Are we “in range” of any potential supernovae? Has the Earth ever been hit in the past? And what about gamma-ray bursts from across the galaxy, are we safe from those? I discuss these questions and more in today’s Ask a Spaceman!

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EPISODE TRANSCRIPTION (AUTO GENERATED)

When Beetlejuice goes off, it's going to be the show of a lifetime. And I'm sure most of you are familiar with the star. And if you aren't, you should be. It's the shoulder of Orion, a red giant sitting about 500 light years away. It's it's huge, weighing somewhere between 15 20 solar masses, but so extended and bloated that if you plopped it down in our own solar system, it would stretch to roughly the orbit of Mars.

And I'm not sure how to put this delicately, so I'll just get it out there. It's not doing so great. Massive stars do not live very long lives with their precise lifetimes depending on a host of factors like their mass, their metallicity, that's the proportion of elements heavier than helium, in spin rate. On the low side, we're talking only a few 100000 years. On the high side, we've got a few million.

But either way, as stars go, that's not a lot. Our own sun will outlive multiple generations of such giants, and red dwarfs, the smallest stars in the universe, can stretch for 1,000,000,000,000 of years at a time. In fact, just fun side note, red dwarfs live for so long that the entire universe isn't even old enough for them to start dying yet. But that's that's a different show. No matter how you slice it, Betelgeuse is on its last legs.

It's in what's called the red giant phase, and it's pretty obvious to see why astronomers picked that name for this phase in a stellar life cycle. It's red, and it's gigantic. And it is so close to being dead that it is in an incredibly unstable phase. In fact, we saw some very dramatic dimming episodes a few years ago where it dimmed by, like, 15% out of nowhere over the course of a few weeks. And then just after a few months, it popped back up and then it got dim again.

And it's just nasty. And and the reason it's nasty is that what's happening in the core is not sustainable. Like our sun, the core is very stable. It's just doing its thing. It's burning hydrogen and turning it into helium.

And but when a star is near the end of its life, sometimes it's fusing hydrogen, sometimes it's fusing helium, sometimes it will shut down for a while, sometimes it'll start back up. The outer edges of the atmosphere are so far away from the central core that they start getting a mind of their own. It just gets complicated. The point is any day now, it's going to go boom. And, you know, any day now means something different to astronomers than it does to everybody else.

But estimates based on its mass, its rotation rate, the group of stars, it's what's it was born with, the amount of metals we can measure in the upper layers of its atmosphere, suggest that it's somewhere in the neighborhood of a few 100000 years from now, it's gonna go supernova. But honestly honestly, it could be tomorrow. In fact, because it's 500 light years away. It could have been a 100 years ago, and we won't find out for a while. It may already be dead.

We don't know. When Betelgeuse goes off as a supernova, it will be a sight to behold. Keep in mind that typical supernova can outshine entire galaxies, Like, one star exploding can outshine 100 of billions of normal stars that is a number that I have a hard time wrapping my mind around. And at a distance of a few 100 light years, Betelgeuse is going to put on an impressive show. It will be visible during the day.

It will be brighter than any planet. It will be almost as bright as the full moon. You're we're talking somewhere between 75 and 95% of the full moon. You know, depending on exactly how close Beetlejuice is, and and exactly the mechanisms of explosion, etcetera, etcetera, etcetera. The the deal is it's gonna be bright.

You'll be able to read a book by the light of the Betelgeuse Supernova at midnight. But it will actually be painful to look at because unlike the full moon that this is this gorgeous disc in the sky, Betelgeuse is still going to be a tiny pinprick of lights. Just going to be a dot of light that in in that dot, it is brighter than or as bright as the full moon. So it won't be comfortable to look at, and it will last a few months before fading away as all supernova do. But as impressive as it is, it won't be dangerous.

What saves us and saves us from most supernova dangers is that as bright as they are, as much radiation as they pour into the universe, stars are really really stinking far apart. What helps here is something called the inverse square law. So there's a fixed amount of light that a star or a supernova or any radiating object in the universe gives off. Yes. A supernova is giving off more light than a typical star, but it's still a fixed amount.

And so that light as it moves away from the star, that same amount of light that is given off by the star has to cover more and more area. You can imagine drawing a sphere around the star and say you're right up against that star and you draw a sphere around it. You can measure the all the light, you know, punching through that sphere, and and that's a lot. But then if you extend that sphere, if you look at a greater distance, that same amount of light now has to stretch out and paint a much much larger sphere. So as you get further away from an object, the radiation really really drops off because it's the fixed amount of radiation having to cover a greater and greater area at that distance.

