# Will Earth lose the Moon before the Sun goes into supernova?

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Ive read on some sites and saw on youtube videos that the moon is getting away from earth by 1-3 cm a year.

Is this enough to make the Earth lose the Moon before the Sun goes into Supernova?

Im asking because I would like to do the calculations for Earths magnetic pull on this subject. It appears to me that it should be a non-linear function (there are also other factors, like if the Moon could be captured by another planet or having asteroids messing with its orbit on Earth).

Thank you.

As HDE 226868 noted in his answer, the Sun is not going to go supernova. That's something only large stars experience at the end of their main sequence life. Our Sun is a dwarf star. It's not big enough to do that. It will instead expand to be a red giant when it burns out the hydrogen at the very core of the Sun. It will continue burning hydrogen as a red giant, but in a shell around a sphere of waste helium. The Sun will start burning helium when it reaches the tip of the red giant phase. At that point it will shrink a bit; a slight reprieve. It will expand to a red giant once again on the asymptotic red giant branch when it burns all the helium at the very core. It will then burn helium in a shell surrounding a sphere of waste carbon and oxygen. Larger stars proceed beyond helium burning. Our Sun is too small. Helium burning is where things stop.

The Sun has two chances as a red giant to consume the Earth. Some scientists say the Sun will consume the Earth, others that it won't. It's all a bit academic because the Earth will be dead long, long before the Sun turns into a red giant. I'll have more to say on this in the third part of my answer.

The current lunar recession rate is 3.82 cm/year, which is outside your one to three centimeters per year window. This rate is anomalously high. In fact, it is extremely high considering that dynamics says that $$frac {da}{dt} = ( ext{some boring constant})frac k Q frac 1 {a^{11/2}}$$ Here, $a$ is the semi major axis length of the Moon's orbit, $k$ is the Earth-Moon tidal Love number, and $Q$ is the tidal dissipation quality factor. Qualitatively, a higher Love number means higher tides, and a higher quality factor means less tidal friction.

That inverse $a^{5.5}$ factor indicates something seriously funky must be happening to make the tidal recession rate so very high right now, and this is exactly the case. There are two huge north-south barriers to the flow of the tides right now, the Americas in the western hemisphere and Afro-Eurasia in the eastern hemisphere. This alone increases $k/Q$ by a considerable amount. The oceans are also nicely shaped so as to cause some nice resonances that increase $k/Q$ even further.

If something even funkier happens and the Moon recedes at any average rate of four centimeters per year over the next billion years, the Moon will be at a distance of 425,000 km from the Earth (center to center). That's less than 1/3 of the Earth's Hill sphere. Nearly circular prograde orbits at 1/3 or less of the Hill sphere radius should be stable. Even with that over-the-top recession rate, the Moon will not escape in the next billion years.

What about after a billion years? I chose a billion years because that's about when the Moon's recession should more or less come to a standstill. If the Earth hasn't already died before this billion year mark, this is when the Earth dies.

Dwarf stars such as our Sun get progressively more luminous throughout their life on the main sequence. The Sun will be about 10% more luminous than it is now a billion years into the future. That should be enough to trigger a moist greenhouse, which in turn will trigger a runaway greenhouse. The Earth will become Venus II. All of the Earth's oceans will evaporate. Water vapor will reach well up into what is now the stratosphere. Ultraviolet radiation will photodissociate that water vapor into hydrogen and oxygen. The hydrogen will escape. Eventually the Earth will not only be bare of liquid water on the surface, it will be bare of water vapor in the atmosphere.

Almost all of the Moon's recession is a consequence of ocean tides. Without oceans, that tunar recession will more or less come to a standstill.

The Sun does not have nearly enough mass to become a supernova. Instead, it will swell to become a red giant, enveloping Mercury, Venus, and possibly Earth. After that, it will shed its outer layers as a planetary nebula, and settle down to become a white dwarf. Wikipedia, apparently, says the exact same things I had though of:

The Sun does not have enough mass to explode as a supernova. Instead it will exit the main sequence in approximately 5.4 billion years and start to turn into a red giant. It is calculated that the Sun will become sufficiently large to engulf the current orbits of the Solar System's inner planets, possibly including Earth.

