Even Stars Can Get the Hiccups

An artist’s impression of Pulsational Pair Instability, credit: Gemini Observatory/NSF/AURA/ illustration by Joy Pollard

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 Fred Lawrence Whipple Observatory’s 48-inch telescope captured this visible-light image of the Pinwheel galaxy (Messier 101) in June 2023. The location of supernova 2023ixf is circled. The observatory, located on Mount Hopkins in Arizona, is operated by the Center for Astrophysics | Harvard & Smithsonian. Hiramatsu et al. 2023/Sebastian Gomez (STScI)

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.

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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.

This new picture from the VLT Survey Telescope (VST) at ESO’s Paranal Observatory shows the remarkable super star cluster Westerlund 1 (eso1034). This exceptionally bright cluster lies about 16 000 light-years from Earth in the southern constellation of Ara (The Altar). It contains hundreds of very massive and brilliant stars, all of which are just a few million years old — babies by stellar standards. But our view of this cluster is hampered by gas and dust that prevents most of the visible light from the cluster’s stars from getting to Earth. Now, astronomers studying images of Westerlund 1 from a new survey of the southern skies [1] have spotted something unexpected in this cluster. Around one of the stars — known as W26, a red supergiant and possibly the biggest star known— they have discovered clouds of glowing hydrogen gas, shown as green features in this new image. Such glowing clouds around massive stars are very rare, and are even rarer around a red supergiant— this is the first ionised nebula discovered around such a star. W26 itself would be too cool to make the gas glow; the astronomers speculate that the source of the ionising radiation may be either hot blue stars elsewhere in the cluster, or possibly a fainter, but much hotter, companion star to W26. W26 will eventually explode as a supernova. The nebula that surrounds it is very similar to the nebula surrounding SN1987A, the remnants of a star that went supernova in 1987 [2]. SN1987A was the closest observed supernova to Earth since 1604, and as such it gave astronomers a chance to explore the properties of these explosions. Studying objects like this new nebula around W26 will help astronomers to understand the mass loss processes around these massive stars, which eventually lead to their explosive demise. Notes [1] This picture forms part of a detailed public survey of a large part of the Milky Way called VPHAS+ that is using the power of the VST to search for new objects such as young stars and planetary nebulae. A spectacular recent picture of the Prawn Nebula was made using observations from the same survey. [2] This nebula is thought to have surrounded SN1987A’s progenitor star since before it went supernova. Links Research paper Photos of the VLT Survey Telescope Other images from the VST

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

Mark Thompson