Even Stars Can Get the Hiccups

Stars come in all manner of sizes and temperatures. Many of the massive ones are nearing the end of their lives and at some point in the next few million years, will detonate as supernova explosions. Observing the early stages of these events is tricky though as we can never be sure when they will go pop! It would be great if we could narrow down the timeframe to help hone our search. One theorised phase is that massive stars can ‘hiccup’ with its core expanding and contracting rapidly. This is known as ‘pulsational pair-instability’ and finally a team of astronomers have actually caught a star having the hiccups!

A supernova marks the end of the life of a massive star. The event is one of the most energetic processes in the universe, releasing immense amounts of energy. There are two  types of supernova, Type I occurs in a binary star system and Type II at the end of a stars life. Stars that are more than 8 times the mass of the Sun run out of nuclear fuel and suddenly the outward pressure generated by fusion ceases. The core collapses under gravity causing a violent explosion that ejects the outer layers of the star. 

The process is an essential step in the evolution of life since all the heavy elements needed to form life have been synthesised inside massive stars and it is the supernova process that liberates them to spread throughout the universe. What remains is dependent on the mass of the progenitor star and will either be a neutron star or black hole. 

Before the star goes supernova however, there has for some time, been a theorised phase during which, the star undergoes what has been described as the ‘hiccups!’ Until now though, they have just remained a theory. The events are perhaps even more rare than a supernova happening so infrequently and only to exceptionally large stars between 60-150 times the mass of the Sun. 

The team published the details of their observations in the Astrophysical Journal where they also describe the process called ‘Pulsational Pair Instability,’ (PPI.) In massive stars, their core develops to a very high temperature which contracts and expand in rapid succession. This might occur in the last few years, or even days, the timescales are still not clear. Each time the stellar core pulsates, a shell of material is ejected causing the star to slowly loose mass. On occasions, the ejected shell of material collides with other shells creating the intense burst of energy that we should be able to see as hiccups. 

The rarity of the event and the relative faintness of the hiccup is what has made them hard to detect, until now! In December 2020, the team detected a supernova (SN2020acct) in a galaxy called NGC2981 and as expected, the light from it faded. Two months later, they detected light from the same region of the galaxy, that’s unusual since it is very unusual for a Type II supernova to repeat itself. 

Further study revealed the supernova the team had thought they had detected was light being produced by slow moving shells of material colliding near the star. It wasn’t a supernova. It turns out, the second burst of radiation was the supernova, the first was one of the first observations of a star suffering with hiccups!

Source : ‘Hiccuping’ stars caught in action in world first

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