Context: A new study has provided insights on the origins of the carbon in our galaxy. The study is published in ‘Nature Astronomy’ and it is an analysis of white dwarfs — the dense remnants of a star after its death.


  • Carbon is essential for life: It is the simple building block of all the complex organic molecules that organisms need. 
  • It is also known that all the carbon in the Milky Way came from dying stars that ejected the element into their surroundings. 
  • However, What has remained debated is what kind of stars made the major contribution.

How does carbon come from stars?

  • Ejections from stars:
    • Most stars except the most massive ones are eventually turned into white dwarfs. When the massive stars die, they go with a spectacular bang known as the supernova
    • Both low-mass and massive stars eject their ashes into the surroundings before they end their lives. 
    • And these ashes contain many different chemical elements, including carbon.
  • Carbon synthesis in stars
    • Both in low-mass stars and in massive stars carbon is synthesized in their deep and hot interiors through the triple-alpha reaction
      • Triple-alpha reaction is the fusion of three helium nuclei.
  • Process after the synthesis
    • In low-mass stars: Here the newly synthesized carbon is transported to the surface [from the interiors] via gigantic bubbles of gas and from there injected into the cosmos through stellar winds. 
    • In massive stars: They enrich the interstellar medium with carbon mostly before the supernova explosion, when they also experience powerful stellar winds.
  • Ongoing debate
    • It is regarding whether the carbon in the Milky Way originated from low-mass stars before they became white dwarfs, or from the winds of massive stars before they exploded as supernovae.
    • The new research suggests that white dwarfs may shed more light on carbon’s origin in the Milky Way.

Findings of the new study

  • Origin and transportation of Carbon 
    • Study shows how and when carbon was produced by stars of our galaxy, and ended up trapped in the raw material from which the Sun and its planetary system were formed.
    • In the last phases of their life, stars that were about 2 solar masses produced new carbon atoms in their hot interiors. And it was transported to the surface, and finally spread into the interstellar medium through gentle stellar winds.
  • Fixed the minimum initial mass for the production of carbon in low-mass stars 
    • The study found out that 1.65-Msun [1.65 times the mass of the Sun] represents the minimum mass for a star to spread its carbon-rich ashes upon death.
  • Initial-final mass relation of a star
    • It is a key astrophysical measure that integrates information of the entire life cycles of stars.
    • The researchers found out that the more massive the star at birth, the more massive the white dwarf left at its death.
    • They found contradictory results as the masses of those white dwarfs were notably larger than what hitherto astrophysicists believed.
  • Contradiction to the earlier belief
    • So far, stars born roughly 1.5 billion years ago in our galaxy were thought to have produced white dwarfs about 60-65% the mass of our Sun. 

Instead, they were found to have died leaving behind more massive compact remnants, about 70-75% solar masses.

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