Going ‘Nova: The Dying Breaths of a Star

Robert Lea
Mar 3 · 6 min read

Scientists have modelled the supernova that results as a pulsating supergiant’s life ends, with the potential of showing us what is happening with Betelgeuse.

Whilst astronomers are still diligently studying Betelgeuse with the hope of discovering what is causing the red supergiant’s surface to rapidly dim, physicists from UC Santa Barbara have devised a model to show the process that a dying star undergoes as it reaches the end of its life and goes supernova.

When a star the size of Betelgeuse — ten times that of the Sun — reaches the end of its life cycle its ‘death’ is marked by a spectacular and powerful explosion. These supernova events can be so luminous that they often outshine the entire light output of the galaxy in which the star sits. Betelgeuse’s dipping brightness has some astronomers theorising that this is the process which it is currently undergoing, but there are other explanations currently being posited.

Supernovae are stupendously energetic; many can briefly outshine an entire galaxy. Artist’s impression. (ESO/M. KORNMESSER)

It is very probable that Betelgeuse will go supernova within the next million years, sooner if this group of astronomers are correct, resulting in a spectacular display that will be visible from Earth, even during daylight.

Betelgeuse is a member of a family of stars calling pulsating semiregular variable stars. Even before this recent period of extreme dimming was observed, dimming as a result of the pulsating nature of the red supergiant had been observed. In fact, it is a well-known characteristic amongst red supergiants.

So much so, that researchers at UC Santa Barbara had already begun work making predictions about the brightness of the supernova that would result when a pulsating star explodes before the Betelgeuse was speculated to be undergoing such a process.

“We wanted to know what it looks like if a pulsating star explodes at different phases of pulsation,” explains physics graduate student Jared Goldberg. “Earlier models are simpler because they don’t include the time-dependent effects of pulsations.”

Goldberg, a National Science Foundation graduate research fellow, has authored a paper detailing a new model of how a star’s pulsation will affect the ensuing explosion as it reaches the end of its life with ars Bildsten, director of the campus’s Kavli Institute for Theoretical Physics (KITP) and Gluck Professor of Physics, and KITP Senior Fellow Bill Paxton. The paper is published in the Astrophysical Journal.

The Death of a Giant

When a red supergiant runs out of material to fuse within its core there is no longer an outward pressure to balance out the inward pull of the star’s own gravity. This usually occurs when the core is pure iron, the heaviest element that can be produced by fusion in the cores of normal stars. What results is a rapid gravitational collapse — often occurring in less than half a second — of the core. The immense speed of this core-collapse means that outer ‘puffy’ layers of the supergiant are too slow to react.

As the core of iron collapses in on itself, the atoms within it are broken down into their constituent electrons and protons. These particles are forced together by the immense pressure in the collapsing core, forming neutrons and in the process releasing high energy particles called neutrinos.

Neutrinos are often nicknamed ‘ghost particles’ due to the fact that as virtually massless and chargeless particles they barely interact with matter. In fact, neutrinos are normally so placid that around 100 trillion of these particles stream through your body every second.

Images in the top row depict an artist’s conception of the supernova explosion process. The corresponding images below were taken with the Hubble Space Telescope. Left: Just before the supernova explosion. A yellow supergiant is shining. Middle: The supernova exploding (the bottom image shows the fading supernova after the explosion). Right: A bright blue star observed.

Despite this, because supernovae are so-powerful and the numbers and energies of the neutrinos produced are so great that even though only a tiny fraction of them interact with the surrounding matter, it is still enough to trigger the onset of a massive shockwave blasting out through the outer layers.

The shocked material slams into these outer layers with such tremendous energy that the resultant explosive burst is of such a magnitude it is capable of outshining the entire galaxy that inhabits.

The explosion remains bright for a period of around 100 days, dying down because the radiation it emits can only escape when all the free electrons have been grabbed back by ionised hydrogen. This process, which results in the present hydrogen becoming neutral again progresses from the outer limits of the supernova towards its core. as a result, this means that astronomers can see deeper and deeper with the supernova and time goes by, with the light from its centre being the last to escape.

By this time, all that remains of the supernova is a dim glow which remains shining for years but gives little obvious indication of the violent process that occurred to create it.

How Does this Apply to Betelgeuse?

The characteristics of a supernova depend heavily on parameters of the star that gives rise to it. These parameters include the star’s mass, its radius, and the total energy of its explosive end. Consequently, this means that the fact that Betelgeuse is undergoing pulsations makes predicting just what its supernova explosion will look like, a challenge.

This comparison image shows the star Betelgeuse before and after its unprecedented dimming. The observations, taken with the SPHERE instrument on ESO’s Very Large Telescope in January and December 2019, show how much the star has faded and how its apparent shape has changed. (ESO/M. Montargès et al.)

Goldberg and his team have discovered that if an entire star is pulsing in unison — almost akin to it ‘breathing’ in and out — its supernova will progress as if it was produced by a star with a static radius. Contrary to this, if different layers of the star are pulsating out of sync with each other, the outer layers expand whilst the inner layers contract and vice versa.

The team used the synchronised pulsation case to create a model that gave them similar results to models that didn’t account for any pulsation at all. It just looks like a supernova from a bigger star or a smaller star at different points in the pulsation,” says Goldberg. “It’s when you start considering pulsations that are more complicated, where there’s stuff moving in at the same time as stuff moving out — then our model actually does produce noticeable differences.”

In examples in which the pulsations between the outer and inner layers did not align, the team found that as light escapes from progressively deeper layers the emissions should begin to resemble emissions from different sized stars.

“Light from the part of the star that is compressed is fainter,” Goldberg says, “just as we would expect from a more compact, non-pulsating star. Meanwhile, light from parts of the star that were expanding at the time would appear brighter, as though it came from a larger, non-pulsating star.”

Goldberg lays out the next steps for the team’s research, submission of their model and a summation of their results to Research Notes of the American Astronomical Society with Andy Howell, a professor of physics, and KITP postdoctoral researcher Evan Bauer. This will specifically include the simulations the team ran on the red supergiant Betelgeuse.

Original research: Goldberg. J. A, Bildsten. L, and Paxton. B, ‘A Massive Star’s Dying Breaths: Pulsating Red Supergiants and Their Resulting Type IIP Supernovae,’ The Astrophysical Journal, (2020), https://iopscience.iop.org/article/10.3847/1538-4357/ab7205

Rob is freelance science journalist from the UK, specialising in physics, astronomy, cosmology, quantum mechanics and obscure comic books.

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Robert Lea

Written by

Freelance science journalist. BSc Physics. Space. Astronomy. Astrophysics. Quantum Physics. SciComm. ABSW member. WCSJ Fellow 2019. IOP Fellow.

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