Why do stars with masses greater than 1.2M(sun) have convective cores?

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In summary, large stars with masses greater than 1.2M(sun) have convective cores due to the highly temperature dependent CNO cycle. This is because the central regions of the star experience a large energy flux, leading to a steep radiative gradient and an instability against convection. Even if our sun were just above this limit, we would still have a star with a convective core, which may cause fluctuations in luminosity due to turbulent flow.
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from Carol and Ostie (textbook):

"stars with masses greater than 1.2M(sun) have convective cores due to the the highly temperature dependent CNO cycle."

QUESTION:

why does the fact the CNO cycle is sensitive to temperature mean that the core is convective?
 
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bump:)
 
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Tere is just one answer, no. The core is thermodynamic system with spherical interfasis. The core takes part in the star cicle.
 
  • #4
Hydrogen burning processes in large stars, as you've pointed out, is dominated by the CNO cycle. This is confined to the central regions of the star and so there's a large energy flux which naturally favours a convective central region.

The resulting steep radiative gradient towards the centre makes the core unstable against convection (http://en.wikipedia.org/wiki/Schwarzschild_Criterion).
 
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  • #5
astrorob said:
Hydrogen burning processes in large stars, as you've pointed out, is dominated by the CNO cycle. This is confined to the central regions of the star and so there's a large energy flux which naturally favours a convective central region.

The resulting steep radiative gradient towards the centre makes the core unstable against convection (http://en.wikipedia.org/wiki/Schwarzschild_Criterion).

yes, i see it now. thanks:)
 
  • #6
vertices said:
"..stars with masses greater than 1.2M(sun) have convective cores...

A factor of 1.2 is pretty close... Suppose our sun would be just above that limit, and the increased luminosity would be compensated by a more distant Earth orbit - would we notice that we have a star with a convective core ? As I understand it, there would be a turbulent flow, would these turbulences cause (high) fluctuations of the luminosity of the sun ?
 

1. What are convective cores?

Convective cores are regions in stars where heat is transported through convection, which is the movement of hot gas rising and cool gas sinking. This process helps to mix the star's interior and maintain its energy balance.

2. Why do convective cores form in stars?

Convective cores form in stars because of the high temperatures and pressures in their interiors. This causes the gas to become less dense and rise towards the surface, creating convection currents that help distribute heat throughout the star.

3. How do convective cores affect the evolution of stars?

Convective cores play a crucial role in the evolution of stars. They help mix the materials in the star's interior, allowing elements created through nuclear fusion to be transported to the surface. This also prolongs the star's lifespan by replenishing its fuel.

4. What determines the size of a convective core?

The size of a convective core is determined by the mass and composition of the star. More massive stars have larger convective cores because they have a higher rate of energy production, leading to more convection. The composition of the star also affects its core size as different elements have different opacities, which can impact the efficiency of convection.

5. Can convective cores be observed?

Convective cores cannot be directly observed, as they are located deep within the interior of stars. However, their presence can be inferred through observations of stellar properties, such as surface temperature and luminosity. These properties can provide clues about the convection happening in a star's interior and the presence of a convective core.

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