A black hole is formed when a massive star dies at the end of its life. The outer layers of the star are launched into space by the huge amount of energy generated in the core collapse. The debris from the blast wave can light up with as much energy as billions of times the energy of the Sun, making it the most powerful wave in the universe.
A supernova is born. Not all supernovae are created equal.
Some are more powerful than others. Changing conditions from star to star can change the way one explodes, and more technical explanations can be found online. It depends on things like the mass of the star when it explodes, the mass it had when it was born, and how much of its outer layers blow off before the explosion.
Even though there are a lot of explosions, there are still some that are exceptional. Astronomers think they can explain what happened with one particular one.
The Last Alert System detected a supernova in a galaxy 200 million light years away. Astronomers are dorks just like everyone else, so the convention dubbed it AT2018cow, and it became known as the COW.
Astronomers were surprised to see that the explosion was 100 billion times brighter than the Sun, and other observatories started watching it as well. It is in the rare class of superluminous supernovae, ones which are more powerful than average.
It acted oddly. It rose in brightness much faster than usual. This one rose in brightness by a factor of 100 in a single day. It faded more quickly than typical supernovae. The new type of event was called a Fast Blue Optical Transient, because the COW was blue in the early days. If you want the details, I wrote a synopsis after it happened.
In the years since, there has been a lot of work trying to figure out how to make a star explode. A white dwarf torn apart by a black hole is one of the ideas. A team of astronomer think they have the answer.
Massive stars can blow a lot of their outer layers away. Betelgeuse blows out a slow dense wind of matter, and sometimes it is more violent. The material surrounding the pre-supernova star is called the circumstellar matter.
We have models for how the interiors of stars behave, and equations that can be solved to understand what is happening inside a star. They can be used to see what happens when a star explodes, and try to match the energies emitted to what is actually observed.
The COW was modeled to be 80 times the mass of the Sun. It is very rare for stars to get that big. A star like that loses a lot of its hydrogen outer layers during its life.
The Sun's mass is mostly helium, but the core is still crushingly-hefty. The star's core is like an onion, with ever-heavier elements in its center. Half of a solar mass was surrounding the core when it blew up, and the best model fit to the observations shows that. It is still about 150,000 times the mass of the Earth.
A bad thing happened inside the massive core. The super-high-energy rays of light are produced when the reaction is so energetic. The interior of the core is heating up thanks to the help of these rays.
There are unstable gamma rays at these energies. Pair production is a process where they can convert themselves into matter. The core is smaller because gravity squeezes it. The temperature goes up and the core expands. The process repeats when the gamma rays start converting again. This causes the core to blow material off its surface.
It can now be measured in days or weeks. Helium blows off its surface, and the pulsations get bigger and bigger until gravity wins. The core collapses, the temperature goes up, and it explodes.
The amount of energy released in a few days is much more than the Sun gives off over its lifetime. The blast wave from the explosion slams into the ejected helium, lighting it up. It would take many days to light it up and even longer for it to fade if there was a lot of this material around the star. It faded quickly because it was only half a solar mass.
The model they used reproduces the rapid change in brightness of the first 20 days of the supernova. After that, the usual type of supernova models work, with about 0.6 solar mass of radioactive nickel created in the nuclear furnace decaying into cobalt, generating so much light that it explains the supernova's brightness thereafter.
The total energy of the explosion doesn't affect the brightness over time, but what really dominates here is the material around the star. That is the key to understanding fast supernovae.
The core explosion of a massive star is so violent it tears itself apart, leaving nothing behind. Material falling back on this object may explain the bright light many weeks after the explosion. The first few weeks are dominated by the circumstellar matter.
It will be interesting to see how these models apply to any future FBOT supernovae. It is one thing to fit a specific case, but it is even more important to explain it to others.
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