, the gigantic explosions seen at large distances and that are really important to understand the nature of the universe.”

Incorporating cosmology with stellar evolution allowed the researchers to conclude that carbon-enriched white dwarfs that were close to their death provide an extensive amount of light released from far away galaxies.



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