19 January 2022
We regularly consider supernova explosions as inevitable for big stars. Large star runs out of gas, gravity collapses its core and BOOM! However astronomers have lengthy thought a minimum of one sort of enormous star didn’t finish with a supernova. Referred to as Wolf-Rayet stars, they had been thought to finish with a quiet collapse of their core right into a black gap. However a brand new discovery finds they may change into supernovae in any case.
Wolf-Rayet stars are among the many most large stars recognized. They’re on the finish of their brief lives, however somewhat than merely working out of gas and exploding, they push out their outer layers with an especially highly effective stellar wind. This produces a surrounding nebula wealthy in ionized helium, carbon, and nitrogen, however nearly no hydrogen. The floor temperature of the remaining star might be over 200,000 Ok, making them essentially the most luminous stars recognized. However as a result of most of that gentle is within the ultraviolet vary, they don’t seem to be significantly vivid to the bare eye.
Even with the outer layers of a Wolf-Rayet star solid off, the central star continues to be far more large than the Solar. So that you’d determine it’s solely a matter of time earlier than it turns into a supernova. Irrespective of how far up the periodic desk fusion happens, it can finally run out of gas, resulting in a core-collapse supernova. However we are able to see the spectra of components inside a supernova, and we’d by no means seen a spectrum that matched a Wolf-Rayet star. As our discovery of supernovae turned commonplace, some astronomers started to marvel if Wolf-Rayet stars had a quiet demise as an alternative. The thought was that they might solid off sufficient outer layers that the remaining core would finally simply collapse instantly right into a black gap. No big explosion wanted. A silent demise to an enormous star.
This newest examine exhibits that a minimum of some Wolf-Rayet stars do change into supernovae. The staff seemed on the spectrum of a supernova generally known as SN 2019hgp, which was found by the Zwicky Transient Facility (ZTF). The supernova’s spectrum had vivid emission gentle indicating the presence of carbon, oxygen, and neon, however not hydrogen or helium. When the staff seemed on the information extra carefully, they discovered these specific emission strains weren’t brought on by components of the supernova instantly. As an alternative, they had been a part of a nebula increasing away from the star at greater than 1,500 km/s.
In different phrases, earlier than the supernova occurred, the progenitor star was surrounded by a nebula wealthy in carbon, nitrogen, and neon, whereas missing the lighter components of hydrogen and helium. The growth of the nebula will need to have been pushed by sturdy stellar winds. This matches the construction of a Wolf-Rayet star extraordinarily effectively. So it seems to be like SN 2019hgp is the primary instance of a Wolf-Rayet supernova. Since then, related supernovae have additionally been detected.
As a result of this supernova was recognized by spectra of the encircling nebula, it isn’t clear whether or not the explosion was a easy supernova, or whether or not it was a extra complicated hybrid course of the place the higher layer of the star exploded whereas the core collapsed on to a black gap. It is going to take extra observations to find out the small print. What’s clear is that a minimum of some Wolf-Rayet stars don’t go silently into the night time.
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