Still, that does put a deadline for the Earth and the Moon to evolve to the point you're looking for. Wikipedia addresses the possibility that the Earth and Moon survive:

Today, the Moon is tidally locked to the Earth; one of its revolutions around the Earth (currently about 29 days) is equal to one of its rotations about its axis, so it always shows one face to the Earth. The Moon will continue to recede from Earth, and Earth's spin will continue to slow gradually. In about 50 billion years, if they survive the Sun's expansion, the Earth and Moon will become tidally locked to each other; each will be caught up in what is called a "spin-orbit resonance" in which the Moon will circle the Earth in about 47 days and both Moon and Earth will rotate around their axes in the same time, each only visible from one hemisphere of the other.

So the Moon will certainly still be around by the time the Sun becomes a red giant, and for many billions of years after that.

Here's the math: $$frac{1 ext{ cm}}{1 ext{ year}} imes frac{1 ext{ m}}{100 ext{ cm}} imes frac{1 ext{ km}}{1000 ext{ m}} imes 5,400,000,000 ext{ years}=54,000 ext{ km}$$

That's about one-seventh the current distance to the Moon - at its closest pass.

## When Betelgeuse Goes Supernova, What Will it Look Like From Earth?

If you stargaze on a clear winter night, it’s hard to miss the constellation Orion the Hunter, with his shield in one arm and the other arm stretched high to the heavens. A bright red dot called Betelgeuse marks Orion’s shoulder, and this star's strange dimming has captivated skygazers for thousands of years. Aboriginal Australians may have even worked it into their oral histories.

Today, astronomers know that Betelgeuse varies in brightness because it’s a dying, red supergiant star with a diameter some 700 times larger than our Sun. Someday, the star will explode as a supernova and give humanity a celestial show before disappearing from our night sky forever.

That eventual explosion explains why astronomers got excited when Betelgeuse started dimming dramatically in 2019. The 11th-brightest star dropped in magnitude two-and-a-half-fold. Could Betelgeuse have reached the end of its life? While unlikely, the idea of a supernova appearing in Earth’s skies caught the public’s attention.

And now new simulations are giving astronomers a more precise idea of what humans will see when Betelgeuse does eventually explode sometime in the next 100,000 years.

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## If the Sun Explodes, We'll Have Just Enough Time to Scream

The sun will blow up one day. And when that happens, the warm yellow orb at the center of our solar system will transform into a cooler — but still broiling — red giant. It will consume Mercury, Venus, and Earth. It’ll probably make Mars into a dessert. And yes, this will happen quickly. One day Earth will be there, and the next it will be gone. But don’t worry, there will be enough time for some suffering.

There’s a few things you ought to remember about the sun. Light that’s emitted from our host star takes eight minutes and 20 seconds to hit our planet. If the sun suddenly blew up, we actually wouldn’t know it happened for — you guessed it — eight minutes, 20 seconds — since even that explosive light show would only be traveling, at maximum, the speed of light. The death and destruction would follow very, very shortly after that.

But when the sun does blow up, it’s not simply going to extinguish like a small flame in a candle. It’s going to shoot out some really gnarly, very powerful stuff. All that energy — about as much as you would observe if you blew up a few octillion nuclear warheads — will almost instantly kill all life on Earth. Chances of survival will near zero.

Even if the Earth miraculously survived, and life found a way to go on without the energy from the sun, the resulting radiation would decimate the planet. A supernova 30 light years away would probably result in a destruction of the ozone layer and mass extinctions. A supernova 8.3 light-minutes away? Annihilation. Explosions would vaporize the surface of the planet facing the sun. The other side would hit temperatures 15 times hotter than the surface of the sun right now. The entire planet would probably disintegrate in a few days.

So, yeah, we’re not going to be able to walk it off.

Now, it’s also important to remember that perhaps we might actually feel the effects of the sun’s departure, even during those eight minutes of impending doom. If the sun exploded, Earth would no longer have a celestial body to rotate around. And when the center ceased to hold, the gyre would widen. There are more nuanced ways of considering how this would actually work from a math and physics perspective, but the general notion is that Earth would go from planet to spaceship to nothingness over the course of relatively little time.

Of course, you’re not supposed to worry about any of this. Our sun has only made it about halfway through its expected 10 billion-year lifespan. The sun is not going supernova tomorrow.

