Stars twice the size of the Sun produced the majority of carbon in the Milky Way

White Dwarfs, the dead remnants of a Sun-like star, are responsible for most of the carbon in the Milky Way – the element essential for life as we know it, a study found. 

Astronomers from the US and UK including the University of Warwick, studied star clusters throughout the Milky Way to track down the main sources of carbon.  

A White Dwarf star is the dense stellar remnant of a Sun-like star – it is the cool dim phase after it runs out of fuel and expels its matter including carbon atoms.

Every carbon atom in the universe was created in a star – some from energetic supernovae explosions and some from White Dwarfs before they begin cooling. 

Researchers discovered that the majority of carbon in our galaxy came from stars twice the size of the Sun expelling large amount of mass and becoming small White Dwarf stars – smaller than the remnant our Sun will eventually become. 

About 90 per cent of stars end their lives as White Dwarfs and our Sun will go the same way when it has run out of fuel and expanded to become a Red Giant. 

With their final few breaths before they collapse, these stars leave an important legacy, spreading their ashes into the surrounding space through stellar winds.

These ashes are enriched with chemical elements, including carbon, newly synthesised in the star’s deep interior during the last stages before its death.

Every carbon atom in the universe was created by stars, through the fusion of three helium nuclei under intense pressure and heat. 

Astrophysicists still debate which types of stars are the primary source of the carbon in our own galaxy – massive supergiants like Betelgeuse or smaller stars like our Sun. 

Some studies favour low-mass stars that blew off their envelopes in stellar winds and became white dwarfs, while others favour massive stars that go supernovae.

In the new study, published in Nature Astronomy, an international team of astronomers analysed white dwarfs in open star clusters in the Milky Way, and their findings help shed light on the origin of the carbon in our galaxy. 

Open star clusters are groups of up to a few thousand stars, formed from the same giant molecular cloud and roughly the same age.

They are held together by mutual gravitational attraction, the astronomers said.

The research reveals an inconsistency in the relationship between the masses of certain white dwarfs and their initial masses before their collapse – what scientists refer to as the initial-final mass relation.

Co-author Dr Pier-Emmanuel Tremblay of the University of Warwick said one of most exciting aspects of this research is that it impacts the age of known white dwarfs.

He said these stars are an ‘essential cosmic probe’ to understand 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,’ he explained.

The study was based on astronomical observations conducted in 2018 at the W. M. Keck Observatory in Hawaii and led by coauthor Enrico Ramirez-Ruiz, professor of astronomy and astrophysics at UC Santa Cruz.

‘From the analysis of the observed Keck spectra, it was possible to measure the masses of the white dwarfs,’ Ramirez-Ruiz explained.

‘Using the theory of stellar evolution, we were able to trace back to the progenitor stars and derive their masses at birth.’ 

The relationship between the initial masses of stars and their final masses as white dwarfs is known as the initial-final mass relation.

This is a fundamental diagnostic in astrophysics that integrates information from the entire life cycles of stars, linking birth to death. 

In general, the more massive the star at birth, the more massive the white dwarf left at its death, and this trend has been supported in observations and in theories. 

But analysis of the newly discovered white dwarfs in old open clusters gave a surprising result, the team explained.

The masses of these white dwarfs were notably larger than expected, putting a ‘kink’ in the initial-final mass relation for stars with initial masses in a certain range.

‘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,’ said lead author Paola Marigo at the University of Padua in Italy.

In the last phases of their lives, stars twice as massive as our Sun 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. 

The team’s 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 appreciably in mass.

Analysing the initial-final mass relation around the kink, the researchers concluded that stars bigger than two solar masses also contributed to the galactic enrichment of carbon, while stars of less than 1.5 solar masses did not. 

In other words, 1.5 solar masses represents the minimum mass for a star to spread carbon-enriched ashes upon its death.

These findings place stringent constraints on how and when carbon, the element essential to life on Earth, was produced by the stars of our galaxy.

This carbon eventually ends up trapped in the raw material from which the Sun and its planetary system were formed 4.6 billion years ago. 

‘Now we know that the carbon came from stars with a birth mass of not less than roughly 1.5 solar masses,’ said Marigo.

However, while the carbon helped the Sun form, when the Sun eventually dies and becomes a White Dwarf it won’t contribute carbon back into the galaxy.  

The research has been published in the journal Nature Astronomy.