Israeli Scientists Observe New Kind of Supernova

It hadn’t been clear whether super-massive Wolf-Rayet stars even go nova. They do, but some may collapse into black holes as well

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A Wolf–Rayet star and the gas around it, as seen by the Hubble Telescope
A Wolf–Rayet star and the gas around it, as seen by the Hubble Telescope
Gid'on Lev
Gid'on Lev

“Live fast, die young and leave a beautiful corpse,” the writer Willard Motley advised in 1947. Among the multitude of celestial bodies, there is a group of particularly massive specimens called Wolf-Rayet stars, which in astronomical terms burn very fast and leave behind a particularly spectacular corpse.

Stars of this type were first observed as early as 1867 by the Paris Observatory, but we did not know how they end their days. Now a team of scientists from the Weizmann Institute of Rehovot, led by Prof. Avishay Gal-Yam, has confirmed a supernova created by the death of a Wolf-Rayet star. The findings were published on Wednesday in the journal Nature

Until about a decade ago, observing a supernova, an exploding star, was a rarity. Improvements in measurement and analysis technology now make it possible to detect up to 50 per day, and also improved the likelihood of spotting rarer types of supernovae that had been hypothetical, until now.

At the core of every star is a constant process of nuclear fusion. Under extreme pressure, light elements fuse and create heavier elements. Hydrogen is the lightest element; two hydrogen atoms fusing become one helium atom; further fusions create carbon and oxygen, and so on. Iron is an end product because it is a particularly stable element. No nuclear energy can be extracted from it because its fusion consumes energy, and does not release any.

When atoms fuse in the star’s core, energy in the form of heat is produced. As long as the process of fusion continues, heat being created in its core causes the star to expand, countering the force of gravity, which causes the star’s mass to contract into the core.

The Hubble telescope in space, which has helped study Wolf–Rayet stars

When the amount of iron in a star exceeds a certain threshold, the star stops producing energy and this balance is disturbed. Then a black hole develops in the star’s core and it collapses into itself; or the rapidly contracting core heats up, creating shock waves and resulting in an explosion, disseminating the star matter into space. That is a supernova.

During its first seconds, the explosion may release more energy than is produced under normal conditions by all the stars in the galaxy combined, and the star’s brightness increases a billionfold or more.

Nature’s wasteful engine

High-mass stars are rare. The bigger they are, the rarer. Wolf-Rayet stars are among the biggest known, probably several tens of times the mass of the sun. They probably only survive a few million years, at most, less than a thousandth of the life expectancy of our sun, which is expected to slog on about 10 billion years.

“You might expect that because there is more mass, that is, more fuel, the star would live longer,” says Gal-Yam, dean of the physics department at the Weizmann Institute. “But it’s the other way around. Because massive stars fuse light elements into heaver ones at a higher temperature, the speed at which the fuel is used increases dramatically. You could say that small stars have less fuel but an economical engine, and the massive ones have a wasteful engine.”

The telescope used to see the super-massive star go nova

The fusion process in the massive stars winds up creating layers. Heavy elements formed through fusion concentrate in the core, where they formed, and are surrounded by layers of increasingly lighter elements. A star starting with hydrogen (the lightest element) will develop two layers intime: a core of helium surrounded by a hydrogen shell; and so on.

“Every time a new inner layer of heavier elements is added, like an onion,” says Gal-Yam.

Massive Wolf-Rayet stars are characterized by the absence of one or more of the outer layers. Where there is usually hydrogen on the surface, as is the case with most stars, Wolf-Rayet stars have helium, carbon or an even heavier element on their surface.

This might be because strong wind blowing on the star’s crust disperses the outermost layer of matter into space. Thus, these stars lose another layer every few hundred thousand years. “A massive star burns a very large amount of fuel in the core,” explains Gal-Yam. This fire creates radiation, which is emitted from the surface of the star and pushes out the atoms in the crust, a phenomenon known as “stellar wind”. The more massive the star, the stronger the radiation and thus in a fairly short period of time it peels off a layer of the star.

Due to the rarity and relatively short lifespan of Wolf-Rayet Stars, no supernovas of them have been observed so far, despite the rapid increase in the supernova detection of other stars. This engendered a hypothesis that Wolf-Rayet Stars do not explode, for whatever reason.

And then on June 8, 2019, the Zwicky Transient Facility in California detected a supernova on the fringes of a galaxy a billion light-years from Earth. Since this first discovery, another Wolf-Rayet Star supernova has been observed. The research team analyzed the spectral signature of the explosion to detect the extant elements the star had had; and discovered that the star had contained heavy element such as carbon, oxygen, and neon. It was the first time neon had been observed in any supernova.

The beautiful corpse

Since this was the first observation, it can’t be determined whether all Wolf-Rayet stars go nova. “Some may collapse and create a black hole,” Gal-Yam qualifies.

According to the researchers’ calculations, the mass disseminated by the blast was roughly equivalent to the mass of the sun, though the star’s original mass had been at least ten times greater than the sun’s mass. Gal-Yam thinks both possibilities might apply: after the star’s nuclear fusion process stops, an explosion takes place that ejects some mass into space, while at the same time, the rest of the mass collapses into the core and forms a black hole.

“We do not quite understand how the collapse and the explosion occur simultaneously,” the scientist says. According to him, one of the hypotheses for explaining the process, which is now being investigated by Dr. Doron Kushnir and Dr. Boaz Katz from the institute, is that when the core collapses, a nuclear explosion occurs simultaneously following uncontrolled nuclear fusion in the outer layers that causes their propulsion into space.

This is perhaps where the third element in Motley’s advice comes into play: leaving a beautiful corpse.

Supernova explosions disperse elements formed in the core of stars throughout the galaxy, ultimately creating new stars and planets in a sort of cosmic version of the life cycle. Indeed, the Earth itself and everything that exists in it, every mineral, plant and animal, every cell in our bodies is made up of elements created in this process. We are stardust, and to stardust we shall return.

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