## 'Oddball supernova' appears strangely cool before exploding

IMAGE: Hubble Space Telescope (HST) imaging showing the explosion site of 2019yvr from 2.5 years before its explosion. Upper left: the supernova itself is seen in an image from the Gemini-South. view more

Credit: Charles Kilpatrick / Northwestern University

A curiously yellow star has caused astrophysicists to reevaluate what's possible within our universe.

Led by Northwestern University, the international team used NASA's Hubble Space Telescope to examine the massive star two-and-a-half years before it exploded into a supernova. At the end of their lives, cool, yellow stars are typically shrouded in hydrogen, which conceals the star's hot, blue interior. But this yellow star, located 35 million lightyears from Earth in the Virgo galaxy cluster, was mysteriously lacking this crucial hydrogen layer at the time of its explosion.

"We haven't seen this scenario before," said Northwestern's Charles Kilpatrick, who led the study. "If a star explodes without hydrogen, it should be extremely blue -- really, really hot. It's almost impossible for a star to be this cool without having hydrogen in its outer layer. We looked at every single stellar model that could explain a star like this, and every single model requires that the star had hydrogen, which, from its supernova, we know it did not. It stretches what's physically possible."

The team describes the peculiar star and its resulting supernova in a new study, which was published today (May 5) in the Monthly Notices of the Royal Astronomical Society. In the paper, the researchers hypothesize that, in the years preceding its death, the star might have shed its hydrogen layer or lost it to a nearby companion star.

Kilpatrick is a postdoctoral fellow at Northwestern's Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and member of the Young Supernova Experiment, which uses the Pan-STARSS telescope at Haleakalā, Hawaii, to catch supernovae right after they explode.

Catching a star before it explodes

After the Young Supernova Experiment spotted supernova 2019yvr in the relatively nearby spiral galaxy NGC 4666, the team used deep space images captured by NASA's Hubble Space Telescope, which fortunately already observed this section of the sky.

"What massive stars do right before they explode is a big unsolved mystery," Kilpatrick said. "It's rare to see this kind of star right before it explodes into a supernova."

The Hubble images showed the source of the supernova, a massive star imaged just a couple years before the explosion. Although the supernova itself appeared completely normal, its source -- or progenitor star -- was anything but.

"When it exploded, it seemed like a very normal hydrogen-free supernova," Kilpatrick said. "There was nothing outstanding about this. But the progenitor star didn't match what we know about this type of supernova."

Direct evidence of violent death

Several months after the explosion, however, Kilpatrick and his team discovered a clue. As ejecta from the star's final explosion traveled through its environment, it collided with a large mass of hydrogen. This led the team to hypothesize that the progenitor star might have expelled the hydrogen within a few years before its death.

"Astronomers have suspected that stars undergo violent eruptions or death throes in the years before we see supernovae," Kilpatrick said. "This star's discovery provides some of the most direct evidence ever found that stars experience catastrophic eruptions, which cause them to lose mass before an explosion. If the star was having these eruptions, then it likely expelled its hydrogen several decades before it exploded."

In the new study, Kilpatrick's team also presents another possibility: A less massive companion star might have stripped away hydrogen from the supernova's progenitor star. The team, however, will not be able to search for the companion star until after the supernova's brightness fades, which could take up to 10 years.

"Unlike it's normal behavior right after it exploded, the hydrogen interaction revealed it's kind of this oddball supernova," Kilpatrick said. "But it's exceptional that we were able to find its progenitor star in Hubble data. In four or five years, I think we will be able to learn more about what happened."

The study, "A cool and inflated progenitor candidate for the type Ib supernova 2019 yvr at 2.6 years before explosion," was supported by NASA (award numbers GO-15691 and AR-16136), the National Science Foundation (award numbers AST-1909796, AST-1944985), the Canadian Institute for Advanced Research, the VILLUM Foundation and the Australian Research Council Centre of Excellence. In addition to the Hubble Space Telescope, the researchers used instruments at the Gemini Observatory, Keck Observatory, Las Cumbres Observatory, Spitzer Space Telescope and the Swope Telescope.

Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.

## Across the Universe, a star exploded so violently that it *completely* annihilated itself

A billion light years away, a monster star tore itself to shreds.

