Astronomers have glimpsed the inner workings of a dying star in a rare type of cosmic explosion called an “extremely stripped supernova.”
In a paper published in natureSteve Schultz of Northwestern University and colleagues describe supernova 2021yfj and the thick shell of gas that surrounds it.
Their findings support existing theories about 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 elements
Stars are powered by nuclear fusion. Fusion is a process in which lighter atoms are crushed into heavier atoms, releasing energy.
Nuclear fusion occurs in stages over the life of a star. In a series of cycles, hydrogen (the lightest element) first fuses into helium, followed by the formation of heavier elements such as carbon. The heaviest stars are neon, then oxygen, then silicon, and finally iron.
Each write cycle is faster than the previous one. The hydrogen cycle can last millions of years, but the silicon cycle lasts only a few days.
As the core of a massive star continues to burn, the gas outside the core acquires a layered structure, with successive layers recording the composition as the combustion cycle progresses.
While all this is happening at the star’s core, the star is emitting gas from its surface, which is carried into space by stellar winds. Each fusion cycle creates an expanding shell of gas containing a different mixture of elements.
core collapse
What happens to a massive star if its core is filled with iron? Great pressures and temperatures cause the iron to fuse, but unlike the fusion of lighter elements, this process absorbs energy rather than releasing it.
The release of energy from nuclear fusion is what has been supporting the star against gravity, so now the iron core will collapse. Depending on its initial size, the collapsed nucleus becomes a neutron star or a black hole.
The collapse process causes a “bounce”, which causes energy and matter to fly outward. This is called a nuclear collapse supernova explosion.
The explosion causes layers of gas previously ejected from the star to glow, allowing us to see what they are made of. In all supernovae known to date, this material has been either hydrogen, helium, or carbon layers produced in the first two nuclear combustion cycles.
The inner layers (neon, oxygen, and silicon) all form within just a few hundred years before the star explodes. That means it doesn’t have time to travel far from the star.
An exploding mystery
But that’s what’s interesting about the new supernova SN2021yfj. Schulz and his colleagues discovered that the star’s outer material comes from a final layer of silicon, just above the iron core, that forms over a timescale of months.
Before the explosion occurred, the stellar wind must have blown all the layers down to the silicon layer. Astronomers do not understand how stellar winds become powerful enough to allow this.
The most likely scenario is that a second star was involved. If another star was orbiting the exploding star, its gravity could have rapidly pulled out the deep silicon layer.
The explosion of stars created the present universe
Whatever the explanation, this view deep into the star supported our theory of fusion cycles inside massive stars.
Why is this important? Because the stars are where all the elements are born.
Carbon and nitrogen are primarily produced by low-mass stars similar to our Sun. Some heavy elements, such as gold, are produced in the exotic environments of neutron star collisions and mergers.
However, oxygen and other elements such as neon, magnesium, and sulfur primarily come from nuclear collapse supernovae.
We are what we are because of the inner workings of the stars. The constant production of elements within stars causes the universe to change continuously. Stars and planets that formed later were very different from those that formed earlier.
When the universe was young, there were far fewer “interesting” elements. Everything worked somewhat differently. Stars may have burned hotter and faster, and planets may have formed fewer, differently shaped, or not at all.
How much a supernova explodes and what it releases into interstellar space are important questions in understanding why our universe and world are the way they are.
