Original | BRIAN KOBERLEIN
Translate | Linvo
We often think of supernova explosions as inevitable for massive stars. The star runs out of fuel, the core collapses due to gravity, and eventually explodes! But astronomers have long believed that at least one type of large star won't end up as a supernova. Stars known as Wolf-Rayet are thought to end with their cores quietly collapsing into black holes. But a new study has found that they may eventually become supernovae as well.
Wolf-Raye star is one of the largest known stars. When their short lives are coming to an end, instead of simply running out of fuel and exploding, they push the outer layer away with an extremely powerful stellar wind. This creates a nebula surrounded by ionized helium, carbon and nitrogen, with almost no hydrogen. The remaining stars can have surface temperatures of more than 200,000 K, which makes them the brightest known stars. But because most of the light is in the ultraviolet range, they're not particularly bright for the naked eye.
In a massive star, different elements are layered before the star explodes.
Even if the outer layer of Wolf-Raye star is stripped away, the central star will still have a much larger mass than the Sun. So you'd think it's only a matter of time before it becomes a supernova. Wherever fusion occurs on the periodic table, it will eventually run out of fuel, resulting in the formation of a supernova with a collapsed core. But by observing the spectra of elements inside supernovae, we never see spectra that match Wolf-Rayet's. As we discover more and more supernovae, some astronomers begin to wonder if Wolf-Rayet will die quietly. The idea is that they will peel off enough of the outer layer that the remaining cores will eventually collapse directly into a black hole without the need for a big bang. Massive stars will die silently.
This latest study suggests that at least some Wolf-Rayet stars do become supernovae. The team studied the spectra of a supernova called SN 2019hgp, which was discovered by the Zwicky Transient Facility (ZTF). The supernova's spectrum has bright emitted light, indicating the presence of carbon, oxygen, and neon, but not hydrogen and helium. When the team looked more closely at the data, they found that these particular emission lines were not directly caused by elements of the supernova. Instead, they are part of a nebula that is expanding outward at a rate of more than 1500 km/s.
Spectrum from SN 2019hgp
In other words, before the supernova explosion, the former star was surrounded by a nebula rich in carbon, nitrogen, and neon, lacking lighter hydrogen and helium elements. The expansion of the nebula must have been driven by strong stellar winds, which dovetails very well with the structure of Wolf-Rayet star. Therefore, SN 2019hgp seems to be the first example of a Wolf-Raye supernova. Since then, similar supernovae have been detected.
Because the supernova was determined by the spectrum of the surrounding nebulae, it's unclear whether the explosion was a simple supernova or a more complex hybrid process in which the star's upper layer explodes while the core collapses directly into a black hole, requiring more observation to determine the details. But what is certain is that at least some Wolf-Rayet stars will not disappear silently into the night.
original:
https://www.universetoday.com/154116/a-new-kind-of-supernova-has-been-discovered/