# How quickly does a supernova heat up/expand?

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Suppose there's a star out there that's a lot more massive than the Sun.

Suppose further that orbiting this star is a planet not unlike Earth. Water, oxygen, civilization, and all.

Now the star decides to go supernova. How quick, or slow, is the process?

How long will it take it to heat up to make life on that planet impossible? A month? A year? A hundred years? A thousand years?

And how long will it take for the supernova to engulf the planet? A day? A month? A year? A million years?

Ricky, it is very rapid. The core collapse and initial neutrino burst takes seconds to tens of seconds. We don't normally think about neutrino interactions, but so many are released that even this might be a problem for a nearby habitable planet. It then takes a few hours for the shockwave from the core collapse and bounce to make it out to the surface, accompanied by an intense flash of UV light that would likely sterilise anything in its planetary system. The outer layers of the exploding star are hurled out at around 1000 km/s, so could travel an astronomical unit in a day or so.

The supernova continues to become more luminous for about a week thereafter, increasing in luminosity, from what must have already been enormous (thousands of solar luminosities) by another factor of $10^5$. The equilibrium temperature of any planet scales roughly as $L^{1/4}$, so temperatures would rise by more than an order of magnitude in a week.

## How quickly does a supernova heat up/expand? - Astronomy

I'm not sure if this question directly relates to supernovae, but could you tell me how long nebulas last for? I know they are formed when a star goes supernova, but do they ever die or fade?

There are many different types of nebulae in astronomy, none of which actually have much to do with each other! But I'll answer your question specifically in regard to nebulae that come from material ejected in a supernova, which are often called "supernova remnants" (SNR). A famous example of a SNR is the Crab Nebula.

SNR do fade away and eventually become invisible. The time for this to happen is on the order of tens of thousands to a hundred thousand years. The reason that SNR eventually fade is simply that they only have a finite amount of energy input to them at their formation -- this energy comes from the material that was ejected by the central star in the supernova explosion. As this material moves away from the center and collides with gas in the region surrounding the star, it will lose some of its energy as it heats up the gas. The heated gas then releases this energy in the form of light, so eventually all the available energy will be released and the SNR will not shine any more.

We can estimate how long a SNR will shine if we measure the temperature of the gas that is being heated up by the shock wave from the central source. If we know the temperature and can estimate the amount of gas there is, we can calculate the rate at which the gas is emitting energy as well as the total amount of energy available for it to radiate therefore, we can estimate how long it will shine for.

Another effect to keep in mind is that as the shock wave from the explosion moves farther away from the center of the SNR, it will sweep up a lot of the surrounding gas. Some of the energy of the shock wave goes into accelerating the "new" material that is being swept up, so that overall, the speed of the shock has to go down. This means that eventually, far away from the center of the SNR, the shock wave won't be moving that fast and so it won't heat up the new gas that it encounters too much, and the SNR will not be as bright.

For more discussion of the phases of expansion in a SNR, have a look at this page from Imagine the Universe.

One final complication to the above discussion that is worth pointing out is the effect that the remnant of the exploded star's core, at the center of the SNR, might have on the SNR emission as a whole. If, for example, the supernova explosion left behind a rapidly rotating, magnetized neutron star (i.e. a pulsar), the pulsar can continue to contribute energy to the SNR long after the supernova explosion occurs. Astronomers think that this process is currently going on in the Crab Nebula.

This page was last updated on July 18, 2015.

### About the Author

#### Dave Rothstein

Dave is a former graduate student and postdoctoral researcher at Cornell who used infrared and X-ray observations and theoretical computer models to study accreting black holes in our Galaxy. He also did most of the development for the former version of the site.

## How quickly does a supernova heat up/expand? - Astronomy

I'm not sure if this question directly relates to supernovae, but could you tell me how long nebulas last for? I know they are formed when a star goes supernova, but do they ever die or fade?

There are many different types of nebulae in astronomy, none of which actually have much to do with each other! But I'll answer your question specifically in regard to nebulae that come from material ejected in a supernova, which are often called "supernova remnants" (SNR). A famous example of a SNR is the Crab Nebula.

SNR do fade away and eventually become invisible. The time for this to happen is on the order of tens of thousands to a hundred thousand years. The reason that SNR eventually fade is simply that they only have a finite amount of energy input to them at their formation -- this energy comes from the material that was ejected by the central star in the supernova explosion. As this material moves away from the center and collides with gas in the region surrounding the star, it will lose some of its energy as it heats up the gas. The heated gas then releases this energy in the form of light, so eventually all the available energy will be released and the SNR will not shine any more.

We can estimate how long a SNR will shine if we measure the temperature of the gas that is being heated up by the shock wave from the central source. If we know the temperature and can estimate the amount of gas there is, we can calculate the rate at which the gas is emitting energy as well as the total amount of energy available for it to radiate therefore, we can estimate how long it will shine for.

Another effect to keep in mind is that as the shock wave from the explosion moves farther away from the center of the SNR, it will sweep up a lot of the surrounding gas. Some of the energy of the shock wave goes into accelerating the "new" material that is being swept up, so that overall, the speed of the shock has to go down. This means that eventually, far away from the center of the SNR, the shock wave won't be moving that fast and so it won't heat up the new gas that it encounters too much, and the SNR will not be as bright.