And by that I mean it tore itself to shreds. In general exploding stars — supernovae — leave behind a neutron star or black hole, but in this case it’s possible that the explosions was so over-the-top ridiculously violent that even the star’s core was ripped apart. It’s difficult to exaggerate how violent an event this was… but then, when huge amounts of antimatter are involved, that’s what happens.

The event is called SN2016iet, a supernova that was detected on November 14, 2016. It was first spotted in data taken by the space-based Gaia observatory, and was followed-up by the Catalina Real-Time Transient Survey, then Pan-STARRS, and eventually the huge Gemini Telescope to get deep spectra of it. But it didn’t take long to determine that this particular supernova was weird.

And then they found it was really weird.

Most exploding stars get bright over the course of a few days, peak, then decay away over the next few months. SN2016iet didn’t do that: It peaked twice, which right away is bizarre. The second peak occurred about 100 days after the first, and both were phenomenally energetic, blasting out more than ten billion times the Sun’s energy for days at a time. Holy yikes.

But even then it didn’t behave properly. Instead of fading away into obscurity, the supernova continued to shine, fading much more slowly than usual. The astronomers were still able to observe it in spring of this year, more than two years after the initial explosion.

The supernova SN2016iet is at least 55,000 light years from what appears to be its host galaxy. Some very faint object is seen at its potion before the event, perhaps an even fainter satellite galaxy. Credit: CfA

The more they observed it, the stranger it got. One of the strangest bits is its location: The explosion doesn’t seem to have happened inside a galaxy. The nearest obvious galaxy is an unnamed dwarf, only about 1/50th as luminous as our Milky Way (and about a billion light years away from Earth). But the supernova is at least 55,000 light years from the galaxy! That’s well outside the usual distance. Once the supernova faded enough, the astronomers detected the faint glow of hydrogen underneath the supernova’s own emission, meaning it’s either in a very faint satellite galaxy to the dwarf, or in a star cluster. If it’s in a galaxy it’s really dinky, and a cluster that far from the host galaxy is dang odd as well.

So what’s going on? Well, strap in. This is a doozy.

The team ran through a whole bunch of physical models trying to figure out what the heck SN2016iet is. Pretty much every normal supernova model failed in one way or another, and what they were left with is a demon.

When the progenitor star of SN2016iet was born, probably 3–4 million years ago, it was a true monster: It probably had a mass 120–260 times the Sun’s. That’s enormous. I mean, incredibly enormous. We don’t think there are any stars that massive in the Milky Way (though a few come close), and in fact we don’t think any can be.

Stars born in the recent Universe are contaminated with heavy elements like iron and magnesium, created when previous generations of massive stars exploded. Those elements are really good at absorbing the high-energy light inside stars, which heats them up. If a star gets really massive it generates a lot of energy, and if it can’t shed that energy it gets so hot it tears itself apart.

A star with more than about 150 times the Sun’s mass like that wouldn’t be able to exist… unless it doesn’t have those heavy elements in it, in which case they can shed the energy rapidly enough. Stars in the early Universe only had hydrogen and helium in them, so they may have been huge. The only way you can even get a star like that now is if it exists in an environment with very few heavy elements. Interestingly, the nearby galaxy (assumed to be the host to the star) is what we call a “metal-poor” galaxy, lacking those heavy elements, and therefore it’s possible it could make a star like this.

So. This extraordinarily massive star was born, and probably tore through its core hydrogen supply, converting it all to helium in just a few million years. Then it started fusing helium into carbon and oxygen, then finally carbon into neon. This is where its huge mass comes into play I’ve explained this before in detail, but in a nutshell the neon fusion in an über-massive star like this proceeds at furious rates, and creates very high-energy gamma rays.

Usually these gamma rays help support the core against its own fierce gravity, but in this case the energies are so high that the gamma rays can convert into matter. This is called pair production, because each gamma ray makes two subatomic particles, one matter, one antimatter. This actually removes support in the core! It was relying on those gamma rays to keep it inflated, like a balloon full of hot air. Remove them, and the core collapses.

If the core has enough mass, the collapse generates a vast amount of energy as the fusion rate goes through the roof. The energy released is enough to tear the core apart, and that energy then rips through outer layers of the star. KABOOM. This is called a pair instability supernova. If the core still has lots of mass but not quite that much, you get a series of energetic pulses which then die off, until finally the core can’t contain them anymore and it explodes anyway. This is called a pulsational pair instability supernova.

