A recent study has provided new insights on the origins of the carbon in our galaxy.

Try this question from CSP 2016:

Q.Consider the following:

1. Photosynthesis
2. Respiration
3. Decay of organic matter
4. Volcanic action

Which of the above add carbon dioxide to the carbon cycle on earth?

(a) 1 and 4 only

(b) 2 and 3 only

(c) 2, 3 and 4 only

(d) 1, 2, 3 and 4

Why study Carbon?

• Carbon is essential for life: It is the simple building block of all the complex organic molecules that organisms need.
• It is known that all the carbon in the Milky Way came from dying stars that ejected the element into their surroundings.
• What has remained debated, however, is what kind of stars made the major contribution.
• The study shows the analysis of white dwarfs — the dense remnants of a star after its death.

How does carbon come from stars?

• Most stars — except the most massive ones — are doomed to turn into white dwarfs.
• When the massive ones 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.

How is it synthesized?

• Both in low-mass stars and in massive stars carbon is synthesized in their deep and hot interiors through the triple-alpha reaction that is the fusion of three helium nuclei.
• In low-mass stars, 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.
• Massive stars enrich the interstellar medium with carbon mostly before the supernova explosion, when they also experience powerful stellar winds.

Findings of the news research

• It was earlier debated that 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.
• The researchers measured the masses of the white dwarfs, derived their masses at birth, and from there calculated the “initial-final mass relation”.
• The IFMR is a key astrophysical measure that integrates information of the entire life cycles of stars.
• They found that the relationship bucked a trend — that the more massive the star at birth, the more massive the white dwarf left at its death.
• 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.

What explains this?

• From an analysis of the initial-final mass relation around the little kink, the researchers drew their conclusions about the size range for the stars that contributed carbon to the Milky Way.
• Stars more massive than 2 solar masses, too, contributed to the galactic enrichment of carbon.
• Stars less massive than 1.65 solar masses did not. In other words 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.

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