Why don't high mass stars expand and cool?

[tovisonnenberg]

When a high mass star starts to fuse heavier elements, the core heats up the outer layers enough to for them to begin fusion of their own. My question is: because the star hasn't gained any mass, but it has increased in temperature a lot, why doesn't the star expand and cool like a red giant, ceasing fusion in the outer layers?

 

[stefan r]

The proton-proton chain starts at temperatures over 4 million K. Type O stars have surface temperatures closer to 30,000K . So the surface of the largest stars are more than a hundred times colder than what is needed for fusion.

 

[tovisonnenberg]

The surface is too cool, however when silicon fusion starts in the core, there are several layers above the core fusing lighter elements. Neon fusion alone requires temperatures exceeding a billion K.

 

[lomidrevo]

The massive stars do expand and cool as well. The expansion or contraction of a star is basically a result of constant competition between the pull of gravity and the pressure generated inside the star. Do you refer to some particular evolution stage of a particular stellar class?

 

[lomidrevo]

The surface is too cool, however when silicon fusion starts in the core, there are several layers above the core fusing lighter elements. Neon fusion alone requires temperatures exceeding a billion K.

When the massive star starts to burn silicon in the core, it is very close to core collapse - as far as I know just matter of days. In this stage, the star is not expanding.

 

[lomidrevo]

When a high mass star starts to fuse heavier elements, the core heats up the outer layers enough to for them to begin fusion of their own.

It is quite opposite. Even the massive stars begin the fusion with converting the hydrogen into helium in their core, via CNO cycle. When the central temperatures are high enough and the density of helium is sufficient, the burning of helium can start to produce carbon via tripple-alpha reaction. This will happen inside the shell of hydrogen burning. The process is being repeated with continually heavier elements while the central temperature is being increased.

 

[Ken G]

When a high mass star starts to fuse heavier elements, the core heats up the outer layers enough to for them to begin fusion of their own. My question is: because the star hasn't gained any mass, but it has increased in temperature a lot, why doesn't the star expand and cool like a red giant, ceasing fusion in the outer layers?

What you are forgetting is that gravity doesn't just depend on mass-- it also depends on radius. When the radius gets smaller, the gravity gets stronger, so it can support a higher pressure. This is sometimes expressed as the rather bizarre fact that stars have a "negative heat capacity"-- meaning that if you rob a star of heat, all else being equal (and it rarely is, but carry on), the star will shrink, gravity will strengthen, pressure will rise, and the star will be hotter. This means that when the temperature rises, it is much more natural for the star to shrink than expand. It is actually the expansion of a red giant that requires explanation! (And that's a whole story of its own, believe me.)

Your comments are actually relevant to why stars like the Sun self-regulate the fusion in their cores to match the luminosity of the star. The difference is in how a star reacts to perturbations, rather than how it sets its prevailing global properties (the latter being related to what I said above). When you have a perturbation in core temperature, that's when you get the behavior you describe-- and temperature rises are indeed compensated by expansion, shutting down the excess fusion and establishing stable self-regulation.

 

[alantheastronomer]

Massive stars that leave the main sequence - start fusing heavier elements in their cores - do have their outer layers expand and become a supergiant star. Fusion of successive heavier elements in the core produce successively less and less energy, thus leading to more and more gravitational core contraction, till production of iron is reached and producing any more energy is no longer possible and the star eventually goes supernova.

 

[Drakkith]

When a high mass star starts to fuse heavier elements, the core heats up the outer layers enough to for them to begin fusion of their own.

It's not so much that the core directly heats up the layers just above it, rather it is that the contraction of the core shrinks the inside of the star. This contraction happens because, as the star runs out of fuel in its core, the energy radiated out of the core to the rest of the star and eventually into space cannot be replenished bu fusion. This loss of energy cools the core, reducing the outward pressure it exerts on the rest of the star, and this reduced outward pressure can no longer balance out the inward pressure and the core contracts. But, at the same time, the contraction of a gas or plasma heats it up. Since both of these are happening at the same time, the end result is a slow decrease in the radius of the core and a slow increase in its temperature and density.

The contraction also causes the temperature and density of the shell just outside the core to rise, which brings material in the shell into a region dense and hot enough to sustain fusion. Further contraction progressively shrinks the inside of the star, regardless of what its outer layers are doing, and more and more material is brought into the fusion region over time. This would continue indefinitely until virtually the entire star was a small, super-dense object except that iron and nickel do not release energy from fusion.

Note that a star like a red giant has its outer layers expanded drastically because of the huge increase in energy generation from fusion in the shell, but the region surrounding the core actually contracts. So even though the star as a whole got larger, the core and surrounding layers actually shrank.