White dwarfs are a big source of carbon in the Universe

Artist’s impression of the crystallization in the interior of a white dwarf star. Credit: University of Warwick/Mark Garlick.

In their nuclear hearts, stars fuse elements heavier than hydrogen, creating the ingredients necessary to make planets, oceans, and people.

Tracing the origins of individual elements in the Milky Way has been a challenge, but a new analysis of white dwarf stars reveals that they may be responsible for one of the most essential elements of all: carbon.

An average star, no matter the size, starts off with roughly 75% hydrogen, 25% helium, and the barest fraction of other (with “other” meaning the rest of the periodic table).

For most of a star’s life, it happily churns through that hydrogen, fusing it into helium and releasing the energy needed to power itself for billions of years.

But eventually the hydrogen in the core runs out, forcing the star to turn to helium fusion to keep the lights on.

And once the helium supply exhausts itself, the star either quits living (if it’s around the same size as our sun) or goes on to process even heavier elements (if it’s much more massive).

For stars like our sun, the leftover waste from that helium fusion is carbon and oxygen, which steadily builds up in the core.

In the last stages of a star’s life, it vomits almost all of its atmosphere into the surrounding system, creating what’s called a planetary nebula.

But in that process, the carbon and oxygen core is left behind, leaving what astronomers call a white dwarf. And the bigger the original star, the bigger the resulting white dwarf.

Almost.

A team of astronomers recently used the W. M. Keck Observatory to survey white dwarfs inside open clusters (loose clumps of stars that likely came from the same original nebula) scattered around the Milky Way, with their results published in Nature Astronomy.

Using those samples of dead white dwarfs, the astronomers reconstructed the demographics of the original star population.

Overall, the results were as expected: smaller parent stars eventually led to smaller white dwarfs, and larger parent stars left behind larger white dwarfs. But that relationship had an odd feature: parent stars with masses between 1.65 and 2.1 times the mass of the sun didn’t quite fit the trend.

This offset from the general trend was especially prominent for stars with masses between 1.8 and 1.9 solar masses, which coincides with an interesting cutoff point in stellar evolution.

For stars smaller than this, when they run out of hydrogen the helium in their cores is able to support itself through the weird quantum mechanical effect known as degeneracy pressure, whereas larger stars cannot.

This means that stars in this mass range are producing larger white dwarfs than expected. And since white dwarfs are made of a lot of carbon, it means that stars in this mass range are producing more carbon than expected.

Most of that carbon ends up in the final white dwarf, but some also gets spread throughout the galaxy in the final phases of the star’s life.

In short: this result suggests that stars in a specific mass range may be responsible for the majority of carbon in the universe.

Including the carbon that makes up you.

Written by Paul M. Sutter, Astrophysicist at Ohio State | Agent to the Stars | pmsutter.com.