For more discussion of the phases of expansion in a SNR, have a look at this page from Imagine the Universe.

One final complication to the above discussion that is worth pointing out is the effect that the remnant of the exploded star's core, at the center of the SNR, might have on the SNR emission as a whole. If, for example, the supernova explosion left behind a rapidly rotating, magnetized neutron star (i.e. a pulsar), the pulsar can continue to contribute energy to the SNR long after the supernova explosion occurs. Astronomers think that this process is currently going on in the Crab Nebula.

This page was last updated on July 18, 2015.

### About the Author

#### Dave Rothstein

Dave is a former graduate student and postdoctoral researcher at Cornell who used infrared and X-ray observations and theoretical computer models to study accreting black holes in our Galaxy. He also did most of the development for the former version of the site.

## Superluminous Supernova Puzzles Astronomers

Supernovae are surprisingly dependable. These brilliant and powerful explosions that mark the end of massive stars’ lives tend to shine anywhere from one hundred million to a few billion times brighter than the Sun for weeks on end. And their intrinsic brightness is always well known.

But in recent years a rare class of cosmic explosions, which are tens to hundreds of times more luminous than ordinary supernovae, has been discovered. And now one of these odd superluminous supernovae is mystifying astronomers further, with characteristics that simply don’t add up.

The Dark Energy Survey (DES) came online in August 2013 in order to investigate millions of galaxies for the subtle effects of weak lensing, the phenomenon where intervening invisible matter causes distant galaxies to appear minutely sheared and stretched.

The survey started off with a bang its first images revealed a rare superluminous supernova, dubbed DES13S2cmm, 7.8 billion light-years away.

“Fewer than forty such supernovae have ever been found and I never expected to find one in the first DES images,” said Andreas Papadopoulos from the University of Portsmouth in a press release. “As they are rare, each new discovery brings the potential for greater understanding or more surprises.”

The problem is this: DES13S2cmm doesn’t easily match the typical characteristics of a superluminous supernova. The stellar explosion could be seen in the data six months later, much longer than most other superluminous supernovae observed to date.

“Its unusual, slow decline was not apparent at first,” said Mark Sullivan from Southampton University. “But as more data came in and the supernova stopped getting fainter, we would look at the light curve and ask ourselves, ‘what is this?’ ”

So Sullivan decided to investigate further. But understanding its origins are proving difficult.

For some supernovae, the optical light we see is actually created by radioactivity. In fact, supernovae tend to create large amounts of radioactive elements, which don’t occur naturally on Earth. Nickel-56, with a half-life of roughly six days, is a common example.

As the nickel decays into cobalt, it releases gamma rays, which are trapped by the other material ejected by the supernova. These trapped rays heat up the surrounding material until it radiates in the optical. In this case, the peak magnitude of the supernova is directly proportional to the amount of nickel-56 created in the explosion.

“We have tried to explain the supernova as a result of the decay of the radioactive isotope nickel-56,” said coauthor Dr Chris D’Andrea of the University of Portsmouth. “But to match the peak brightness, the explosion would need to produce more than three times the mass of our Sun of the element. And even then the behavior of the light curve doesn’t match up.”

So the team is now investigating other explanations. In one intriguing scenario the supernova was relatively normal but created a magnetar — an extremely dense and highly magnetic neutron star that’s millions of times more powerful than the strongest magnets on Earth — whose energy made the explosion exceptionally bright.

But this explanation doesn’t match the data either.

A few months ago a team of astronomers led by Robert Quimby explained a superluminous supernovae, PS1-10afx, by a chance cosmic alignment, where intervening matter worked like a lens to deflect and intensify the background light for a typical Type Ia supernova. D’Andrea, however, doesn’t believe this is the case here.

“DES13S2cmm looks nothing like a normal type of supernova, either in its photometric evolution or its spectroscopy,” D’Andrea told Universe Today. “So while we can never be sure that a very faint but very massive galaxy lies between us and another object and is serendipitously brightening the object, there is no need to adopt that assumption in the case of DES13S2cmm.”

So astronomers are heading back to the drawing board.

“With so few known, it’s hard to really understand their properties in detail,” said Bob Nichol from the University of Portsmouth. “DES should find enough of these objects to allow us to understand superluminous supernovae as a population. But if some of these discoveries prove as difficult to interpret as DES13S2cmm, we’re prepared for the unusual.”

The results will be presented today at the National Astronomy Meeting 2014 in Portsmouth.

## Amateur Astronomer Captures Supernova’s First Light

By: Javier Barbuzano February 21, 2018 6

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In 2016 an amateur astronomer was testing his camera — and captured the first flash of a supernova.

Observing the exact moment that a supernova explodes has long been a challenge for astronomers. Initiatives are underway to catch this fleeting moment, but it’s an amateur astronomer who now claims the earliest detection of a supernova.

This series of negative images obtained at the moment of discovery show the supernova, a faint and rapidly brightening object in the southern, outer regions of the spiral galaxy NGC 613.
Víctor Buso & Gastón Folatelli

On September 20, 2016, amateur astronomer Víctor Buso was testing a new CCD camera in the homemade observatory he had built atop his home in Rosario, Argentina. He pointed his 40-centimeter Newtonian telescope to the spiral galaxy NGC 613 because it was near zenith and he knew it had a beautiful shape with an interesting structure full of bright and dark clouds — and also because he could see it without moving his heavy dome late at night, which would disturb the neighbors.

