Relationship between stellar mass and core mass?

[bbbl67]

Is there an astrophysical relationship between the how large a star's core gets and how large the star itself gets? Is it a simple linear percentage, or something more complex? For example, red dwarfs can fuse their entire hydrogen allocation, so the whole star is the core. But the Sun has a core that is only 35% of its mass. How about larger stars like blue giants?

 

[Drakkith]

For example, red dwarfs can fuse their entire hydrogen allocation, so the whole star is the core.

Red dwarfs can fuse their entire supply of hydrogen because they are convective throughout their entire volume instead of only part of it like larger stars. I'm fairly certain that the core is still a small area near the center where the overwhelming majority of fusion is taking place.

I'm afraid I don't know the answer to your question though, but I hope to find out.

 

[nikkkom]

Depends on the definition what "core" is.

 

[Ken G]

I believe the solar value of around 30% of the mass in the core is fairly ubiquitous for main-sequence stars, where "core" means the region where central fusion is happening when it is happening, or the region inside the shell fusion when it isn't. For stars that eventually achieve core helium burning, most have that occur when the core mass (of pure helium) is about a half a solar mass, so in the core helium fusing phase, it's not the fractional mass but the total mass of the core that tends to be similar. Then shell helium fusion adds even more to that, so what ends up being the core when the envelope is shed and a white dwarf is made can be more like a solar mass (for stars that have had time to make white dwarfs).

 

[bbbl67]

Depends on the definition what "core" is.

Well, you tell me what definitions of "core" there are that you know of.

 

[bbbl67]

I believe the solar value of around 30% of the mass in the core is fairly ubiquitous for main-sequence stars, where "core" means the region where central fusion is happening when it is happening, or the region inside the shell fusion when it isn't. For stars that eventually achieve core helium burning, most have that occur when the core mass (of pure helium) is about a half a solar mass, so in the core helium fusing phase, it's not the fractional mass but the total mass of the core that tends to be similar. Then shell helium fusion adds even more to that, so what ends up being the core when the envelope is shed and a white dwarf is made can be more like a solar mass (for stars that have had time to make white dwarfs).

So what you're saying is that during the main sequence phase, all large stars (i.e. not red-dwarfs), from the Sun on up to R136a1, will have an approximately 0.3 solar mass core? That the largest stars just burn through their 0.3 SM cores faster?

 

[Ken G]

I think even the red dwarfs have cores like that, in rough terms-- the exact fraction varies with mass and age. As mentioned above, the fusing cores of highly convective stars are always pulling in new gas, so for them, the core is not a region of different composition, but it can be for stars with quieter radiative cores like our Sun. But yes, the more luminous stars simply burn through their cores faster.

 

[stefan r]

Well, you tell me what definitions of "core" there are that you know of.

Waves travel through an object and refract at phase boundaries. The core is the region on the inside. We know that the Earth and moon have cores because of earthquakes (moonquakes). Astroseismologists study resonant frequencies in stars.