What can you do in 15 minutes? Maybe read a few articles or watch a few videos online. But as astrophysicist and Space.com columnist Paul Sutter explains, 15 minutes is enough time for a particular kind of ordinary star to transform into an ultra-dense neutron star.
In Episode 9 of his Facebook Watch series made in partnership with Space.com, "Ask a Spaceman," Sutter explains how this process works. "I hate to break it to you, but it's not just the most massive stars in the universe that die. It's all stars. Every single star in our universe only has a finite life, including our own sun," he says.
Earth's sun fuses hydrogen into helium in its core. This process not only generates heat and energy for nearby planets such as Earth, but also stops the sun from collapsing into itself. Most stars spend the majority of their life spans converting hydrogen to helium. But when stars run out of hydrogen in their cores, interesting things happen. [Inside a Neutron Star (Infographic)]
Sutter took the example of a star that is about 20 to 25 times more massive than the sun. This kind of star spends about 10 million years fusing hydrogen into helium. (That sounds like a long time, but the sun has been doing this very thing for about 4.5 billion years and will continue to do so for roughly 2.5 billion years more. So, big stars burn through their fuel much more quickly.)
As the larger star runs out of hydrogen in its core, it builds up a core of helium. The helium eventually ignites due to nuclear processes, and the core starts fusing again. However, helium fuses at a higher temperature, and the gas must burn at a faster rate to keep the star from collapsing onto itself. Or, as Sutter puts it, "there is less explosive energy per reaction to resist gravity." (He explained this process is more detail in a 2017 Space.com column.)
Helium, therefore, takes much less time to run out in the core — only 1 million years. What's left behind now are carbon and oxygen, and (upon ignition) it takes an even shorter amount of time to burn through these elements: about 600 years. That's because they are even more inert than helium and so require even more energy for fusion, Sutter says.
Each succeeding element requires even more fusion energy to overcome how inert it is. Next comes neon, which takes a year to burn through, while oxygen takes about six months and silicon, just one day. So, how about that 15 minutes we were talking about at the beginning of the article?
That refers to the very last reaction this massive star will attempt to produce, which is fusing iron. But iron cannot be fused. Some weird physics take place. Little particles called electrons (which normally orbit the core of atoms, the building blocks of all matter) crash into protons (which normally reside in the core of atoms). Electrons carry a negative charge and protons a positive charge. As these two particle types crash into each other, they create particles known as neutrons, which have a neutral charge.
It takes only 15 minutes to convert this dying star — the one with iron in its center — into a huge ball of neutrons. Scientists refer to this kind of star as a neutron star. These stars are stable, but only because, as Sutter explains, "you can only fit so many neutrons in a box." Through a process called degeneracy pressure, the individual neutrons stop the star from collapsing. (Degeneracy pressure can be overridden, as Sutter explains in this 2015 column, but a star of the size Sutter is discussing is too small to do it.)
To be sure, this is the life cycle for only one particular kind of star. More-massive stars will eventually explode into supernovas, spreading gas throughout their neighborhoods. Our own sun, a less-massive type of star, will one day lose its outer layers of gas and fade into a white dwarf.
"Ask a Spaceman" episodes are released weekly on Wednesdays at 12 p.m. EDT (1600 GMT), so "like" the show's Facebook page or check back later to see more. Sutter also responds to reader questions in every episode. On that page, you can learn more about past topics the show has covered, such as the Big Bang, Pluto and galaxy collisions.
Sutter is a cosmologist at The Ohio State University and chief scientist at Columbus Ohio's Center of Science and Industry. He has a long-running podcast, also called "Ask a Spaceman." You can catch all past episodes of that podcast here.