Studying the world of cosmic explosions
Just as every firework is different, so too are cosmic explosions. “It turns out that stars explode in many different ways, with each type of star exploding in its own unique manner,” says Avishay Gal-Yam, an astrophysicist at the Weizmann Institute of Science. With the support of the EU-funded Fireworks project, Gal-Yam and his team took a deep dive into the world of cosmic explosions – work that led to the discovery of a new type of supernova explosion. This first-of-its-kind exploding star, which was previously thought to have existed only in theory, arises from the death of a massive Wolf-Rayet star. According to Gal-Yam, spectroscopic analysis of the light emitted from the explosion led to the discovery of spectral signatures that are associated with specific elements. In this way, researchers were able to show that the explosion contained carbon, oxygen, and neon atoms, the latter an element that had not been observed in this manner in any supernova to date. The team also identified that the matter spouting cosmic radiation did not itself participate in the blast but rather originated from the space surrounding the volatile star. This, in turn, strengthened their hypothesis in favour of strong winds that took part in stripping the star of its outer envelope. “While we can’t say at this stage whether all Wolf-Rayet stars end their lives with a bang or not, and it might be that some of them do collapse quietly into a black hole, one thing is for certain, this one star did not undergo the ‘silent’ collapse often referred to in the past,” adds Gal-Yam.
Studying cosmic explosions shortly after they occur
The Fireworks project, which received support from the European Research Council (ERC), was also one of the first to study cosmic explosions shortly after they occur. In one case, the team witnessed an explosion just three hours after the fact. According to Gal-Yam, this timing allowed researchers to observe the debris and understand what happened right before the star exploded. “Not only were we able to learn that most massive stars explode inside thick clouds of gas, it also let us measure the size and composition of the star before the explosion,” he explains. Researchers also contributed to the discovery and characterisation of a new class of cosmic explosions. Sometimes called rapidly evolving transients, or RETs, these explosions are unique in their rapid evolution.
Tidal disruption events and high-energy neutrino particles
The project also contributed to the discovery of a possible association between tidal disruption events – when a star is ripped apart by a black hole – and the detection of high-energy neutrino particles. The European Southern Observatory (ESO) observed such an event, during which the extreme gravitational forces of the supermassive black hole ripped apart a sun-like star that had the misfortune of getting too close. This triggered the tidal disruption event, during which the star was ‘spaghettified’. According to an ESO press release, shocks and colliding debris as well as heat generated in accretion led to a burst of light. It possibly also produced high-energy neutrino particles. “This gave the event the appearance of being a very bright supernova explosion, even though the star would not have become a supernova on its own due to a lack of mass,” adds Gal-Yam. The Weizmann Institute of Science research team plans to continue their research into cosmic explosions via a new observatory in Israel and through the forthcoming ULTRASAT space mission.
Keywords
Fireworks, stars, cosmic explosions, supernova, cosmic radiation, massive stars, tidal disruption events, black hole, energy neutrino particles, European Southern Observatory
