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

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Discussion Overview

The discussion revolves around why stars with masses greater than 1.2 solar masses (Msun) possess convective cores, particularly in relation to the temperature sensitivity of the CNO cycle. It explores theoretical aspects of stellar structure and energy generation processes, with a focus on the implications of these processes for stellar behavior.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants reference the CNO cycle's temperature dependence as a reason for the convective nature of the cores in massive stars.
  • One participant describes the core as a thermodynamic system with spherical interfaces, suggesting that it participates in the overall stellar cycle.
  • Another participant notes that the energy flux from hydrogen burning processes in large stars favors a convective central region due to the steep radiative gradient, which may lead to instability against convection.
  • A later reply questions the implications of a star just above the 1.2 Msun threshold, speculating on whether such a star would exhibit turbulent flow and fluctuations in luminosity if it were positioned at a greater distance from Earth.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between the CNO cycle and convective cores, with some supporting the connection while others introduce alternative considerations. The discussion remains unresolved regarding the implications of a star's mass and its core dynamics.

Contextual Notes

There are limitations in the assumptions made about the relationship between mass, core convection, and the CNO cycle, as well as the implications of distance on luminosity fluctuations. These aspects are not fully explored or resolved in the discussion.

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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:)
 
Tere is just one answer, no. The core is thermodynamic system with spherical interfasis. The core takes part in the star cicle.
 
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).
 
Last edited:
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:)
 
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 ?
 

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