What happens to the Core of a main sequence star as additional mass is added?

[MikeeMiracle]

TL;DR Summary

What happens to the core of a main sequence star is additional mass is added

So there is a post going around the facebook groups about what would happen if we could pour a Sun sized bucket of water onto the sun, the claim being that the sun would gain mass and become a bigger, hotter burning blue star. I know this cannot happen but I was just curious as to what would likely occur in such a scenario.

While I can appreciate that more mass would make a bigger and hotter star if a sufficiently sized dust cloud collapsed into a bigger star straight away, I have suspicions on if this would happen in the case above.

Namely as far as I understand it the core is it's own exclusive convection zone which has no access to the rest of the hydrogen in the suns outter layers. Can the size of the core change once established without a collision with another star because of this?

I am suspecting that the core would be unaffected by the additional mass added, is that correct?
Would the water even make it to the surface to become part of the sun or just get boiled off into space by the suns atmosphere?

Any idea to the possible outcome of such an event? I would say the water boils off into space without even getting to the surface.

 

[Drakkith]

Namely as far as I understand it the core is it's own exclusive convection zone which has no access to the rest of the hydrogen in the suns outter layers. Can the size of the core change once established without a collision with another star because of this?

Adding mass increases the weight of material that the core is holding up. This compresses the core and the regions surrounding it, bringing the shell of material just outside the core up to a density and temperature capable of sustaining fusion. Normally this happens only once the star exhausts the supply of hydrogen in its core and it enters the red giant phase, where fusion only happens in this shell and not in the core. But since we've artificially added material we have both the core and the shell undergoing fusion simultaneously.

Note that the core of the Sun is not convective. The entire region from the middle of the core out to about 0.7 solar radii is radiative, and this increases as we add mass up until about 1.2 solar masses, so our new shell of fusing material is essentially part of a new, larger core. So my guess is that yes, the core can change size (and probably does over time as the Sun slowly contracts and heats up during its main sequence life).

Above 1.2 solar masses the core becomes partially convective, with a radiative region above that. As we continue to add mass this convective region increases and the rate of fusion skyrockets as the temperature of the core heats up under the compression of the material we've added.

Once the Sun reaches an equilibrium after we add our solar mass of material it should be roughly 11 times as luminous as it is now.

Really all this talk of radiative and convective regions is just irrelevant details of the larger fact that if we double the mass of a star then the inner regions MUST increase in density and temperature. More mass = more weight that must be supported. More weight compresses the star and this compression heats up the gas (we say gas, but we mean both gas and plasma since both follow similar rules when it comes to compression heating). A hot gas exerts more pressure than a cold gas (See bottom of post), so the combination of increased density and higher temperatures is where the outward pressure comes from that counteracts the weight of the outer layers. This increase in temperature and density also increases the rate of fusion and expands the zones already supporting fusion.

Would the water even make it to the surface to become part of the sun or just get boiled off into space by the suns atmosphere?

I think you underestimate the Sun's gravity. At the photosphere, the Sun's outermost layer that we could consider its 'surface', the gravitational force is almost 30x what it is here on Earth's surface. The water could boil all it wants, it's not escaping the Sun. If the material falls into the Sun, it's staying there. Remember that water is made up of hydrogen and oxygen, the same materials that the Sun is made up of (just different proportions). There's no reason for the water to boil off into space if the hydrogen and other materials in the photosphere aren't.*Home experiment: Put a plastic bottle full of air inside your fridge. Let it cool off, then open it inside the fridge again momentarily to let a bit more cold air get in. Then take it out and leave it to warm on the counter for a while. Once it has warmed it will be pressurized and you should be able to hear the gas escape once you crack the lid open.

Or, once you finish your milk, close the lid on the empty milk jug and let it sit and warm up. Come back later and you'll notice that it's swelled up from the increased pressure from the warmed gas inside.

 

[bob012345]

Summary:: What happens to the core of a main sequence star is additional mass is added

So there is a post going around the facebook groups about what would happen if we could pour a Sun sized bucket of water onto the sun, the claim being that the sun would gain mass and become a bigger, hotter burning blue star. I know this cannot happen but I was just curious as to what would likely occur in such a scenario.

While I can appreciate that more mass would make a bigger and hotter star if a sufficiently sized dust cloud collapsed into a bigger star straight away, I have suspicions on if this would happen in the case above.