And in fact, it goes like this. If you double the distance, the radiation in any one spot gets cut to 25%. If you go to 10 times the distance, then you get a factor of a 100 drop off. It goes as the square of the distance. Or or as you as the distance goes further and further, the distance gets bigger and bigger.

You square that, and then you take one over that number. That's why it's inverse square law. But the I the basic idea is the farther away you get, the better off you are in a really big way. This inverse square law is absolutely savage, and it definitely has its downsides. You know, if you're using one solitary light to illuminate a dark room or you're out in the middle of the woods at night and you've got your flashlight, man, that light drops off really quickly.

If you're trying to stay warm by a fire, you will notice if you stand really close to the fire. It's maybe a little bit too hot, but then you take one step back, and all of a sudden, it's it's barely hot enough, or you might even be cold. That's because of that inverse square law of the infrared radiation being emitted by the fire. But in this case, in this one case, we're going to be grateful for the inverse square law. Because we're talking about a giant star turning itself into an uncontrolled nuclear bomb and detonating with enough energy to overwhelm an entire galaxy's worth of starlight.

And with Betelgeuse, this massive star in our backyard that's going supernova even though it's only a few 100 light years away. From our perspective, because of that inverse square law, because of the radical dropping off of the amount of light, in any one spot as a function of distance. From our perspective, Betelgeuse will go from a dot of light in the night sky to a brighter dot of light in the night sky. It's not going to be a threat. So what does it have to take?

How close does a supernova have to be to do some serious damage to the Earth? To estimate this, and you will not be surprised to learn that some bored slash morbidly curious astronomers have actually gone and calculated all this, We have to look at what the actual destructive capabilities are of a supernova. As in, what does a supernova produce? How deadly are those products and what is their range? Okay.

Let's do a quick survey. First off, there's the shock wave from the explosion itself. You have a large chunk of a dead star accelerating away from it and reaching speeds as healthy fraction of the speed of light. And that is going to slam into you, and that's going to be really, really bad. But trust me, if you're close enough to a supernova to be worried about the shock wave, then you're close enough to the pre supernova star to get a lethal dose of radiation, and you really should have moved away a long time ago.

So that's nobody's fault but your own. Basically, if you're close enough to be hit by the shockwave and for it to be a concern, you are so close to the star before it turned into a supernova that you are just soaking up all of its deadly radiation already before it even explodes. So I can't help you there. Next, there's visible light, which while impressive and may lead to temporary or, you know, permanent blindness, it just isn't going to be a factor. Now you might imagine some fanciful scenario where there's so much radiation, like, the volume and intensity of radiation is so much that, I don't know, it just rips the skin off of you like some category 47 hurricane.

But but considering that visible light never accounts for more than 1% of the energy output of a supernova, I'm going to go ahead and call this a nonissue at any reasonable interstellar distance. Like, if you're close enough that the sheer quantity of light, the pressure and momentum from the light itself is going to harm you, the shockwave is gonna get you too. Visible light is not a problem. Speaking of energy output, by far, most of the energy emitted by a supernova is in the form of neutrinos. You know, those ghostly particles that hardly ever interact with matter.

In fact, there are trillions of neutrinos passing through your body every single second, like, right now and now again and now again. Trillions and trillions and trillions of neutrinos passing through you, and I bet you didn't even notice them. Across your entire lifetime, like we're talking 70, 80, 90 years of exposure to trillions of neutrinos every single second, you're going to interact with roughly one of them in your entire life. So even if you got a face full of supernovas worth of neutrinos, it's not going to bother you. At interstellar distances, the neutrinos are not a problem.

Well, what about other wavelengths of light like x rays and gamma rays? Well, the good thing here is that supernova tend, and I'm emphasizing that word because we're going to come back to attend not to produce copious amounts of high energy radiation. But the bad thing is that's only in a relative sense. You know, relative to the other kinds of radiation, there's not an exceptional amount of X rays and gamma rays. But on any reasonable absolute scale, like just how many gamma rays are going to pass through my atmosphere or my liver.