Buso observed the galaxy for approximately 90 minutes, taking 20-second exposures to avoid saturation by the bright city sky. The first series of images didn’t show anything unusual, but after a 45-minute break, Buso noticed that a pixel near the end of one of the galaxy’s spiral arms had brightened and was becoming brighter with each shot.

The pixel was initially so faint that Buso didn´t recognize it as a supernova right away. Nevertheless suspicious that the bright spot might be something interesting, he reached out to some professional astronomers — only to find that none were available. Then he called another amateur astronomer, Sebastian Otero, a member of the American Association of Variable Star Observers (AAVSO). Otero helped Buso send an international warning for other astronomers to follow up. Both amateurs are receiving credit as coauthors of the research article published in the February 22nd Nature.

Once the supernova was confirmed, receiving the official designation SN 2016gkg, extensive monitoring began, including with the Neil Gehrels Swift Observatory, which took X-ray, ultraviolet, and visible-light observations. However, Buso had captured the most valuable information in the earliest hours of the stellar explosion. What he imaged was what astronomers call the shock-breakout phase of the supernova, the moment when the shockwave traveling from the collapsing core of the star reaches the outer layers and breaks through the surface. The star’s outer layers of gas heat up as they’re ejected, and they brighten rapidly — in this case, at a rate of 40 magnitudes per day!

Until now, the shock-breakout phase was largely theoretical. Although hints of the phenomenon had been observed, it had never been definitively detected at visible wavelengths due in large part to its ephemeral nature — the shock wave only takes two to five hours to break out of the star. Moreover, much of the immediate emission is at high-energy wavelengths rather than visible light — that’s probably why Buso didn’t realize he had discovered a supernova and stopped observations before dawn.

Amateur astronomer Víctor Buso with his 40-centimeter Newtonian telescope at the Busonian Observatory, the homemade observatory Buso built on top of his house in Rosario, Argentina.

“Actually, some researchers were starting to question if the models were right,” says lead author Melina Bersten (Instituto de Astrofísica de La Plata, Argentina). “[Buso’s] observations are extremely invaluable winning the lottery is more likely to happen than what he did.” Bersten estimated the chance of such a serendipitous observation at 1 in a million, and given the bright city lights and other observing conditions, the chances might have been even lower than that.

Based on the discovery and follow-up observations, Bersten and her team determined that the exploding object had been a star in a binary system that had lost its outer layers of gas, leaving behind a helium-dominated core. This star, once about 20 times as massive as our Sun, had lost most of its mass to its companion star. By the time it exploded, it had shrunk to around five solar masses. Further analysis of the data will enable researchers to learn more about the structure of the star before it exploded, and about the physical processes that occur during supernovae.

Buso, who works as a locksmith, affirms that the excitement of finding something that none had observed before has brought him and his family and friends extreme joy. “Sometimes I wonder why I do this, why I put so many hours and so much passion into this . . . Now, I have found the answer.”

## This Is How We’d All Die Instantly If The Sun Suddenly Went Supernova

The ultra-massive star Wolf-Rayet 124, shown with its surrounding nebula, is one of thousands of … [+] Milky Way stars that could be our galaxy’s next supernova. It’s also much, much larger and more massive than you’d be able to form in a Universe containing only hydrogen and helium, and may already be onto the carbon-burning stage of its life.

Hubble Legacy Archive / A. Moffat / Judy Schmidy

As far as raw explosive power goes, no other cataclysm in the Universe is both as common and as destructive as a core-collapse supernova. In one brief event lasting only seconds, a runaway reaction causes a star to give off as much energy as our Sun will emit over its entire 10-12 billion year lifetime. While many supernovae have been observed both historically and since the invention of the telescope, humanity has never witnessed one up close.

Recently, the nearby red supergiant star, Betelgeuse, has started exhibiting interesting signs of dimming, leading some to suspect that it might be on the verge of going supernova. While our Sun isn’t massive enough to experience that same fate, it’s a fun and macabre thought experiment to imagine what would happen if it did. Yes, we’d all die in short order, but not from either the blast wave or from radiation. Instead, the neutrinos would get us first. Here’s how.

An animation sequence of the 17th century supernova in the constellation of Cassiopeia. This … [+] explosion, despite occurring in the Milky Way and about 60-70 years after 1604, could not be seen with the naked eye due to the intervening dust. Surrounding material plus continued emission of EM radiation both play a role in the remnant’s continued illumination. A supernova is the typical fate for a star greater than about 10 solar masses, although there are some exceptions.

NASA, ESA, and the Hubble Heritage STScI/AURA)-ESA/Hubble Collaboration. Acknowledgement: Robert A. Fesen (Dartmouth College, USA) and James Long (ESA/Hubble)

A supernova — specifically, a core-collapse supernova — can only occur when a star many times more massive than our Sun runs out of nuclear fuel to burn in its core. All stars start off doing what our Sun does: fusing the most common element in the Universe, hydrogen, into helium through a series of chain reactions. During this part of a star’s life, it’s the radiation pressure from these nuclear fusion reactions that prevent the star’s interior from collapsing due to the enormous force of gravitation.