Namely as far as I understand it the core is it's own exclusive convection zone which has no access to the rest of the hydrogen in the suns outter layers. Can the size of the core change once established without a collision with another star because of this?

I am suspecting that the core would be unaffected by the additional mass added, is that correct?
Would the water even make it to the surface to become part of the sun or just get boiled off into space by the suns atmosphere?

Any idea to the possible outcome of such an event? I would say the water boils off into space without even getting to the surface.

I think it would put the sun out.

 

[Drakkith]

I think it would put the sun out.

Not a chance.

 

[Ibix]

It'd be a very peculiar star, though. The Sun is almost entirely H and He - Wiki says 75:25 by mass in the photosphere and around 40:60 in the core. This would be about 44% oxygen, and I very much doubt that anyone has constructed a remotely rigorous model of such a thing. Also, how the new matter gets distributed depends on how you "pour a Sun sized bucket of water" on the Sun, bearing in mind that $10^{31}$kg of water is probably a star in its own right, and it'll probably bring a lot of kinetic energy to the party however you add it.

I agree that it's unlikely to put the star out since 44% oxygen by mass is still 90%+ hydrogen by particle count. But I could see a lot of short term (on stellar timescales) disruption and some very weird end-of-life behaviour.

 

[MikeeMiracle]

Thank you for the feedback and not dismissing / ridiculing the post.

 

[bob012345]

Not a chance.

Well, I was joking but if you did have a sun sized bucket of water, which itself would ultimately turn into a star of some kind but not immediately, and it completely surrounded the sun, it would take a very long time to see light again at the surface I think.

 

[Devin-M]

Once the sun becomes a white dwarf I thought adding that much mass will put it over the chandrasekhar limit…

 

[Devin-M]

“ If a main-sequence star is not too massive (less than approximately 8 solar masses), it eventually sheds enough mass to form a white dwarf having mass below the Chandrasekhar limit, which consists of the former core of the star.

…

Type Ia supernovae derive their energy from runaway fusion of the nuclei in the interior of a white dwarf. ”

https://en.m.wikipedia.org/wiki/Chandrasekhar_limit

“Physically, carbon–oxygen white dwarfs with a low rate of rotation are limited to below 1.44 solar masses (M☉).[2][3] Beyond this "critical mass", they reignite and in some cases trigger a supernova explosion; this critical mass is often referred to as the Chandrasekhar mass, but is marginally different from the absolute Chandrasekhar limit, where electron degeneracy pressure is unable to prevent catastrophic collapse. If a white dwarf gradually accretes mass from a binary companion, or merges with a second white dwarf, the general hypothesis is that a white dwarf's core will reach the ignition temperature for carbon fusion as it approaches the Chandrasekhar mass. Within a few seconds of initiation of nuclear fusion, a substantial fraction of the matter in the white dwarf undergoes a runaway reaction, releasing enough energy (1–2×1044 J)[4] to unbind the star in a supernova explosion.[5]”

“…the individual particles making up the white dwarf gain enough kinetic energy to fly apart from each other. The star explodes violently and releases a shock wave in which matter is typically ejected at speeds on the order of 5,000–20,000 km/s, roughly 6% of the speed of light.”

https://en.m.wikipedia.org/wiki/Type_Ia_supernova

 

[PeterDonis]

there is a post going around the facebook groups

Which is not exactly the best place for learning about science.

That said, @Drakkith's response is a good summation of the relevant physics.

 

[PeterDonis]

Once the sun becomes a white dwarf

As I understand the OP, it is asking about adding mass to the Sun in its current state, not about what happens if we add mass to a white dwarf.

 

[Devin-M]

Yes sorry, I meant that's what would happen if you add too much water to the future sun.

 

[Bandersnatch]

To be fair, a solar mass-sized bucket of water is pretty much what a white dwarf is. One that is particularly homogeneous in composition, but still. So one way to answer the question would be - if you add too much Sun to the water bucket, it'll explode.

 

[PeterDonis]

a solar mass-sized bucket of water is pretty much what a white dwarf is

If by "water" we mean "a fluid in hydrostatic equilibrium", then yes, this is true. At white dwarf densities and pressures, all matter behaves like a fluid.