If if I'm just trying to count the raw radiation, it's still a ton of high energy radiation. So we're going to have to keep track of that. And lastly, we have to contend with the cosmic rays, which are not rays at all. I know it's a naming thing. Cosmic rays are these charged particles that are accelerated to nearly the speed of light.

We're talking protons, helium nuclei, sometimes other nuclei, where they're just hanging out, minding their own business, and then boom, they get this massive energy injection from the supernova, and then they go flying out like crazy. The universe is absolutely soaked in cosmic rays. Our magnetic field and our atmosphere protect us from most of the cosmic rays, but still, if you're standing around on the surface of the earth, about one cosmic ray passes through you every single second, which honestly is uncomfortable if you think about it for too long. Biologically, cosmic rays are nasty little buggers. They have a tendency to snip apart DNA.

They can also cause some ionization damage within a cell, and they're just kind of bad news. There's a certain percentage, like, somewhere around 3% of all cancers on earth are triggered by cosmic rays. So we're going to have to deal with that. We have to deal with the high energy products, the cosmic rays, the x rays, and the gamma rays. You got all the rays, and they're all a problem.

But what do all these x rays and gamma rays and cosmic rays do to the earth that make them so nasty? Well, it's actually not what you might think. It's not like we get blasted like a heat wave or disintegrated by a shock wave. No. Again, we're talking interstellar distances.

We're talking 100 or 1000 of light years. And yes, these are extremely energetic particles extremely energetic radiation but they're not going to disintegrate the earth. We are simply too far away from any supernova for that to ever be a problem. What happens is that these forms of radiation pack enough energetic punch that they can tear apart molecular nitrogen and oxygen. Those elements like nitrogen and oxygen in our atmosphere prefer to float around as molecules.

You know, n 2, o 2, ozone o 3. But then once they get hit by x rays, gamma rays, and cosmic rays, they get broken apart. And then they recombine in interesting and fascinating ways like various nitrogen oxides including everybody's favorite nitrous oxide AKA laughing ass. And while everyone's laughing, having a good time, our ozone layer gets stripped away. That's the issue.

That's the problem caused by these this high energy radiation, the gamma rays, the x rays, and the cosmic rays. It breaks up our ozone layer. And without an ozone layer, it means the earth is vulnerable to ultraviolet radiation from this. I remember, our ozone layer protects us from the vast majority of ultraviolet radiation. There's a couple specific bands of wavelengths that do sneak through, which is why we need to wear sunscreen here on the surface, to so we don't get nasty tans and sunburns and skin cancer and all that.

But imagine no ozone layer, then you get all the UV radiation, the full output, and it's bad. And it's not just a matter of quicker tans and faster burns and higher rates of skin cancer. The problem is, photosynthetic microorganisms like algae become vulnerable. They get cooked and then they die. And since they form the very base layer of the food chain, you end up with whole ecosystem collapse and a mass extinction.

That that that's a bad thing. So we've done the math. And it says that generally, for the typical strengths of supernova that tend to occur in our galaxy, a dying star has to be within roughly no more than 25 to 30 light years of the Earth to be able to strip away at least half of our ozone layer, which would be enough to trigger, all the aforementioned bad things. And I do have some good news to help you sleep at night. You know, there are no known supernova candidates within 30 light years of the Earth.

The nearest candidate, the nearest star that is about to go supernova is Spica or Spica. And it's about 250 light years away. So like 10 times further away than that. And then because of that inverse square law, it's 10 times further which means it's a 100 times weaker. Okay.

That's great. There are no supernova candidates in the danger zone. And just we felt like checking and we did. And it's a good thing we did. There are no stars that will evolve to become supernova candidates that will approach within 30 light years of the earth in their lifetime.

So if we look at some giant stars that are not supernova candidates yet, but might be a candidate in like a 1000000 years or 10000000 years. And then we plot out their their motions, their trajectories, their velocities through the galaxy, and they are not going to pass within 30 light years of the earth. So on that score too, we're safe. At least as far as our calculations can take us. It doesn't look like any time in the next, you know, few 1000000 years, there's going to be a blast that's going to strip away our ozone layer.