The core masses you need for this are ridiculous… but the observations of SN2016iet indicate that when the core of the star exploded it had 55–120 times the Sun’s mass. Ye-freaking-GADS. That puts it somewhere in the range of these two kinds of explosion mechanisms, and this is the first time a supernova has been seen unambiguously to have exploded this way.

Still, that’s less mass than the star started with in total. It turns out the huge luminosity of the star even before it exploded means it shed vast amounts of mass in a super-solar wind for a long time before the end. And this is where it gets really weird.

That second peak in brightness? That’s probably when the supernova material screaming outwards at several thousand kilometers per second crashed into the material previously shed by the star. That caused a vast increase in brightness, getting up to about half as bright as the initial peak. The amount of that material out there was probably about 35 or so times the mass of the Sun. But the weird part is its location: It was pretty close to the exploded star, indicating it was ejected recently, and didn’t get far. In fact, it looks like it was all shed in the past decade before the explosion.

A decade. To lose 35 times the Sun’s mass worth of matter. To put this in some sort of scale that will still be nearly impossible to grasp, in the ten years before the ultimate explosion this star was blasting out matter equal to about Earth’s mass every thirty seconds. For a decade. Before it exploded.

It was pretty much at this point reading the journal paper that my brain wanted to leap out of my skull and run around in panicked circles screaming. I’ve run out of adjectives to describe an event like this.

Artwork depicting the explosion of a massive star: a supernova. Credit: Gemini Observatory/NSF/AURA/ illustration by Joy Pollard

A final weirdness is how long it’s taking the supernova to fade. Sometimes an extremely energetic neutron star called a magnetar is left by a pair instability explosion, and that pumps energy into the expanding debris, keeping it glowing for a long time. But in this case the amount of material in the core was far too large to make a neutron star. Also, in a normal pair instability event, the material in the outer layers is shed over many thousands of years, not in a single decade.

So in the end, nothing with this supernova fits. No one model seems to explain everything it’s doing, which means it truly is one of a kind. Nothing like it has ever been seen before, and we can’t fully explain its behavior.

I wonder though, just how long this will remain a unique event. We now observe thousands of supernovae every year. Even if this event is extremely rare, we’re likely to find another one eventually. Maybe not exactly like it, but close enough that we can compare them, see how they differ. That will help astronomers understand how these catastrophic events occur in the first place. Although these kinds of supernovae are at the tippy-top of the scale, they provide checks on our understanding of the physics of exploding stars under extraordinarily extreme conditions.

## The Final Countdown Before a Supernova

I’m sometimes asked what I think the next exploding star in our galaxy will be. Most people expect I’ll say Betelgeuse, the red supergiant marking Orion’s right shoulder.

But Betelgeuse may not go supernova for another million years, which is a long, long time. There are several stars much closer to The End, and I recently learned of a new one: SBW1.

Photo by ESA/NASA acknowledgement: Nick Rose

The star is a blue supergiant, a hot, energetic beast probably about 20 or so times the mass of the Sun. Stars like that don’t live long, just a few million years tops. But we know (we think) it’ll explode much sooner than that, because of that ring you see in the Hubble picture above. How does that ring tell us anything? Ah, glad you asked.

We’ve seen another star like this: Sanduleak -69 202. That was a blue supergiant that blew up, and its light reached Earth in February 1987, so we called it Supernova 1987A, (or just SN87A). It too was a massive star, but slightly less so than SBW1. (That’s important so keep it in mind.)

SN87A also had a ring of gas around it, ejected by the star about 20,000 years before it exploded we know the age due to how fast the gas was moving and how far it had expanded (like saying you know how long a car has been on the road by knowing its speed and distance). That means once the ring formed, Sanduleak -69 202 had just 20 millennia to live.

Massive stars run through their fuel faster than less massive ones. Since we already see a ring around SBW1, that means all things being equal, it most likely has less than 20,000 years before it goes kablooie.

That’s pretty soon, on a galactic scale. But are all things really equal? It’s a good question.