 

[Bandersnatch]

I was going at it from the composition standpoint. Say we have 1 solar mass of literal water in an immaterial bucket that would magically appear in space. The water would start to collapse under its own gravity. It'd dissociate into H and O, stratify, and turn degenerate. You'd end up with an inert, degenerate oxygen core and marginal hydrogen atmosphere. A white dwarf.

 

[bob012345]

I was going at it from the composition standpoint. Say we have 1 solar mass of literal water in an immaterial bucket that would magically appear in space. The water would start to collapse under its own gravity. It'd dissociate into H and O, stratify, and turn degenerate. You'd end up with an inert, degenerate oxygen core and marginal hydrogen atmosphere. A white dwarf.

How long would that take?

 

[PeterDonis]

Say we have 1 solar mass of literal water in an immaterial bucket that would magically appear in space.

This is not possible since stress-energy can't just appear out of nowhere. One could consider a (highly idealized) scenario in which there was a cloud of water with a mass of around one solar mass, starting out highly diffuse, and gradually contracting under its own gravity.

It'd dissociate into H and O, stratify, and turn degenerate.

The hydrogen would also fuse at some point, since the collapse process would heat up the object and would be expected to produce temperatures and densities suitable for fusion.

 

[MikeeMiracle]

Which is not exactly the best place for learning about science.

That said, @Drakkith's response is a good summation of the relevant physics.

Tell me about it but it's fun at times seeing peoples misconception and silly claims. This site is by far my best resource for learning actual science.

As it happens this silly post got me thinking of what might happen in this scenario which brought me here to formulate more in depth questions which do answer actual science questions.

I have always been confused as to why exactly main sequence stars form a core which does not convect with the rest of the star which I guess led to this post. Why is this the case?

 

[Drakkith]

I have always been confused as to why exactly main sequence stars form a core which does not convect with the rest of the star which I guess led to this post. Why is this the case?

Most stars, or at least a significant portion of stars, do have a convective core. It's only stars between 0.3 and 1.2 solar masses that have a radiative core. Stars above 1.2 solar masses have a convective core with a radiative region above that.

Per wiki:
The radiation zone is stable against formation of convection cells if the density gradient is high enough, so that an element moving upwards has its density lowered (due to adiabatic expansion) less than the drop in density of its surrounding, so that it will experience a net buoyancy force downwards.

Also:
The Schwarzschild criterion expresses the conditions under which a region of a star is unstable to convection. A parcel of gas that rises slightly will find itself in an environment of lower pressure than the one it came from. As a result, the parcel will expand and cool. If the rising parcel cools to a lower temperature than its new surroundings, so that it has a higher density than the surrounding gas, then its lack of buoyancy will cause it to sink back to where it came from. However, if the temperature gradient is steep enough (i.e. the temperature changes rapidly with distance from the center of the star), or if the gas has a very high heat capacity (i.e. its temperature changes relatively slowly as it expands) then the rising parcel of gas will remain warmer and less dense than its new surroundings even after expanding and cooling. Its buoyancy will then cause it to continue to rise. The region of the star in which this happens is the convection zone.

There's no simple explanation I can give you. Whether a zone is convective or radiative depends on a complex interplay between the density gradient, temperature gradient, heat capacity, and some other effects. Unfortunately I don't know nearly enough about stellar dynamics or thermodynamics to do more than link some articles for you to read.

See these articles for more info:
https://en.wikipedia.org/wiki/Radiation_zone
https://en.wikipedia.org/wiki/Convection_zone

 

[Devin-M]

Would the oxygen from that much water displace the hydrogen & helium in the core?

 

[Drakkith]

Would the oxygen from that much water displace the hydrogen & helium in the core?

It probably depends on how you introduce the water. If you do it somewhat gradually the oxygen never mixes with the core at all. If you take a star-sized ball of water and 'drop' it into the Sun from a couple of AU out such that they approach each other at a free-fall and head on, then the kinetic energy might nearly blow the two stars apart. A back of the envelope calc gives me a kinetic energy somewhere in the ballpark of the Sun's gravitational binding energy, so it looks very much plausible.

An in-between method might lead to some mixing of oxygen with the core, but you'd certainly never have the oxygen displace all of the hydrogen, since even in the case that the water somehow becomes the new core it is still 2/3 hydrogen by particle number.

 

[Devin-M]

What happens if all the Oxygen somehow gets to the core?

 

[PeterDonis]

If you do it somewhat gradually the oxygen never mixes with the core at all.

