The Shape of Very Massive Stars: Spheres or Something Else?

[windy miller]

What is the shape of very massive stars? Are they simply spheres or something else?

 

[phinds]

Why would you expect them to be anything other than spheres?

 

[phyzguy]

If they are rotating rapidly, they will be oblate spheroids. Otherwise, they are spheres.

 

[windy miller]

Why would you expect them to be anything other than spheres?

I seem to recall someone saying they weren't , but I want to be clear I don't have any reference for that. Always best to ask i think if one has some doubts.

 

[windy miller]

If they are rotating rapidly, they will be oblate spheroids. Otherwise, they are spheres.

thanks

 

[windy miller]

Thanks for the help, could you clear up the surface deformrity issue though. When I think of a surface of a neutron star I thin of it being very smooth , but our sun has lots of bubbling convection flows I imagine. SO what about massive stars, will they have more surface irregularitites?

 

[Nik_2213]

Some red giants have really, really big sun-spots.
https://en.wikipedia.org/wiki/Starspot

 

[phyzguy]

Thanks for the help, could you clear up the surface deformrity issue though. When I think of a surface of a neutron star I thin of it being very smooth , but ur sun has lots of bubbling convection flows I imagine. SO what about massive stars, will they have more surface irregularitites?

Of course they will have surface irregularities. The Earth is very close to a sphere, but it has mountains and canyons. The height of any surface irregularities will depend on the surface gravity. The higher the surface gravity, the more potential energy it will take to produce a "bump". A massive supergiant like Betelgeuse has a relatively low surface gravity because of its large radius (about 0.5 m/s^2 according to Wikipedia), so we would expect relatively large surface irregularities. A massive main sequence star like Theta Orionis A1 is much smaller, so has a much larger surface gravity (20,000 m/s^2 according to Wikipedia), so we would expect much smaller surface irregularities. A neutron star has an enormous surface gravity so any irregularities will be extremely small.

 

[windy miller]

Of course they will have surface irregularities. The Earth is very close to a sphere, but it has mountains and canyons. The height of any surface irregularities will depend on the surface gravity. The higher the surface gravity, the more potential energy it will take to produce a "bump". A massive supergiant like Betelgeuse has a relatively low surface gravity because of its large radius (about 0.5 m/s^2 according to Wikipedia), so we would expect relatively large surface irregularities. A massive main sequence star like Theta Orionis A1 is much smaller, so has a much larger surface gravity (20,000 m/s^2 according to Wikipedia), so we would expect much smaller surface irregularities. A neutron star has an enormous surface gravity so any irregularities will be extremely small.

Thanks that makes a lot off sense.

 

[lomidrevo]

Of course they will have surface irregularities. The Earth is very close to a sphere, but it has mountains and canyons. The height of any surface irregularities will depend on the surface gravity. The higher the surface gravity, the more potential energy it will take to produce a "bump". A massive supergiant like Betelgeuse has a relatively low surface gravity because of its large radius (about 0.5 m/s^2 according to Wikipedia), so we would expect relatively large surface irregularities. A massive main sequence star like Theta Orionis A1 is much smaller, so has a much larger surface gravity (20,000 m/s^2 according to Wikipedia), so we would expect much smaller surface irregularities. A neutron star has an enormous surface gravity so any irregularities will be extremely small.

just to add, one should also realize that average density of supergiant stars like Betelgeuse, can be as low as $10^{-8} \rho_{sun}$, where $\rho_{sun} = 1410 \ kg \ m^{-3}$ is the average solar density. That means that average density of Betelgeuse is approximately $10^5$ times less then the density of air we breathe at sea level. It might be quite difficult to define any surface in such conditions :)