Where Does Carbon Come From? Massive Stars and AGB Stars Shape the Milky Way

Carbon is the fourth most abundant element in the universe. It is found everywhere—from the air we breathe and the carbohydrates in food to the oil in Earth’s crust. Unlike many lighter elements created during the Big Bang, carbon was forged inside stars through nuclear fusion.

How stars create carbon

Massive stars, with at least eight times the Sun’s mass, fuse three helium atoms into carbon atoms in their cores through the triple-alpha process. They eventually release this carbon into space through stellar winds or at the end of their lives in a core-collapse supernova.

Small- and medium-sized stars, with between half and eight times the Sun’s mass, can also produce carbon through the triple-alpha process. However, they create it not in their cores, but in dense helium shells during a later stage of evolution called the asymptotic giant branch, or AGB phase. AGB stars repeatedly expand and contract, ejecting some of their material into space.

Which stars supply most of the Milky Way’s carbon?

Although scientists understand how these stellar processes work, the relative contribution of massive stars and AGB stars to the universe’s carbon supply remains uncertain. To investigate the question, a team of scientists modeled how the Milky Way’s chemical composition changed over time as different types of stars produced and released carbon.

The researchers then tested their models against the Milky Way’s observed carbon abundance. Their results showed that massive stars are the primary source of the galaxy’s carbon, while AGB stars contribute a smaller but significant share.

Using APOGEE stars as a record of galactic history

To establish a benchmark for the Milky Way’s carbon content, the team used stars from the Apache Point Observatory’s Galaxy Evolution Experiment (APOGEE) survey. They selected 14,066 large, bright stars with reliable measurements.

These stars were not AGB stars or massive stars. Instead, they were at a stage of life in which newly produced carbon remained trapped in their cores. The researchers measured the carbon in the stars’ outer layers, using the composition of these subgiant stars as a record of the gas from which they formed.

Modeling the Milky Way’s chemical evolution

The scientists began with a computer program developed by another research team to model how the Milky Way’s chemistry changes over time. They used estimates from previous studies to calculate how much carbon different types of stars might produce.

They then incorporated those estimates into the Versatile Integrator for Chemical Evolution, or VICE, a simulation program that models how stars form, elements are fused, and newly created material is released into galaxies. The team ran 14 versions of the model, with each one assuming a different contribution from AGB stars compared with massive stars.

In the simulation, the Milky Way began as 200 concentric rings. Each ring had its own stellar population, star-formation rate, and supply of gas available to form new stars. The researchers then modeled the movement of stars between the rings and allowed the simulated galaxies to evolve for more than 10 billion years.

Finally, they randomly selected 14,066 simulated stars and compared their carbon content with that of the same number of observed APOGEE subgiant stars.

AGB stars contribute up to 40% of the Milky Way’s carbon

The model that best matched the observed carbon content indicated that AGB stars contribute between 10% and 40% of the Milky Way’s carbon. The strongest agreement with the APOGEE observations occurred when AGB stars contributed between 15% and 30%.

The findings confirmed that massive stars are the galaxy’s primary carbon producers. They also showed that lower-mass AGB stars may create carbon more efficiently than previously thought, even though they take longer to become significant carbon sources than more massive stars.

What the findings mean for future research

The researchers concluded that their estimates provide a benchmark for future studies of stellar carbon production and the chemical history of the Milky Way. The relationship between how elements form inside stars and how stars lose mass is not yet fully understood.

Future research could examine larger samples of stars, investigate other galaxies, and use additional data to develop a deeper understanding of how carbon and other elements spread through the universe.

Post views: 92