How much of a star's radiation would you have to reflect back at it to blow it up?

I tried to ask this question a long time ago in another thread but managed to botch the phrasing of the question so horribly (and with so many, many, unnecessary words) that many people were just annoyed at me for putting it in GQ. I apologize for that.

It’s been a very long time and I think I’ve figured out how to pare it down to it’s fundamental point, though, so I’m trying again:

How much of a star’s radiation would you have to reflect back at it to blow it up?

A shitload.

I think what you suggest would have the opposite effect of what you intend. The increased energy would make the star hotter, causing it to expand, resulting in more diffuse internal burning patterns that would ultimately make the star somewhat more stable in the near term. Stars go supernova when their fuel runs low and they star burning (fusing) heavier elements, until the force of gravity overcomes the heat pressure and the star collapses in a supersonic shockwave. Adding energy back to a star at a rate comparable to the star’s output would not make it explode. Of course, IANAAP, so I could be wrong.

Any process by which you do this is going to take millions of years, at least for the Sun. To “blow up” a star, by which I mean “give enough energy to its pieces so that they don’t re-collect and re-form into a star”, the energy you impart to the star would have to be greater than its gravitational binding energy. This would take about 19 million years for the Sun.

It would probably take less time for a larger star, since the binding energy scales more slowly than the square of the mass (the radius increases with mass too), while the luminosity (i.e., total power output) of stars generally increases more quickly than the square of the mass. My intuitive guess is that you might get this down to tens of thousands of years for the largest, hottest stars; but that’s really just a WAG.

I was assuming a process that caused some dramatic acceleration of the star’s internal fusion. Stars that supernova already contain enough energy to blow themselves up, after they’ve consumed the majority of their fuel, and the explosion does not take millions of years.

And, arguably, the vast majority of exploded stars do have some of their “pieces” “re-collect and re-form into a star”: that’s why we’ve got heavy elements here on Earth, because our star and everything in its system is the “re-collected” ashes of a previous star.

Reflecting radiation back at the star won’t change the rate of fusion though. (If there is a change it will be as MikeS said, to slow down the fusion). Fusion is a product of both density/pressure and temperature. Stable stars have reached an equilibrium between a tendency to collapse under gravity, and a tendency to expand under heat. Your strategy to reflect energy back would increase the heat without changing the gravity.

In fact, any strategy to increase fusion will have the opposite effect. Fusion increases heat, which makes the star expand, which reduces the speed of fusion. So all you’d change is the star’s point of equilibrium.

As for supernovae, I’m not sure what you mean when you say it doesn’t take millions of years. While it’s true that the supernova itself happens very quickly, the star took an extremely long time getting into that state - it had to burn through hydrogen, helium, etc. until its core was extremely dense, extremely hot, and running out of burnable fuel. The supernova is the final step in the process, but not one you can induce without going through the whole process.

In fact, this should highlight what you really want: in order to blow up a star, you want to stop fusion. The best way to achieve that is probably by throwing a giant mass of iron into it. (I don’t know exactly how much, but a lot. There might not be enough iron in the entire solar system.) The iron can’t fuse, so once it displaces enough fuel in the core, you’ll have a nova/supernova.

Is this true? I seem to remember that iron would fuse, under tremendous, even by stellar standards, pressure and heat, but it took more energy to make it do so than it would produce. Thus there was a net loss of energy which lead to novahoodness.

I’m quite prepared to have my ignorance fought though.

It’s my recollection that getting elements past iron requires the supernova. Supernova first, which provides the energy to produce the heavier elements, rather than iron atoms fusing first.

I thought the same thing too, and made a post almost exactly like yours just last week (link). Apparently, the elements higher than iron are formed by neutron bombardment, not proper fusion.

I was responding to MikeS, whose response and math seemed to indicate he thought I wanted to spend millions years using mirrors to puff up a star like a marshmallow in a microwave in order to get it to eventually dissolve. That is definitely not what I’m asking about.

I’m loathe to elaborate too much on my question because of my previous experience of elaborating way, way too much, but what I wanted to know is how much energy, expressed as a percentage of a star’s radiative output, would have to be reflected back into that star in order for that energy to accelerate its fusion so dramatically it experienced a nova or nova-like event. My hypothetical scenario is one in which the star’s own energy is used, not energy from some ridiculously huge external source. Although, given some of the speculative descriptions I’ve encountered about stars very close to each other in the galactic core possibly heating each other up until they all nova together, maybe that’s relevant too.

There’s a lot of assumptions in here, the first being that more heat equals more fusion. I understand what For You and you are saying- and it’s a good point- that the tendency is for stars to relieve the pressure caused by increased heat by expanding. So, I guess an important requirement for this to happen is that energy is reflected back into a star at a high enough rate that it heats up faster than it can expand to relieve the pressure. If 100% percent of a star’s radiative output, reflected back into it, would still cause it to expand to relieve pressure faster than it heated up and increased the pace of fusion, than my hypothetical scenario couldn’t happen at all.

Why would a star with no fusion blow up? It’s gravity holding it together, not fusion. Fusion is what’s making it hot, and heat is what’s driving it to expand. A star with no fusion would collapse. Or are you describing a scenario in which fusion is only stopped temporarily, the star then becomes much more dense during that time, and then fusion resumes at a much higher rate?

Moving around that much iron sounds interesting but is not the sort of thing I’m looking for, no. I’m thinking of something that uses mirrors or magnetic fields or some other field effect that is reflective.