So what happens, then, when the star burns through all the hydrogen in its core? The radiation pressure drops and gravity starts to win in this titanic struggle, causing the core to contract. As it contracts, it heats up, and if the temperature can pass a certain critical threshold, the star will start fusing the next-lightest element in line, helium, to produce carbon.

This cutaway showcases the various regions of the surface and interior of the Sun, including the … [+] core, which is where nuclear fusion occurs. As time goes on, the helium-containing region in the core expands and the maximum temperature increases, causing the Sun’s energy output to increase. When our Sun runs out of hydrogen fuel in the core, it will contract and heat up to a sufficient degree that helium fusion can begin.

Wikimedia Commons user Kelvinsong

This will occur in our own Sun some 5-to-7 billion years in the future, causing it to swell into a red giant. Our parent star will expand so much that Mercury, Venus, and possibly even Earth will be engulfed, but let’s instead imagine that we come up some clever plan to migrate our planet to a safe orbit, while mitigating the increased luminosity to prevent our planet from getting fried. This helium burning will last for hundreds of millions of years before our Sun runs out of helium and the core contracts and heats up once again.

For our Sun, that’s the end of the line, as we don’t have enough mass to move to the next stage and begin carbon fusion. In a star far more massive than our Sun, however, hydrogen-burning only takes millions of years to complete, and the helium-burning phase lasts merely hundreds of thousands of years. After that, the core’s contraction will enable carbon fusion to proceed, and things will move very quickly after that.

As it nears the end of its evolution, heavy elements produced by nuclear fusion inside the star are … [+] concentrated toward the center of the star. When the star explodes, the vast majority of the outer layers absorb neutrons rapidly, climbing the periodic table, and also get expelled back into the Universe where they participate in the next generation of star and planet formation.

Carbon fusion can produce elements such as oxygen, neon, and magnesium, but only takes hundreds of years to complete. When carbon becomes scarce in the core, it again contracts and heats up, leading to neon fusion (which lasts about a year), followed by oxygen fusion (lasting for a few months), and then silicon fusion (which lasts less than a day). In that final phase of silicon-burning, core temperatures can reach

3 billion K, some 200 times the hottest temperatures currently found at the center of the Sun.

And then the critical moment occurs: the core runs out of silicon. Again, the pressure drops, but this time there’s nowhere to go. The elements that are produced from silicon fusion — elements like cobalt, nickel and iron — are more stable than the heavier elements that they’d conceivably fuse into. Instead, nothing there is capable of resisting gravitational collapse, and the core implodes.

Artist’s illustration (left) of the interior of a massive star in the final stages, pre-supernova, … [+] of silicon-burning. (Silicon-burning is where iron, nickel, and cobalt form in the core.) A Chandra image (right) of the Cassiopeia A supernova remnant today shows elements like Iron (in blue), sulphur (green), and magnesium (red). We do not know whether all core-collapse supernovae follow the same pathway or not.

NASA/CXC/M.Weiss X-ray: NASA/CXC/GSFC/U.Hwang & J.Laming

This is where the core-collapse supernova happens. A runaway fusion reaction occurs, producing what’s basically one giant atomic nucleus made of neutrons in the star’s core, while the outer layers have a tremendous amount of energy injected into them. The fusion reaction itself lasts for only around 10 seconds, liberating about 10 44 Joules of energy, or the mass-equivalent (via Einstein’s E = mc 2 ) of about 10 27 kg: as much as you’d release by transforming two Saturns into pure energy.

That energy goes into a mix of radiation (photons), the kinetic energy of the material in the now-exploding stellar material, and neutrinos. All three of these are more than capable of ending any life that’s managed to survive on an orbiting planet up to that point, but the big question of how we’d all die if the Sun went supernova depends on the answer to one question: who gets there first?

The anatomy of a very massive star throughout its life, culminating in a Type II Supernova when the … [+] core runs out of nuclear fuel. The final stage of fusion is typically silicon-burning, producing iron and iron-like elements in the core for only a brief while before a supernova ensues. Many of the supernova remnants will lead to the formation of neutron stars, which can produce the greatest abundances of the heaviest elements of all by colliding and merging.

When the runaway fusion reaction occurs, the only delay in the light getting out comes from the fact that it’s produced in the core of this star, and the core is surrounded by the star’s outer layers. It takes a finite amount of time for that signal to propagate to the outermost surface of the star — the photosphere — where it’s then free to travel in a straight line at the speed of light.

As soon as it gets out, the radiation will scorch everything in its path, blowing the atmosphere (and any remaining ocean) clean off of the star-facing side of an Earth-like planet immediately, while the night side would last for seconds-to-minutes longer. The blast wave of the matter would follow soon afterwards, engulfing the remnants of our scorched world and quite possibly, dependent on the specifics of the explosion, destroying the planet entirely.

But any living creature would surely die even before the light or the blast wave from the supernova arrived they’d never see their demise coming. Instead, the neutrinos — which interact with matter so rarely that an entire star, to them, functions like a pane of glass does to visible light — simply speed away omnidirectionally, from the moment of their creation, at speeds indistinguishable from the speed of light.