There’s a professional journal paper detailing SBW1 that was a fascinating read for me I studied SN87A and its ring for my Ph.D. thesis. I asked the paper’s lead author, Nathan Smith (he’s the S in SBW1, in fact), if he could send me higher-res versions of the images in the paper, and he kindly sent me this striking shot of SBW1 that’s a combination of Hubble imagery with a ground-based infrared shot from the Gemini observatory:

The pink fuzz is dust, complex carbon molecules like soot, blown out from the star, and the blue is gas. Interestingly, the gas is different than what was in SN87A’s ring: The ring around SBW1 doesn’t have nearly as much nitrogen, whereas SN87A’s had plenty. Nitrogen in the ring would come from the star when it was a red supergiant the deficiency of nitrogen in SBW1’s ring means the star hasn’t turned into a red supergiant (yet).

But with SN87A, we think it had to have been red before it exploded its ring was likely the result of winds of gas blown out from the star when it was red getting blasted from the dins blown out later when it was blue (the reasons for this are subtle, and I’ve explained them before if you’re interested in details). If SBW1 has not been red, then there must be some other reason it has a ring.

This is the part I love about all this. Smith and his co-authors have an idea for what formed SBW1’s ring: It ate a companion star.

It used to be two stars, a massive one and a smaller one. As the massive one expanded, it would have engulfed the smaller one. The smaller one would’ve been orbiting it rapidly, so when it got engulfed it would’ve spun up the big one like a whisk stirring a bowl of eggs. When this happened, the star ejected a huge amount of gas and dust in a single eruptive event. Because the star was spinning rapidly, this material would have been blown out preferentially in the plane of the star’s equator. All that junk flew away from the star where it expanded and cooled, forming a clumpy ring. The tremendous energy of the light coming from the central blue supergiant star lit it up, which is why we see it in the Hubble images.

Now we don’t know this for sure, but it fits the data. It’s different than what’s proposed for the formation of SN87A’s ring, which was winds from the star interacting over thousands of years. So the two rings may be very different, but we don’t know. It’s possible they both formed the same way. They are both about the same size and mass, though, which means they’re similar in some regards. So maybe SBW1 is on the same timeline that Sanduleak -69 202 was before it went off.

That puts 20,000 years tops on SNW1’s clock. But again, we’re not sure it may be longer. I’ll note that two other stars, Sher 25 and HD 168625 (pictured below, also c/o Smith), are also blue supergiants with rings, and both are more massive and hotter than SN87A (before it blew) and SBW1. Either one of them might go first. We just don’t know.

As you can see, this gets a bit confusing. It gets weirder, too: We think, on average, a star should blow up once per century in a galaxy our size, but it’s been many centuries since the last supernova in the Milky Way (SN87A was in a companion galaxy to us). That’s a statistical average, so it’s not like we’re overdue.

But it tells you that there are probably lots of stars that might explode before any of those four (well, three, since 87A already exploded). Or it may very well be one of these guys. I’ll note that all of them are too far away to hurt us when they explode they’re thousands of light years away, and a supernova has to be less than 100 to hurt us significantly. So we’re safe.

The good news is that SN87A was 170,000 light years away when it exploded and we learned a huge amount about how massive stars end their lives (answer: not well). SBW1 is about 20,000 light years away, so we’ll have a much better view. It seems weird to say, but I rather hope it blows up soon. It’s in the constellation of Carina, which isn’t visible from where I live, but still, the pictures would be spectacular, and the science would be fantastic.

The heavy elements in the Universe were literally created in such explosions the calcium in your bones and the iron in your blood were forged in the hearts of supernovae. Without such destruction there would be no creation. In that sense, studying exploding stars is just another way of studying ourselves. We are literally a part of the Universe, and the Universe is in us. It’s funny to think that looking outward helps us see inward, but astronomy is full of delightful ironies like that.

## “Pre-supernova” Neutrinos: What Happens Before a Star Explodes and Dies?

A recent study on ‘pre-supernova’ neutrinos—tiny cosmic particles that are extremely hard to detect—has brought scientists one step closer to understanding what happens to stars before they explode and die. The study, co-authored by postdoctoral researcher Ryosuke Hirai, from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) at Monash University, investigated stellar evolution models to test uncertain predictions.

When a star dies, a huge number of neutrinos are emitted which are thought to drive the resulting supernova explosion. The neutrinos flow freely through and out of the star before the explosion reaches the surface of the star. Scientists can then detect neutrinos before the supernova occurs, in fact, a few dozen neutrinos were detected from a supernova that exploded in 1987, several hours before the explosion was seen in light.

