Weizmann Institute scientists have identified a new type of supernova in a galaxy a billion light years away
In the recent past, spotting a supernova - an exploding star - was considered a rare sight. when Prof. Avishai Gal-Yam, for example, studied for a PhD, he found seven supernovae in four years. Today, improvements in the means of measurement and analysis make it possible to predict about fifty such explosions every day. The improved ability of astrophysicists to witness supernovae may make the celestial event less exciting, but the increase in frequency makes it more likely to witness rarer types of explosions - which until now were only considered theoretical. Recently, Prof. Gal-Yam and his research partners identified a supernova of a type that had not been observed until now. their findings are published today in the scientific journal Nature.
At the core of every star, at any given moment, a process of nuclear fusion takes place: light elements merge and turn, little by little, into heavier elements. Fusion of hydrogen nuclei turns into helium, which turns into carbon and oxygen, and so on. At the end of the process, iron is formed - from which nuclear energy cannot be produced. In a normal state, the energy generated in the star's core creates heat that tends to cause the star to expand, allowing it to maintain balance with gravity - which pushes the star's mass into the core. As soon as a star stops producing energy - the balance is violated, and can lead to one of two results: either a black hole will open in the core of the star - which causes it to collapse in on itself, or it will explode - and scatter the material from which it is made into the vastness of space.
This is, of course, a very long process. The lifespan of massive stars, which Prof. Gal-Yam studies - Dean of the Faculty of Physics at the Weizmann Institute of Science - is considered to be relatively short: a few million years at most. The sun, by comparison, is expected to live for about 10 billion years. In massive stars, the nuclear fusion in the core creates a situation where the star consists of layers - the heavy elements are found in its core, and increasingly lighter elements surround them. Wolf-ray stars are extremely massive stars in which one or more of the upper layers of the light elements are missing, so that instead of hydrogen - the lightest element - the surface will be characterized by helium, carbon or even a heavier element. A possible explanation for this phenomenon is that a strong wind blowing from the outer shell of the star scatters the outermost layer into space, and thus these stars lose another layer every few hundreds of thousands of years. When stars are observed without light elements in their outer layer - this is a still image that records a moment in a long process. But despite their relatively short life span, and the very fact that they are in an advanced disintegration process, so far no supernova originating from a Wolf-Raya star has been observed.
Analysis of the spectral signature of the radiation emitted by the star's explosion revealed the presence of neon - an element not previously observed in any supernova.
The rapid increase in supernova observations has led to the strengthening of the hypothesis that, for unknown reasons, Wolf-Raya stars do not explode - since, if they did, we would already be witnessing this type of explosion. But recently the members of Prof. Gal-Yam's research group managed to disprove the hypothesis and identify, for the first time, a supernova originating from this type of star. The analysis of the spectral signature of the explosion - from which it is possible to learn about the wavelengths of the light emission, and their belonging to one or another element - showed that the explosion contained carbon, oxygen and neon - an element that had not been observed before in any supernova. Beyond that, the scientists recognized that the material from which the cosmic radiation was emitted did not itself participate in the explosion, but came from the star's mantle - something that strengthens the theory of the strong wind.
Since this is a first observation, Prof. Gal-Yam says that it is too early to state unequivocally that this is the end of every star: "We cannot say at this stage whether all Wolf-Raya stars explode. It is possible that some of them do collapse into a black hole," he says. "We estimate that the mass scattered by the explosion was similar to or less than the mass of the Sun, while the star was originally much more massive - at least 10 times the mass of the Sun. So where did the majority of the mass go?" To explain this, he offers an intermediate scenario, in which both possibilities exist at the same time: after nuclear energy ceases to be generated in the star's core, an explosion occurs that throws part of the mass into space, while at the same time the rest of the mass collapses into the star's core and creates a black hole. "For sure," says Prof. Gal-Yam, "this is not the silent collapse they were talking about." It is worth noting that since this first discovery, another explosion of the Wolf-Raya star has been observed - that is, it is not a one-time event. It is possible that as the measurement and detection methods improve, witnessing these types of explosions - which are now considered exotic and rare - will become a matter of routine."
Supernova explosions may be perceived as distant and huge events that have no direct impact on our lives, but the truth is that they are at the core of life itself. The explosions allow the elements formed in the star's core to spread throughout the galaxy, from which new stars are formed. The Earth and everything that exists on it - including us - were created as a result of this process. "We investigate the origin of substances in nature, and look for explanations for phenomena we take for granted," concludes Prof. Gal-Yam. "This is what interests me - where did everything around us come from - and I want to understand this process as well as possible."
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