A Wolf’s Story | by Brian Koberlein



19 January 2022

Hen 2-427, a Wolf–Rayet star.
ESA/Hubble & NASA, Judy Schmidt
Hen 2-427, a Wolf–Rayet star.

We frequently consider supernova explosions as inevitable for giant stars. Massive 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 turn into supernovae in any case.

Wolf-Rayet stars are among the many most huge stars recognized. They’re on the finish of their brief lives, however moderately than merely operating 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 could be over 200,000 Okay, making them essentially the most luminous stars recognized. However as a result of most of that gentle is within the ultraviolet vary, they aren’t notably vibrant to the bare eye.

In a large star, different elements are in layers before the star explodes.
Itai Raveh
In a big star, totally different parts are in layers earlier than the star explodes.

Even with the outer layers of a Wolf-Rayet star solid off, the central star remains to be way more huge 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’s going to ultimately run out of gas, resulting in a core-collapse supernova. However we will see the spectra of parts inside a supernova, and we’d by no means seen a spectrum that matched a Wolf-Rayet star. As our discovery of supernovae grew to become commonplace, some astronomers started to surprise if Wolf-Rayet stars had a quiet demise as a substitute. The thought was that they’d solid off sufficient outer layers that the remaining core would ultimately simply collapse instantly right into a black gap. No large explosion wanted. A silent demise to an enormous star.

A spectra from SN 2019hgp.
Itai Raveh
A spectra from SN 2019hgp.

This newest research exhibits that a minimum of some Wolf-Rayet stars do turn into supernovae. The group regarded on the spectrum of a supernova referred to as SN 2019hgp, which was found by the Zwicky Transient Facility (ZTF). The supernova’s spectrum had vibrant emission gentle indicating the presence of carbon, oxygen, and neon, however not hydrogen or helium. When the group regarded on the information extra intently, they discovered these specific emission strains weren’t attributable to parts 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 parts of hydrogen and helium. The enlargement of the nebula should have been pushed by robust stellar winds. This matches the construction of a Wolf-Rayet star extraordinarily properly. So it appears to be like like SN 2019hgp is the primary instance of a Wolf-Rayet supernova. Since then, comparable 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 advanced hybrid course of the place the higher layer of the star exploded whereas the core collapsed on to a black gap. It’ll 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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