The next generation of neutrino detectors are expected to detect about 50,000 neutrinos from a similar kind of supernova. The technology has become so powerful that scientists predict they will detect the weak neutrino signals that come out days before the explosion just like a supernova forecast, it will give astronomers a heads up to catch the first light of a supernova. It’s also one of the only ways to directly extract information from a star’s core—similar to an X-ray image of your body, except it’s for stars. But an X-ray image is meaningless unless you know what you’re looking at.

Although there is a general understanding of how a massive star evolves and explodes, scientists are still uncertain about the lead up to the supernova explosion. Many physicists have attempted to model these final phases, but the outcomes appear random there is no way to confirm if they’re correct. Since pre-supernova neutrino detections allow scientists to better assess these models. a team of OzGrav scientists investigated the late stages of stellar evolution models and how that might affect pre-supernova neutrino estimates.

OzGrav researcher and co-author Ryosuke Hirai says: ‘This will help us make the most of the information from future pre-supernova neutrino detections’. In this first study, we explored the uncertainty on a single star that is 15 times the mass of the Sun. The neutrino emission calculated from these stellar models differed greatly in the neutrino luminosity. This means that pre-supernova neutrino estimates are very sensitive to these small details of the stellar model.’

The study revealed the significant uncertainty of pre-supernova neutrino predictions, as well as the relationship between the neutrino features and the star’s properties.

‘The next supernova in our galaxy can happen any day, and scientists are looking forward to detecting pre-supernova neutrinos, but we still don’t know what we can learn from it. This study lays out the first steps of how to interpret the data. Eventually, we’ll be able to use pre-supernova neutrinos to understand crucial parts of massive star evolution and the supernova explosion mechanism.’

## The Final Countdown Before a Supernova

Iâm sometimes asked what I think the next exploding star in our galaxy will be. Most people expect Iâll say Betelgeuse, the red supergiant marking Orionâs right shoulder.

But Betelgeuse may not go supernova for another million years, which is a long, long time. There are several stars much closer to The End, and I recently learned of a new one: SBW1.

#### More Bad Astronomy

The star is a blue supergiant, a hot, energetic beast probably about 20 or so times the mass of the Sun. Stars like that donât live long, just a few million years tops. But we know (we think) itâll explode much sooner than that, because of that ring you see in the Hubble picture above. How does that ring tell us anything? Ah, glad you asked.

Weâve seen another star like this: Sanduleak -69 202. That was a blue supergiant that blew up, and its light reached Earth in February 1987, so we called it Supernova 1987A, (or just SN87A). It too was a massive star, but slightly less so than SBW1. (Thatâs important so keep it in mind.)

SN87A also had a ring of gas around it, ejected by the star about 20,000 years before it exploded we know the age due to how fast the gas was moving and how far it had expanded (like saying you know how long a car has been on the road by knowing its speed and distance). That means once the ring formed, Sanduleak -69 202 had just 20 millennia to live.

Massive stars run through their fuel faster than less massive ones. Since we already see a ring around SBW1, that means all things being equal, it most likely has less than 20,000 years before it goes kablooie.

Thatâs pretty soon, on a galactic scale. But are all things really equal? Itâs a good question.

Thereâs a professional journal paper detailing SBW1 that was a fascinating read for me I studied SN87A and its ring for my Ph.D. thesis. I asked the paperâs lead author, Nathan Smith (heâs the S in SBW1, in fact), if he could send me higher-res versions of the images in the paper, and he kindly sent me this striking shot of SBW1 thatâs a combination of Hubble imagery with a ground-based infrared shot from the Gemini observatory:

The pink fuzz is dust, complex carbon molecules like soot, blown out from the star, and the blue is gas. Interestingly, the gas is different than what was in SN87Aâs ring: The ring around SBW1 doesnât have nearly as much nitrogen, whereas SN87Aâs had plenty. Nitrogen in the ring would come from the star when it was a red supergiant the deficiency of nitrogen in SBW1âs ring means the star hasnât turned into a red supergiant (yet).

But with SN87A, we think it had to have been red before it exploded its ring was likely the result of winds of gas blown out from the star when it was red getting blasted from the dins blown out later when it was blue (the reasons for this are subtle, and Iâve explained them before if youâre interested in details). If SBW1 has not been red, then there must be some other reason it has a ring.