Moreover, neutrinos carry an enormous fraction of a supernova’s energy away: approximately 99% of it. In any given moment, with our paltry Sun emitting just

4 × 10 26 joules of energy each second, approximately 70 trillion (7 × 10 13 ) neutrinos pass through your hand. The probability that they’ll interact is tiny, but occasionally it will happen, depositing the energy it carries into your body when it happens. Only a few neutrinos actually do this over the course of a typical day with our current Sun, but if it went supernova, the story would change dramatically.

A neutrino event, identifiable by the rings of Cerenkov radiation that show up along the … [+] photomultiplier tubes lining the detector walls, showcase the successful methodology of neutrino astronomy and leveraging the use of Cherenkov radiation. This image shows multiple events, and is part of the suite of experiments paving our way to a greater understanding of neutrinos. The neutrinos detected in 1987 marked the dawn of both neutrino astronomy as well as multi-messenger astronomy.

Super Kamiokande collaboration

When a supernova occurs, the neutrino flux increases by approximately a factor of 10 quadrillion (10 16 ), while the energy-per-neutrino goes up by around a factor of 10, increasing the probability of a neutrino interacting with your body tremendously. When you work through the math, you’ll find that even with their extraordinary low probability of interaction, any living creature — from a single-celled organism to a complex human being — would be boiled from the inside out from neutrino interactions alone.

This is the scariest outcome imaginable, because you’d never see it coming. In 1987, we observed a supernova from 168,000 light-years away with both light and neutrinos. The neutrinos arrived at three different detectors across the world, spanning about 10 seconds from the earliest to the latest. The light from the supernova, however, didn’t begin arriving until hours later. By the time the first visual signatures arrived, everything on Earth would have already been vaporized for hours.

A supernova explosion enriches the surrounding interstellar medium with heavy elements. The outer … [+] rings are caused by previous ejecta, long before the final explosion. This explosion also emitted a huge variety of neutrinos, some of which made it all the way to Earth.

Perhaps the scariest part of neutrinos is how there’s no good way to shield yourself from them. Even if you tried to block their path to you with lead, or a planet, or even a neutron star, more than 50% of the neutrinos would still get through. According to some estimates, not only would all life on an Earth-like planet be destroyed by neutrinos, but any life anywhere in a comparable solar system would meet that same fate, even out at the distance of Pluto, before the first light from the supernova ever arrived.

The only early detection system we’d ever be able to install to know something was coming is a sufficiently sensitive neutrino detector, which could detect the unique, surefire signatures of neutrinos generated from each of carbon, neon, oxygen, and silicon burning. We would know when each of these transitions happened, giving life a few hours to say their final goodbyes during the silicon-burning phase before the supernova occurred.

There are many natural neutrino signatures produced by stars and other processes in the Universe. … [+] Every set of neutrinos produced by a different fusion process inside a star will have a different spectral energy signature, enabling astronomers to determine whether their parent star is fusing carbon, oxygen, neon, and silicon in its interior, or not.

IceCube collaboration / NSF / University of Wisconsin

It’s horrifying to think that an event as fascinating and destructive as a supernova, despite all the spectacular effects it produces, would kill anything nearby before a single perceptible signal arrived, but that’s absolutely the case with neutrinos. Produced in the core of a supernova and carrying away 99% of its energy, all life on an Earth-like would receive a lethal dose of neutrinos within 1/20th of a second as every other location on the planet. No amount of shielding, even from being on the opposite side of the planet from the supernova, would help at all.

Whenever any star goes supernova, neutrinos are the first signal that can be detected from them, but by the time they arrive, it’s already too late. Even with how rarely they interact, they’d sterilize their entire solar system before the light or matter from the blast ever arrived. At the moment of a supernova’s ignition, the fate of death is sealed by the stealthiest killer of all: the elusive neutrino.

## Best-candidate supernova erupting every year and on the brink of catastrophe

A quasi-true colour Liverpool Telescope image of the region of M31 around the recurrent nova M31N 2008-12a. The yellow backdrop is the unresolved stars of M31, and the dark patches are regions of dust and gas. The bright stars seen in the image are in the foreground and belong to our own Galaxy. Marked by the white arrow, the nova can be seen in eruption in the centre of the image. Credit: M. Darnley / LJMU

Using the robotic Liverpool Telescope, an international team of scientists has found what looks like the best pre-explosion candidate yet for a 'type 1a' supernova, where a massive and extremely dense star in the Andromeda Galaxy is dragging material away from its companion. This star is set to be completely destroyed in the (astronomical) near future in a catastrophic explosion. Matt Darnley of Liverpool John Moores University presented their results today (9 July) at the National Astronomy Meeting at Venue Cymru, in Llandudno, Wales.

Our Sun is expected to have a relatively gentle end to its life. But some stars have a more violent demise in prospect – they are destined to explode as supernovae, briefly shining as brightly as a whole galaxy of stars. One class of these explosions, type 1a supernovae (SN1a), is fundamental to our understanding of the evolution of the Universe.

Some pairs of stars or binary systems are particularly close together. Where one of the stars is a white dwarf (the long extinguished superdense remnant of a star that was once similar to our Sun), and the other is a more normal companion, the gravity of the white dwarf fundamentally changes both objects. The outer atmosphere of the normal star, mostly hydrogen and helium, flows towards the white dwarf, eventually building up as a thick layer on its surface.

