Rate of Solar collapse to Black hole

[halfeye]

He was talking about an Earth-sized volume of material from the core of a star. The matter down there is something like fourteen times as dense as lead due to the enormous pressures, so there's a lot more mass in an Earth-sized chunk of it than in the actual Earth itself.

If I remember correctly (it's a particularly random factiod that seems to have stuck) the area of the even horizon increases in proportion to the mass. So 56 Earth masses gets you 56 times the surface area of a one cm diameter sphere. That's still not a large area.

[Lord Torath]

Just a query, how sure are we that there's no rock (or plasma of those elements) in the centre of the Sun? It seems probable to me that there would be, on first thinking about it.

Here's an article about a pair of stars, one of which has "eaten" 15 earth-masses of rocky planets. Still seems to be fusing nicely.

[keybounce]

Ligo is definitely impressive with that detection ability. It can clearly feel when space is rippling, due to a tensor perturbation, or through a wave of gravitation.

This view of space is rippling through our collective imagination; can you feel the excitation?

[halfeye]

Here's an article about a pair of stars, one of which has "eaten" 15 earth-masses of rocky planets. Still seems to be fusing nicely.

I wasn't thinking that having rock in it would stop it fusing. I was thinking that if there was a core beneath the fusion, the fusion would continue on top of that. However I can also see that the temperatures involved might mean that the "metal" ions would be moving fast enough to get mixed in with the rest of the mass, and I wouldn't expect that to stop fusion either.

I don't see why that star is suppposed to have "eaten" planets to get it's "metals", if the "metals" were present in the original cloud I don't see why they wouldn't have fallen in.

[Leewei]

I wasn't thinking that having rock in it would stop it fusing. I was thinking that if there was a core beneath the fusion, the fusion would continue on top of that. However I can also see that the temperatures involved might mean that the "metal" ions would be moving fast enough to get mixed in with the rest of the mass, and I wouldn't expect that to stop fusion either.

I don't see why that star is suppposed to have "eaten" planets to get it's "metals", if the "metals" were present in the original cloud I don't see why they wouldn't have fallen in.

Something very similar to this is happening with helium in our sun. We have a few billion years before most of our sun's fusion is happening close to its surface, ejecting enormous amounts of hot solar mass and cooking our world to a cinder.

We'd probably need an iron core big enough to make a noticeable difference in our sun's fusion for it to not mix in with the rest of the lighter mass around it. (The convection in our sun is incredibly forceful.) If this was around a singularity, the firewall would not initially blow apart the sun. Instead, the hole would feed until it reached fusible material, then its firewall would blow the rest of the sun apart.

[Lord Torath]

I don't see why that star is supposed to have "eaten" planets to get it's "metals", if the "metals" were present in the original cloud I don't see why they wouldn't have fallen in.

Because it companion star does not have the same concentration of metals, and since they formed from the same molecular cloud, the obvious explanation is that the one "ate" 15 earth-masses of planets sometime after the stars were formed.

[halfeye]

Because it companion star does not have the same concentration of metals, and since they formed from the same molecular cloud, the obvious explanation is that the one "ate" 15 earth-masses of planets sometime after the stars were formed.

That's sort of what the story says, yeah, but where did the planets get the metals if the cloud didn't have them and the planets formed in the cloud?

[Rockphed]

I wasn't thinking that having rock in it would stop it fusing. I was thinking that if there was a core beneath the fusion, the fusion would continue on top of that. However I can also see that the temperatures involved might mean that the "metal" ions would be moving fast enough to get mixed in with the rest of the mass, and I wouldn't expect that to stop fusion either.

I don't see why that star is suppposed to have "eaten" planets to get it's "metals", if the "metals" were present in the original cloud I don't see why they wouldn't have fallen in.

That's sort of what the story says, yeah, but where did the planets get the metals if the cloud didn't have them and the planets formed in the cloud?

Well, after solar ignition, volatile elements (i.e. ones that readily form light gasses) get removed from the planets that originally had them. So you end up with metals that form rocks and heavy polymers being left behind. If those get eaten by the star after they form, they increase the stellar concentration of metals without increasing the stellar concentration of hydrogen and helium.

[factotum]

I wasn't thinking that having rock in it would stop it fusing. I was thinking that if there was a core beneath the fusion, the fusion would continue on top of that.

That can happen in high-mass stars, because it'll end up with an inert iron core underneath an onion arrangement of layers fusing different materials. However, there's absolutely nothing that would cause the vapourised remnants of a rocky planet to sink to the core--the stuff down in the core is far denser than any regular material due to pressure, so any material being introduced from outside would probably float on top of it.

It's also worth noting that convection isn't really a thing in the cores of anything but the tiniest stars. In stars like our Sun all of the energy transfer from the core to the outer layers happens via radiation, and any convection going on occurs entirely in the outer layers.

[Lord Torath]

I think Half-eye's main objection here is: If there're all these heavy elements available for planets to form from, why don't the central stars have just as much or more of them than the planets that form around them from the same cloud? (Did I summarize that correctly?)

I confess I do not know enough about solar system formation to provide an answer.

[halfeye]

I think Half-eye's main objection here is: If there're all these heavy elements available for planets to form from, why don't the central stars have just as much or more of them than the planets that form around them from the same cloud? (Did I summarize that correctly?)

Yes, that's what I was asking. If two stars formed from one cloud, it doesn't seem that unreasonable for the cloud to have areas of slightly different composition.

However, there's absolutely nothing that would cause the vapourised remnants of a rocky planet to sink to the core--the stuff down in the core is far denser than any regular material due to pressure, so any material being introduced from outside would probably float on top of it.

Why would the material on top of any layer be at significantly less pressure than the top of that layer? I'm not saying that iron at Earth pressures wouldn't float on hydrogen at deep solar pressures, but how does the iron remain at Earth pressures if it's so deep in a star?

It's also worth noting that convection isn't really a thing in the cores of anything but the tiniest stars. In stars like our Sun all of the energy transfer from the core to the outer layers happens via radiation, and any convection going on occurs entirely in the outer layers.

Ah, that's something I didn't remember, thanks for that.

[factotum]

Well, as I understand it, the material the Sun and the planets formed from was all the same--five billion years ago or thereabouts a cloud of gas and dust slowly collapsed under its own gravity. As it did so it spun faster, flattening it out into a disc. Eventually, the temperature and density at the centre rose high enough for fusion to start and the Sun came into being. The solar wind from the young Sun would have blown away most of the lighter gasses from the inner planets, which is why they're all rocky and you don't get gaseous ones until you get to Jupiter, which is a fair distance out.

I think the overall proportions of rock and other solid material to gas would still be higher in the remnants of the original disc than they are in the Sun itself, simply because of this scouring of lighter material from the cloud in its early days.

Oh, and it's worth noting that the gas the Sun formed from was about three-quarters hydrogen and one-quarter helium, yet as far as we know the Sun does not (yet) consist of a helium core with hydrogen fusing around it--it's all pretty well mixed up in there. In a few billion years when it leaves the main sequence and becomes a red giant the Sun will start to accumulate an inert helium core, but it's a long way off that yet.