That's not quite true, because any time you have a heavier fluid on top of a lighter fluid (for example, water or oxygen on top of hydrogen/helium), sooner or later random fluctuations will lead to a Rayleigh-Taylor instability. But the time scale for this could be fairly long.

An in-between method might lead to some mixing of oxygen with the core, but you'd certainly never have the oxygen displace all of the hydrogen, since even in the case that the water somehow becomes the new core it is still 2/3 hydrogen by particle number.

At the temperatures at which all this is likely to happen, there won't be any water; there probably won't even be any neutral atoms of oxygen or hydrogen. Everything will be plasma, i.e., electrons and nuclei, with all particle species essentially moving freely in a fluid. So it would in fact be possible for all the oxygen nuclei to end up at the center and all the lighter nuclei to end up further out. But, as noted above, the time scale for this could be fairly long compared to the time scale of the initial "drop water on star" event.

 

[Drakkith]

hat's not quite true, because any time you have a heavier fluid on top of a lighter fluid (for example, water or oxygen on top of hydrogen/helium), sooner or later random fluctuations will lead to a Rayleigh-Taylor instability. But the time scale for this could be fairly long.

Hmmm. I assumed that the lack of convection in the radiative region would keep anything from mixing, but I guess that may not hold when the heavier elements are on top of the lighter ones instead of the reverse.

 

[PeterDonis]

I assumed that the lack of convection in the radiative region would keep anything from mixing, but I guess that may not hold when the heavier elements are on top of the lighter ones instead of the reverse.

Yes, the lack of convection in the radiative region of a normal star is due to the fact that the heavier nuclei (helium) are towards the center, below the lighter nuclei (hydrogen). One can't assume that will still be the case if you dump a huge blob of heavier nuclei on top.

 

[Devin-M]

If one ran a simulation where they “gently” filled the core with oxygen on a relatively short time scale would it supernova? Is a star that size hot enough to burn oxygen or would you get gravitational collapse?

 

[PeterDonis]

they “gently” filled the core with oxygen on a relatively short time scale

How would you do that? There is no direct, unobstructed path to the core from outside.

 

[Devin-M]

Oxygen accretion disc?

 

[PeterDonis]

Oxygen accretion disc?

This will add oxygen to the surface of the star, not the core.

 

[Devin-M]

Earlier you mentioned the Rayleigh Taylor instability process.

 

[PeterDonis]

Earlier you mentioned the Rayleigh Taylor instability process.

That process is not gentle.

 

[Devin-M]

By gentle I meant accretion disc rather than a blob falling from a few AU.

 

[PeterDonis]

By gentle I meant accretion disc rather than a blob falling from a few AU.

So what? What difference does that make?

You are being very vague. Please take a bit to think carefully and ask a specific question about a specific scenario. For example, if the specific scenario you are interested in is "what happens when a Rayleigh-Taylor instability is triggered because we have a layer of oxygen/hydrogen mixture from a sun-sized bucket of water sitting on top of a helium core?", then ask that specific question. Don't leave other people trying to guess what you're asking about.

 

[Devin-M]

Well in “Universe Sandbox” software I could modify the elemental composition and mass of a sunlike star by adding oxygen and mass… in such a simulation would it be expected to supernova? If it did in the simulation would the simulation be accurate?

https://en.m.wikipedia.org/wiki/Universe_Sandbox

 

[Drakkith]

By gentle I meant accretion disc rather than a blob falling from a few AU.

There's not as much of a difference as you might imagine. Most of the acceleration will happen close to the Sun.

For example, the force near the Sun's surface is 7000x as strong as it is near Mercury's orbit (0.387 AU) and 460x as strong as it is at 0.1 AU. Most of the acceleration happens in the last little bit. For comparison, here's Apollo 11's velocity vs distance as it fell back to Earth under gravity.

As you can see, the spacecraft gained 75% of its final velocity in the last 45,000 miles, or last 25% of its fall distance, and half of its final velocity in the last 10,000 miles from Earth, or the last 5%. Anything falling towards the Sun would experience a similar effect.

Well in “Universe Sandbox” software I could modify the elemental composition and mass of a sunlike star by adding oxygen and mass… in such a simulation would it be expected to supernova? If it did in the simulation would the simulation be accurate?

No. Universe Sandbox does not simulate stellar core physics in an accurate enough way to give you a meaningful result for such a unique situation.