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Tuesday, August 04, 2020

Explained: How stars provided the carbon that makes life possible

When a star dies, it releases various elements including carbon into the surroundings. New study on white dwarfs sets size limits for the stars that enriched the Milky Way with carbon

Written by Kabir Firaque , Edited by Explained Desk | New Delhi | Updated: July 10, 2020 6:49:00 pm
The new research analysed white dwarfs in this star cluster. Called NGC 7789, it is about 8,000 light-years away. It is also called Caroline’s Rose, after the astronomer Caroline Lucretia Herschel who discovered it in the late 18th century. (Credit: Guillaume Seigneuret via NASA)

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.

Now, a study has provided new insights on the origins of the carbon in our galaxy. Published in ‘Nature Astronomy’ by an international team of researchers, the study is an 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.

“Both in low-mass stars and in massive stars carbon is synthesised in their deep and hot interiors through the triple-alpha reaction, that is the fusion of three helium nuclei,” the study’s lead author, Paola Marigo of the University of Padua in Italy, told The Indian Express, by email.

“In low-mass stars the newly synthesised 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,” she said.

What astrophysicists debate is 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.

So, what did the study find?

Between August and September 2018 at the Keck Observatory in Hawaii, the researchers analysed a few white dwarfs belonging to open star clusters of the Milky Way. They measured the masses of the white dwarfs, derived their masses at birth, and from there calculated the “initial-final mass relation” — 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. “… We were struck by an unexpected and, in a certain way, bizarre result: the masses of those white dwarfs were notably larger than what hitherto astrophysicists believed. Even more surprising, we realized that their inclusion broke the linear growth, introducing a sort of small ripple in the relationship, a little kink peaking at initial masses around 2 solar masses,” Marigo wrote for ‘Nature’ in an article about the research paper.

So far, stars born roughly 1.5 billion of 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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What explains this?

In their interpretation, Marigo and colleagues pose stringent constraints on 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 4.6 billion years ago.

In the last phases of their life, stars that were about 2 solar masses produced new carbon atoms in their hot interiors, transported them to the surface, and finally spread them into the interstellar medium through gentle stellar winds. “Our detailed stellar models indicate that the stripping of the carbon-rich outer mantle occurred slowly enough to allow the central cores of these stars, the future white dwarfs, to grow considerably in mass,” Marigo wrote.

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,” Marigo wrote.

How does this compare with the existing theories of carbon enrichment?

“Actually, our study is not in favour of either scenario,” Marigo told The Indian Express. “Both sources (low-mass and massive stars) likely contributed, in different proportions (still uncertain). Having fixed the minimum initial mass for the production of carbon in low-mass stars is a valuable result since it helps putting the puzzle pieces together,” she said.

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