Under the right conditions, this material will compress and heat up enough for runaway nuclear fusion to take place, similar to that in a hydrogen bomb, but far more powerful than anything seen on Earth. This explosion is a nova – meaning 'new star' – and for a short period the system will have the brightness of between 100 and 500 thousand Suns. Some, but by no means all, of the accumulated material from the companion star will be ejected into space.

Of the 400 novae seen in our Galaxy, the Milky Way, a handful have been seen to erupt more than once. These 'recurrent novae' erupt frequently as the mass of the white dwarf is already high from the millions of years of transfer of material and its companion star is losing material at a high rate. In the Milky Way, the most active recurrent nova is U Scorpii, which erupts about once a decade.

But the cycle of explosions cannot go on for ever. Once a white dwarf accumulates close to 1.4 times the mass of the Sun – the 'critical mass' – its core temperature will have risen to around 500 million degrees (30 times hotter than the centre of the Sun). The stellar material subsequently undergoes another and much more powerful thermonuclear reaction, in an enormous explosion that destroys the white dwarf in a few seconds, releasing vast amounts of energy in the process. This is a type 1a supernova, and for a number of days it has the brightness of billions of Suns.

In 2008 scientists spotted the eruption of a star, later confirmed to be a nova, in the Andromeda Galaxy (M31), the nearest large galaxy to our own some 2.5 million light years away. Remarkably the same star, catalogued as M31N 2008-12a, erupted again in 2009, 2011, 2012, 2013 and 2014. Darnley and his team initiated a follow up programme in 2013 and 2014, using the Liverpool Telescope and X-ray observations from the orbiting Swift observatory.

Their work shows that in astronomical terms, M31N 2008-12a is on the brink of catastrophe. With explosions in rapid succession, the white dwarf must be just a fraction under the critical mass and could be torn to pieces in a supernova anytime in the next few hundred thousand years.

Darnley commented: "We've never seen anything like this before. Here is a pair of stars that release vast amounts of energy almost every year. The system is right on the cusp of total destruction, so we are getting a first look at how it is changing right before it erupts as a supernova. And that could happen tomorrow, or hundreds of thousands of years in the future – it's very much a star system to watch."

This UK and international team hope to continue to monitor M31N 2008-12a for the foreseeable future. Type 1a supernovae are all thought to have similar brightnesses, so are used as 'standard candles' to gauge the distance to galaxies and measure the properties of the Universe as a whole. Understanding systems like M31N 2008-12a is a key part of that.

## This Early Warning Signal Could Successfully Predict Betelgeuse’s Supernova

As Betelgeuse continues to vary in brightness in the night sky, it reminds us that this is an object that could explode in a spectacular supernova at any point in the foreseeable future. With approximately 20 times the mass of the Sun and already in the red supergiant phase of its life, Betelgeuse is already burning elements heavier than hydrogen and helium in its core. At some point in the not too distant future, whether it’s days, years, or millennia away, we fully expect it to die in the most visually stunning way of all.

While a whole slew of signals will arrive once the supernova actually occurs, from neutrinos to light of all different energies and wavelengths, the outward, visual appearance of the star will not give any surefire clues that a supernova is imminent. But the nuclear reactions powering the star do change over time, and at just 640 light-years away, Betelgeuse’s neutrinos may give us the early warning signal we need to predict its supernova accurately, after all.

In order to become the red supergiant that we observe today, Betelgeuse needed to undertake a slew of important evolutionary steps. It needed for the enormous cloud of gas that it was born from to collapse, with a large amount (maybe 30-to-50 Sun’s worth) of mass contracting down to eventually form a proto-star. It needed for nuclear fusion to ignite in its core, fusing hydrogen to helium like our Sun does, albeit hotter, faster, and over a larger volume of space.

It needed for millions of years to pass and its core to run out of hydrogen, so that the internal radiation pressure drops, the core contracts and heats up farther, and the star swells into a red giant. In this giant phase, helium fusion began to occur, as every three helium nuclei wind up fusing together into a carbon nucleus, while hydrogen-burning continues in a shell around the helium-fusing core. At last, when the core runs out of helium, the star becomes a supergiant.

The reason is straightforward: a star is simply an object where the outward pressure of radiation balances the force of gravity that works so hard to collapse all that mass. When the radiation pressure drops, the star contracts when the radiation pressure increases, the star expands. Whenever the star runs out of whatever core fuel its burning, the core will contract, heat up, and — if it gets hot enough — begin burning the next element in line in its nuclear furnace.

With the transition from helium-burning to carbon-burning, the temperature rises so high that a series of shell burning commences: carbon on the inside, helium surrounding it, and hydrogen outside of that. The radiation pressure increases so significantly that the material outside of the outermost shell begins forming large convective cells, forming plumes of irregular ejecta, and swelling to beyond the size of Jupiter’s orbit around the Sun.

Although there are certainly changes happening inside of Betelgeuse’s core, those changes have a delayed effect in how they propagate to the star’s outer layers. Just as the photons created in the Sun’s interior take on the order of

100,000 years to propagate to the Sun’s photosphere, the energy created in Betelgeuse’s core take on the order of at least thousands of years to propagate to the surface.

