Astronomers have glimpsed the core of a dying star – confirming theories of how atoms are made

Astronomers have glimpsed the inner structure of a dying star in a rare kind of cosmic explosion called an “extremely stripped supernova”.

In a paper published today in Nature, Steve Schulze of Northwestern University in the United States and colleagues describe the supernova 2021yfj and a thick shell of gas surrounding it.

Their findings support our existing theories of what happens inside massive stars at the end of their lives – and how they have shaped the building blocks of the universe we see today.

How stars make the elements

Stars are powered by nuclear fusion – a process in which lighter atoms are squished together into heavier ones, releasing energy.

Fusion happens in stages over the star’s life. In a series of cycles, first hydrogen (the lightest element) is fused into helium, followed by the formation of heavier elements such as carbon. The most massive stars continue on to neon, oxygen, silicon and finally iron.

Each burning cycle is faster than the previous one. The hydrogen cycle can last for millions of years, while the silicon cycle is over in a matter of days.

As the core of a massive star keeps burning, the gas outside the core acquires a layered structure, where successive layers record the composition of the progression of burning cycles.

While all this is playing out in the star’s core, the star is also shedding gas from its surface, carried out into space by the stellar wind. Each fusion cycle creates an expanding shell of gas containing a different mix of elements.

Core collapse

What happens to a massive star when its core is full of iron? The great pressure and temperature will make the iron fuse, but unlike the fusion of lighter elements, this process absorbs energy instead of releasing it.

The release of energy from fusion is what has been holding the star up against the force of gravity – so now the iron core will collapse. Depending on how big it is to start with, the collapsed core will become a neutron star or a black hole.

The process of collapse creates a “bounce”, which sends energy and matter flying outwards. This is called a core-collapse supernova explosion.

The explosion lights up the layers of gas shed from the star earlier, allowing us to see what they are made of. In all known supernovae until now, this material was either the hydrogen, the helium or the carbon layer, produced in the first two nuclear burning cycles.

The inner layers (the neon, oxygen and silicon layers) are all produced in a mere few hundred years before the star explodes, which means they don’t have time to travel out far from the star.

An explosive mystery

But that’s what makes the new supernova SN2021yfj so interesting. Schulze and colleagues found the material outside the star came from the silicon layer, the last layer just above the iron core, which forms on a timescale of a few months.

The stellar wind must have expelled all the layers right down to the silicon one before the explosion occurred. Astronomers don’t understand how a stellar wind could be powerful enough to do this.

The most plausible scenario is a second star was involved. If another star were orbiting the one that exploded, its gravity might have rapidly pulled out the deep silicon layer.

Exploding stars made the universe what it is today

Whatever the explanation, this view deep inside the star has confirmed our theories of the cycles of nuclear fusion inside massive stars.

Why is this important? Because stars are where all the elements come from.

Carbon and nitrogen are manufactured primarily by lower mass stars, similar to our own Sun. Some heavy elements such as gold are manufactured in the exotic environments of colliding and merging neutron stars.

However, oxygen and other elements such as neon, magnesium and sulfur mainly come from core-collapse supernovae.

We are what we are because of the inner workings of stars. The constant production of elements in stars causes the universe to change continuously. Stars and planets formed later are very different from those formed in earlier times.

When the universe was younger it had much less in the way of “interesting” elements. Everything worked somewhat differently: stars burned hotter and faster and planets may have formed less, differently, or not at all.

How much supernovae explode and just what they eject into interstellar space is a critical question in figuring out why our Universe and our world are the way they are.