My sort of mental image of this exploding star is akin to a packaged microwavable meal in a sealed container that the preparer is supposed to break the seal of to vent. When the package is vented, the heat escapes, and the meal is merely warmed. But, if the package is not vented, the heat builds up and the pressure increases, sometimes causing the package to violently explode. In my imagination, reflecting the star’s radiation back at it is like closing the vent, forcing the heat to build up until the pressure becomes overwhelming. I actually did this on accident with a package of rice I forgot to vent about two weeks ago. There was rice everywhere.

How science-fictiony are you wanting to get?

It takes a photon literally hundreds of thousands of years (if not millions) to get from the core to the surface, due to the density of particles in a star. Here’s a simplistic cartoon of it.

Simply reflecting all that energy back isn’t going to produce any immeidate results. Those particles (photons or not) are simply going to come bouncing back out again. Any eventual heat that might get near the core will take hundreds of thousands of years (if not millions) and will, as mentioned above, simply slow the existing fusion process.

So, again, how science-fictiony are you wanting to get? The popcorn bag analogy fails because no reflection process is going to work anywhere near the speed you envision.

The OP is positing a spherical mirror surrounding a star, that can have the level of reflectivity adjusted from 0 to 100%. At what point on that scale can you make the star explode.

The reflected energy is going to increase the heat of the star, causing it to swell and fusion rates to drop. On the other hand, the reflected radiation is going to exert radiation pressure on the surface of the star, reducing the expansion. My question is - what is the magnitude of that radiation pressure - will it confine the star enough to increase internal pressure and fusion reaction rates or not?


Pretty science-fictiony. I’m mostly interested in whether the math is broadly compatible, not whether it’s actually remotely possible.

See, this is the sort of misconception I’m trying to root out. So what you’re saying is that a star is packed so tight that the core is virtually impervious to whatever you’re doing to the outside? I can see I’ve let the “mass of incandescent gas” line skew my perception of how dense stars are. They are fusing, after all, but I guess on some level I still thought of them as being a sphere of elastic fluid all the way down. It sounds like that is definitely not the case.

This is the previous thread where I tried to ask about this: On The Engineering Of Applied Phlebotinum - Factual Questions - Straight Dope Message Board

It’s awful, don’t read it. But yes, it is blatantly science-fictiony, and it has to do with a story idea. I had this idea that posited some lifeform that was native to stellar coronas, ate solar wind, stayed alive with the same sort of shields-and-forcefields mumbo-jumbo that star ships in Star Trek use to defend themselves, and periodically reached population levels that induced an instinct to drive their star to supernova and seed them across the galaxy in the fashion of Larry Niven’s stage trees.

I was willing to accept a lot of implausibility with the idea, but one thing that felt like going too far was botching the math with regard to just how many of the critters there needed to be before they could give their star a sharp kick. If they needed to cover every inch of the surface of a star, well that was just stupid. And it sounds like you’re saying that even if they did cover every inch of a star, and used forcefield mumbo-jumbo to reflect energy inward, it wouldn’t do squat, because the outer layers of the star would expand and relieve the pressure long before the energy managed to get anywhere near where the fusion was happening.

There almost certainly isn’t enough iron in the Solar System, since the vast majority of everything in the Solar System is already in the Sun. And if anything, this would be even more true of heavy stuff like iron.

…Which to me, means that the amount of energy bubbling around inside the sun, right now, is equivalent to that many years of typical output - if that’s anywhere near correct, then reflecting all of the output back for a day, a week, a year, a century, is going to be like pissing into a hurricane.

Thank you, Si, for restating the original question so clearly. It’s longer than mine but it narrows things down even more quite nicely.

It’s been done before by David Brin in Sundiver. Only he used Dark Matter creatures to increase the gravity of the Sun to prematurely age it.

Strain of Thought, I think you have a fundamental misunderstanding of how the nova/supernova process works, so I’d start there. For example, Wikipedia says this of a Core Collapse Supernova:

So you see, the “explosion” is a misnomer - it is an implosion with only the outer layers being thrown outwards. The core collapses, and the collpase is a result of too little fusion to hold it up against gravity.

A star does not explode because of too much energy… except perhaps for a type Ia supernova. In nature, this works only with a white dwarf that is acquiring additional material (such as from a binary partner star) and this additional material raises the internal temperature enough to begin fusing carbon.

So… if the idea of reflecting heat back onto a star has any plausibility, it is only for a white dwarf, to produce a type Ia supernova. I still can’t fathom how much energy is involved, but there’s a problem there too. White dwarf stars aren’t produce new energy from fusion; they’re merely radiating stored energy. Thus, the reflector system can’t raise the temperature - the best you can do is cause it to stop cooling. So you need another source of energy/mass. (Ironically, we’re now back to the solution of throwing a giant chunk of something at the star, though it’s no longer important that you use iron.)

The idea that an explosion is cause by an increase in energy is where Strain of Thought is straining. Gasoline on a fire causes a flare-up, for example, but not an explosion. The physical reality is that matter and energy are all but indistinguishable, adding energy back into a star is not a whole lot different from adding material. What seems to be the key precursor to an explosion is high compression, explosions are a newtonian reaction to having too much heat in too small a space.

As an aside, there are some interesting things going on in a big supernova (is “big supernova” redundant?). In many cases, silicon burning creates an iron core (rather quickly, on the order of a day) which eventually collapses to become a ball of neutrons about as dense as an atomic nucleus (neutron star). Curious thing is that iron is a typical element that is composed of isotopes that are about half protons, like most matter, so what happens to all those protons? Are they cast out of the core, expanding in part through electrical repulsion? Or do they transform into neutrons? The latter case is interesting because it would wash the area with positrons, which might contribute to the expansion of the shell through the energy released as they co-annihilate with local electrons.

The electrons are driven in by the pressure, and they combine with the protons to form neutrons.