Because of the complexities of energy transfer within the interior of a star, the small changes we’re seeing in the outermost layers of Betelgeuse today are most likely unrelated to a transition occurring in Betelgeuse’s core they’re far more likely to be due to instabilities in the tenuous outer layers of the star. Even if Betelgeuse has moved on from carbon fusion to begin burning heavier elements still — elements like neon, oxygen, and silicon — those stages only take a few years to complete.

When your supergiant star begins fusing carbon, that stage takes on the order of 100,000 years to burn to completion, the overwhelming majority of the time a star spends in the supergiant phase. Neon burning takes only a few years at most oxygen burning typically takes merely months silicon burning endures for only a day or two at most. These last stages do not result in any significant temperature changes or photosphere changes that are observable in a meaningful way.

If we want to know what’s going on in the core of a star — our only true indicator of when a supernova is coming — observing the electromagnetic properties of the star won’t give it to us there is no change in a star’s temperature, brightness, or spectrum that occurs after the transition from carbon-burning to heavier elements.

In the lead-up to a supernova, the neutrinos carry away the vast majority of the energy produced in those core fusion reactions. For the carbon burning phase, the neutrinos are emitted with a particular energy signature: a specific luminosity and a specific maximum energy-per-neutrino. As we transition from carbon-burning to neon-burning, oxygen-burning, silicon-burning, and eventually the core-collapse phase, both the energy flux of neutrinos and the energy-per-neutrino increase.

According to a paper by Polish physicist Andrzej Odrzywołek and his collaborators, this leads to an important observable signature. During the silicon-burning phase, neutrinos are produced with higher energies than previously, and as the silicon-burning phase continues, shells of silicon fusion begin forming around the core. In the final few hours of this star’s life, shortly before the core collapses, the neutrinos produced cross a critical energy threshold, labeled E_th above.

What’s going on inside these stars? When you start burning carbon (or anything heavier) inside your star’s interior, the process is energetic enough to begin producing positrons — the antimatter counterpart of electrons — in copious amounts. These positrons annihilate with electrons, which will sometimes lead to the production of neutrinos and antineutrinos, which simply carry energy omnidirectionally away from the star entirely.

When the antineutrinos arrive on Earth, which some of them inevitably will, they’re typically indistinguishable from the “natural” sources of antineutrinos that show up in our detectors: from radioactive processes in Earth’s interior and in nuclear reactors. But when you cross that critical energy threshold, E_th, your antineutrinos can interact with the protons in your detector, producing a unique signature: neutrons and positrons, an unmistakable signal of inverse beta decay.

Under normal circumstances, inverse beta decay events are extreme rarities in neutrino detectors, coming about only when a random neutrino from the Universe strikes our sophisticated neutrino detectors. But if a star were burning silicon in its core and had crossed that critical energy threshold to produce sufficiently energetic antineutrinos, and if it were close enough, we should see a large number of inverse beta decay events that all come from the same direction.

Based on a 2004 calculation, a tank that contained 1,000 tons of water should see approximately 32 events per day from a late-stage silicon-burning star located at the distance of Betelgeuse. Super-Kamiokande, presently the largest water-based neutrino detector, holds 50,000 tons of water, and will be upgraded to Hyper-Kamiokande, holding 260,000 tons. These correspond to 1,600 and 8,300 events-per-day, respectively, enough to give an unambiguous supernova warning.

In the first hour, in fact, Super-Kamiokande alone should see somewhere between 60 and 70 antineutrinos interacting with their detector, producing this specific inverse beta decay reaction with direction data inherent to it. The additional fact that the antineutrinos are expected to arrive in peaks, as the silicon-burning core and the silicon-burning shells outside it oscillate, would provide additional information that Betelgeuse is about to blow.

In fact, this technique is so remarkably good that by the time Hyper-Kamiokande is operational, we should be able to detect any star that would go supernova within about 7,000 light-years very robustly: we’d receive about 3 positron-producing antineutrinos per hour with directional information in our detector. If a star went supernova at the present distance of the Crab Nebula, which itself was created in a supernova explosion about 1,000 years ago, we’d definitely be able to see it coming.

Even stars as far away as the galactic center might emit a handful of detectable neutrinos in time to herald the imminent arrival of a supernova.

Sure, it’s only a few hours of warning time, but it would represent one of the most spectacular achievements of modern science: the ability to know precisely when the most visually stunning astronomical event in centuries would occur. We could have a series of multiwavelength observatories all pointing at Betelgeuse even before the moment of its supernova, just waiting to observe whatever signatures come out, and catch them all in the act of emerging for the first time.

It’s true that the big flux of neutrinos, which occur at the moment of core-collapse, will still arrive and herald the arrival of the supernova itself. But for a brief window beforehand, there’s an observable signature that would tip us off to what’s coming. If you’ve got a spare ton of water lying around and the technology to build a neutrino detector, an impending supernova would deliver you 2-to-3 neutrinos per hour once the critical antineutrino energy threshold was crossed. With the right technology, this fascinating theoretical work demonstrates that even a supernova can be successfully predicted.

## Stripped of coolant

One possibility is that most of the mass in some protogalaxies collapsed into monster black holes, which could then merge to give rise to the earliest supermassive versions at the hearts of galaxies, says Daniel Whalen of the Los Alamos National Laboratory in New Mexico. Most early galaxies started out as clouds of atomic hydrogen that were too hot to form stars. They eventually cooled down and began forming molecular hydrogen, which helped them chill even faster. In these galaxies, dense packets of cool gas were able to collapse and ignite new stars.

