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

[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.

 

[Drakkith]

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?

Short of magic, I can't think of any way to add mass to the core of the star in a short timescale (let's say a million years or less) that doesn't substantially heat up the core.

 

[Devin-M]

So it wouldn’t be essentially a white dwarf way over the Chandrasekhar limit at the start of the sim?

 

[Drakkith]

So it wouldn’t be essentially a white dwarf way over the Chandrasekhar limit at the start of the sim?

A main sequence star? No, not at all.

 

[Devin-M]

I thought you need over 10 solar masses to burn oxygen so why wouldn’t you get a gravitational collapse?

 

[Drakkith]

Main sequence stars don't have cores made out of degenerate matter. The core's not as dense as a white dwarf and there's still plenty of normal gas pressure holding the core up, so no collapse.

 

[PeterDonis]

I thought you need over 10 solar masses to burn oxygen so why wouldn’t you get a gravitational collapse?

Adding water is not just adding oxygen. It's also adding hydrogen. A solar mass of water contains a lot of hydrogen. All of which the star can and will burn.

My guess (and it's just a guess, I haven't done any math) is that the end result of this (after a Rayleigh-Taylor instability had put the densest atoms at the bottom and the least dense at the top) would be something like an oxygen core with a shell of helium around it, and hydrogen around that. The hydrogen would burn, quite possibly the helium would burn; the oxygen probably would not.

Eventually, when the hydrogen and helium burning was exhausted, since the total mass of this thing is over the Chandrasekhar limit but still (probably) less than the Tolman-Oppenheimer-Volkov limit (AFAIK the lower bound on that limit now is at least 2 Suns), then it would collapse to a neutron star.

 

[Devin-M]

Would that final collapse into a neutron star likely involve explosive oxygen burning?

 

[PeterDonis]

Would that final collapse into a neutron star likely involve explosive oxygen burning?

It could, since it would most likely be a supernova.