This is the part I love about all this. Smith and his co-authors have an idea for what formed SBW1âs ring: It ate a companion star.

It used to be two stars, a massive one and a smaller one. As the massive one expanded, it would have engulfed the smaller one. The smaller one wouldâve been orbiting it rapidly, so when it got engulfed it wouldâve spun up the big one like a whisk stirring a bowl of eggs. When this happened, the star ejected a huge amount of gas and dust in a single eruptive event. Because the star was spinning rapidly, this material would have been blown out preferentially in the plane of the starâs equator. All that junk flew away from the star where it expanded and cooled, forming a clumpy ring. The tremendous energy of the light coming from the central blue supergiant star lit it up, which is why we see it in the Hubble images.

Now we donât know this for sure, but it fits the data. Itâs different than whatâs proposed for the formation of SN87Aâs ring, which was winds from the star interacting over thousands of years. So the two rings may be very different, but we donât know. Itâs possible they both formed the same way. They are both about the same size and mass, though, which means theyâre similar in some regards. So maybe SBW1 is on the same timeline that Sanduleak -69 202 was before it went off.

That puts 20,000 years tops on SNW1âs clock. But again, weâre not sure it may be longer. Iâll note that two other stars, Sher 25 and HD 168625 (pictured below, also c/o Smith), are also blue supergiants with rings, and both are more massive and hotter than SN87A (before it blew) and SBW1. Either one of them might go first. We just donât know.

As you can see, this gets a bit confusing. It gets weirder, too: We think, on average, a star should blow up once per century in a galaxy our size, but itâs been many centuries since the last supernova in the Milky Way (SN87A was in a companion galaxy to us). Thatâs a statistical average, so itâs not like weâre overdue.

But it tells you that there are probably lots of stars that might explode before any of those four (well, three, since 87A already exploded). Or it may very well be one of these guys. Iâll note that all of them are too far away to hurt us when they explode theyâre thousands of light years away, and a supernova has to be less than 100 to hurt us significantly. So weâre safe.

The good news is that SN87A was 170,000 light years away when it exploded and we learned a huge amount about how massive stars end their lives (answer: not well). SBW1 is about 20,000 light years away, so weâll have a much better view. It seems weird to say, but I rather hope it blows up soon. Itâs in the constellation of Carina, which isnât visible from where I live, but still, the pictures would be spectacular, and the science would be fantastic.

The heavy elements in the Universe were literally created in such explosions the calcium in your bones and the iron in your blood were forged in the hearts of supernovae. Without such destruction there would be no creation. In that sense, studying exploding stars is just another way of studying ourselves. We are literally a part of the Universe, and the Universe is in us. Itâs funny to think that looking outward helps us see inward, but astronomy is full of delightful ironies like that.

## Waiting for Betelgeuse to Explode

That the star will eventually blow up, nobody denies. Betelgeuse — sometimes pronounced “beetle-juice,” and also known as Alpha Orionis — is at least 10 times and maybe 20 times as massive as the sun. If it were placed in our solar system, its fiery gases would engulf everything out to Jupiter’s orbit.

The star is a so-called red supergiant in the last violent stages of its evolution. It has already spent millions of years burning primordial hydrogen and transforming it into the next lightest element, helium. That helium is burning into more massive elements. Once the core of the star becomes solid iron, sometime within the next 100,000 years, the star will collapse and then rebound in a supernova explosion, probably leaving behind a black hole.

That will be quite a show. Betelgeuse is only 700 light years from Earth, far enough to not kill us when it goes, but close enough to impress the supernova would be as bright as a full moon in our sky.

The star’s current diminution probably does not mark The End, astronomers say. Aging stars are notoriously cranky and moody, coughing out bursts of gas and dust that obscure themselves, or sputtering inside as their cores evolve and change.

Even normal stars are subject to periodic fluctuations in brightness. Betelgeuse endures such cycles of ups and downs, and the most likely explanation for the current episode is that two cycles bottomed out at the same time.

“My money all along has been that Betelgeuse is going through a somewhat extreme, but otherwise normal quasi-periodic change in brightness,” said J. Craig Wheeler, a supernova expert at the University of Texas in Austin.