However, some nearby protogalaxies were then bathed in strong ultraviolet radiation generated by the newborn stars, and this stripped away their molecular hydrogen. Without any coolant, these protogalaxies couldn’t make stars and began to heat up instead. “The gas just gets hotter and hotter and can’t collapse any further,” says Whalen. He and colleagues ran computer simulations that show hot protogalaxies can grow to be 100 million times the mass of the sun without forming a single star.

When a protogalaxy reaches that mass, gas falling in from intergalactic space gets so hot that the hydrogen atoms collide violently, moving their electrons from their lowest energy levels to the next highest rungs on the atomic energy ladder. When these electrons return to their original state, they emit a photon that carries energy away. In other words, the protogalaxy’s gas finally has a way to cool down.

## Lopsided star explosion holds the key to other supernova mysteries

The core of a core-collapse supernova in the pre-explosion. Credit: Ott/Caltech (simulation), Drasco/Calpoly San Luis Obsipo (visualization)

New observations of a recently exploded star are confirming supercomputer model predictions made at Caltech that the deaths of stellar giants are lopsided affairs in which debris and the stars' cores hurtle off in opposite directions.

While observing the remnant of supernova (SN) 1987A, NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, recently detected the unique energy signature of titanium-44, a radioactive version of titanium that is produced during the early stages of a particular type of star explosion, called a Type II, or core-collapse supernova.

"Titanium-44 is unstable. When it decays and turns into calcium, it emits gamma rays at a specific energy, which NuSTAR can detect," says Fiona Harrison, the Benjamin M. Rosen Professor of Physics at Caltech, and NuSTAR's principal investigator.

By analyzing direction-dependent frequency changes—or Doppler shifts—of energy from titanium-44, Harrison and her team discovered that most of the material is moving away from NuSTAR. The finding, detailed in the May 8 issue of the journal Science, is the best proof yet that the mechanism that triggers Type II supernovae is inherently lopsided.

NuSTAR recently created detailed titanium-44 maps of another supernova remnant, called Cassiopeia A, and there too it found signs of an asymmetrical explosion, although the evidence in this case is not as definitive as with 1987A.

Supernova 1987A was first detected in 1987, when light from the explosion of a blue supergiant star located 168,000 light-years away reached Earth. SN 1987A was an important event for astronomers. Not only was it the closest supernova to be detected in hundreds of years, it marked the first time that neutrinos had been detected from an astronomical source other than our sun.

These nearly massless subatomic particles had been predicted to be produced in large quantities during Type II explosions, so their detection during 1987A supported some of the fundamental theories about the inner workings of supernovae.

With the latest NuSTAR observations, 1987A is once again proving to be a useful natural laboratory for studying the mysteries of stellar death. For many years, supercomputer simulations performed at Caltech and elsewhere predicted that the cores of pending Type II supernovae change shape just before exploding, transforming from a perfectly symmetric sphere into a wobbly mass made up of turbulent plumes of extremely hot gas. In fact, models that assumed a perfectly spherical core just fizzled out.

"If you make everything just spherical, the core doesn't explode. It turns out you need asymmetries to make the star explode," Harrison says.

According to the simulations, the shape change is driven by turbulence generated by neutrinos that are absorbed within the core. "This turbulence helps push out a powerful shock wave and launch the explosion," says Christian Ott, a professor of theoretical physics at Caltech who was not involved in the NuSTAR observations.

The core of a core-collapse supernova at the onset of explosion. Neutrinos that are emitted from the protoneutron at the center (bluish sphere) are absorbed by the gas behind the shock front, heat up this gas, and drive turbulence. Eventually an asymmetric explosion develops. Credit: Ott/Caltech (simulation), Drasco/Calpoly San Luis Obsipo (visualization)

Ott's team uses supercomputers to run three-dimensional simulations of core-collapse supernovae. Each simulation generates hundreds of terabytes of results—for comparison, the entire print collection of the U.S. Library of Congress is equal to about 10 terabytes—but represents only a few tenths of a second during a supernova explosion.

A better understanding of the asymmetrical nature of Type II supernovae, Ott says, could help solve one of the biggest mysteries surrounding stellar deaths: why some supernovae collapse into neutron stars and others into a black hole to form a space-time singularity. It could be that the high degree of asymmetry in some supernovae produces a dual effect: the star explodes in one direction, while the remainder of the star continues to collapse in all other directions.

"In this way, an explosion could happen, but eventually leave behind a black hole and not a neutron star," Ott says.

The NuSTAR findings also increase the chances that Advanced LIGO—the upgraded version of the Laser Interferometer Gravitational-wave Observatory, which will begin to take data later this year—will be successful in detecting gravitational waves from supernovae. Gravitational waves are ripples that propagate through the fabric of space-time. According to theory, Type II supernovae should emit gravitational waves, but only if the explosions are asymmetrical.

Harrison and Ott have plans to combine the observational and theoretical studies of supernova that until now have been occurring along parallel tracks at Caltech, using the NuSTAR observations to refine supercomputer simulations of supernova explosions.

"The two of us are going to work together to try to get the models to more accurately predict what we're seeing in 1987A and Cassiopeia A," Harrison says.