Over longer timescales, however, things start to get more interesting as they tend to do with entities posing existential risk to entire biospheres like patreon.patreon.com/pmsutter. It it does potentially, I'm just putting this out here, potentially pose an existential risk to the Earth's biosphere. I'm not exactly sure how it works, but why don't you just go to patreon.com/pmsutter, that's p m s u t t e r, and contribute to help keep this show going, and just make sure we've eliminated that as a possibility. One of the fun things and and fun is definitely in quotes in my notes. Fun things is that our solar system is just now entering the Orion spiral arm of the Milky Way galaxy, and spiral arms are known for their advanced rate of star formation, hence why they tend to stick out in photographs.

But higher rates of star formation means higher rates of star deaths, which means a greater than average chance of getting too close for comfort in the 10000000 years it will take us to cross the arm. So we're in a slightly elevated risk as we cross through the Orion spiral arm of the Milky Way. Still nothing that we're seeing on the radar over the next few 1000000 years but, you know, it's gonna take us 10000000 years to make the crossing so so who knows what's gonna meet us when we're in the middle of it. Once you add everything up, once you calculate, you know, rates, typical rates of supernova in the galaxy, the average distance between stars, average lifetime of stars, you know, estimates of the danger zone. Once you mash that all together, you end up with estimates of a potentially lethal supernova encounter very roughly a small handful of times per 1000000000 years.

So on the low side estimates are, we're gonna get a we get a lethal supernova or a dangerous extinction type supernova once every, you know, 1 to 2000000000 years. And then on the high side, somewhere around once every, say, 2 to 300000000 years. And in fact, it may have happened in the past. There's this mass extinction event called the late Ordovician mass extinction. It was a nasty one, 440000000 years ago.

We lost roughly 85% of all marine species, which no matter how you count it as a bad day, it's possible that that extinction event was triggered by a nearby supernova. It it's it's not the top contender for a cause. We actually don't fully understand what triggered this mass extinction, but based on some radioisotopes, you know, some radioactive elements in aging and dating, there's like a potential that that mass extinction was triggered by a supernova, and there are a few other candidates in the fossil record. When you see these mass extinctions, you're like, oh, you know, that that could have been a supernova going off. So it may have already happened.

You know, it's like one of it it the the level of danger we're here talking about is on the level of asteroid impacts where we certainly have gotten caught in the blast radius of a supernova where we have gotten too much of the high energy radiation, and it has done damage to our ozone layer. Like, that has happened in our past, just like rocks have fallen out of the sky in the past. But it's a different question of whether it has triggered a mass extinction or not or if our ozone layer has been able to repair itself or if there's been die off but it hasn't appeared in the fossil record lay. It's up for debate. It's up for debate.

We know what's happened. But I have to mention that all of this analysis is focused on type 2 supernova. These are the supernova that occur at the end of a massive star's life. And because massive stars are massive and super bright, they're easy to see, they're easy to predict when they're gonna go supernova, and they're easy to estimate our our risk. But there are other varieties of supernova that are a little bit harder to predict, a little bit harder to spot.

For example, there's a kind of type 2 supernova like the usual type 2 supernova where you have a giant star end of the life. Boom. But, there are special cases where the dying star is enshrouded by a thick layer of dust. And then the shock wave from that explosion hits the dust, creates a flood of x rays way more than a typical supernova. I told you we'd come back to, you know, typical rates of high energy radiation.

You get this absolute flood of x rays. Then because of the interaction with the shock wave and the dust, you get a whole lot of cosmic rays that come a few centuries later. And so it's a nasty one two punch where you you get this flood of x rays that can weaken an ozone layer. And then a few 100 years later before the ozone layer has had time to repair itself, the cosmic rays just finished the job. These kinds of supernova can be deadly from up to a 150 light years away, which is far bigger.

That's 5 times greater distance than the 30 light year estimates we had before. So, again, we have to do another sense as we look around. Are there stars that are about to die and are enshrouded in these thick layers of dust? Thankfully, there are no such systems within a 150 light years. But again, over the course of 100 of 1000000 of years, that gets a little bit harder to predict when one of these stars might migrate into the danger zone.

Oh, and there's this whole other kind of supernova, the type one a. These have a completely different mechanism for going boom. This is when a white dwarf is orbited by a companion that nears the end of its life. The companion swells to become a red giant, pours material down onto the surface of the white dwarf until it reaches a critical threshold and then kabluey. The thing about white dwarfs is that they are generally small and dim, so they are much harder to detect.

And their final evolution towards a supernova is much more random. One day, they're just fine just hanging out. You know, material is is accumulating on their surfaces like it's no big deal. We've handled this before and then you blank and they've turned themselves inside out in a nuclear inferno. So type 1 a supernovas are much harder to predict.

We we know on a galaxy scale, they happen, you know, a few times per century. But in our nearby neighborhood, it's harder to predict when the next white dwarf binary system might turn into a supernova. But again, I'm gonna help you sleep tonight. The nearest candidate is the binary white dwarf ika pagacy which is about a 150 light years away well outside the danger zone. But in the future, who knows where these white dwarfs might move?

We don't know. Or white dwarf systems that are totally fine now. And then in, like, 2000000 years, the companion star swells to become a red giant, and now we have to keep a close eye on it. But before you get too complacent, there's an an entire other category of cosmic explosion. These are the gamma ray bursts.

There are 2 kinds of gamma ray bursts. One are triggered by neutron star mergers. When neutron stars merge together, they release an incredible flood of gamma rays, and the other is from hypernovas. These are when the biggest, biggest, biggest stars die, and they form either a neutron star or a black hole in the center. And then the material that was trying to explode out instead gets swallowed in by the black hole, forms an accretion disk, and then this jet of material goes out, and gamma ray bursts are fascinating and awful.

The trouble with gamma ray bursts is that their radiation doesn't always follow the inverse square law. Because that inverse square law works for a source of light that is radiating in uniformly in all directions. So like our sun, our sun is glowing, towards the top into the sides and at weird angles, and then all that radiation is expanding in a sphere around it. But gamma ray burst, when they go, they are beamed. They they're like light houses, almost like these giant cosmic lasers.

They're not really lasers. It's just a metaphor here, but the idea is they're beamed and they're focused. They're also incredibly powerful. Some of the most powerful explosions in the universe. You thought supernova were powerful, but gamma ray burst put them to shame.

So you get a combination of incredibly powerful, high output of high energy radiation like gamma rays, hence the name, and it's focused into a tight beam. Now the good news with gamma ray burst is that most of the time, the beams are not going to be pointed at the earth because they're pointing in all sorts of random directions, and they're directions, and they're pretty tightly focused. Most of them are just gonna miss. We do see them all the time. They come we tend to see them coming from distant galaxies.

You know, just lucky chance to happen to be pointed at the earth. But we've done the math here, and a gamma ray burst can be deadly from about 10,000 light years away. 10000 light years is a pretty decent fraction across the entire Milky Way galaxy. And these are essentially impossible to predict because they are so far away. We can't have a a complete enough senses.

You know, we're talking a census of, like, a 100,000,000,000 stars. We'd have to catalog all those stars, find out which ones are candidates to become gamma ray bursts, Estimate to that the angle, is is face on where we might get full blast or is it edge on where the beam is just gonna go somewhere else. It's hard. We suspect we have likely been blasted by a GRB in the past, potentially with devastating consequences. Again, there may be some hints in the fossil record, that some of the extinction events we faced in the past have been due to gamma ray bursts, but estimating the rate is much much trickier than when it comes to supernova.

They are much more rare. That's for sure. But now we're dealing with a much larger danger zone. Not 30 light years. We're talking 10,000 light years.

In other words, it appears that for the next few 1000000 years, we are safe from supernova of all kinds. But when it comes to gamma ray bursts, we're probably safe. But, you know, it could come any day, any time, from any direction on the sky, and there's absolutely nothing we could do about it. Sweet dreams, kids. This is probably why I'm not allowed to read bedtime stories anymore.

Thank you so much to Steven g for the question that led to today's episode, and thank you to all my top Patreon contributors. Oh, thank you to all my Patreon contributors. That's patreon.com/pmsutter, p m s u t t e r. But I would like to thank my top contributors this month. They are Justin g, Chris l, Alberto m, Duncan m, Corey d, Stargazer, Robert b, Nyla, Sam r, John s, Joshua Scott m, Rob h, Scott m, Louis m, John w, Alexis Gilbert m, Rob w, Jules r, Mike g, Jim l, David s, Scott r, Heather, Mike s, p h, Sebas, Watt WattWord, Lisa r, c, Kevin b, and Michael b.

Thank you so much everyone. Keep those questions coming. Ask a spaceman.com or ask a spaceman atgmail.com. Send me your questions. I will put them on the list like I just did and devote an entire episode.

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I'll see you next time for more complete knowledge of time and space.

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