Increase in Radius when a Star becomes a Red Giant

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Discussion Overview

The discussion revolves around the increase in radius of a star when it enters the red giant phase, focusing on the factors influencing this expansion and the variability in predictions regarding the extent of growth. Participants explore the complexities of stellar evolution and seek to understand how to approximate the radius of a red giant star.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the factor by which a star's radius increases during the red giant phase, noting a range of predictions for the Sun's expansion from 100x to 200x its current radius.
  • Another participant highlights that the evolutionary track of a star is sensitive to its mass and chemical composition, suggesting that determining a simple formula for radius increase may not be straightforward.
  • A participant references a resource (MESA) that visualizes the evolution of stars and mentions that the red giant phase has a log_R of 2.24, equating to approximately 175 times the current solar radius.
  • One participant expresses gratitude for the information provided about the MESA resource and the specific radius estimate.

Areas of Agreement / Disagreement

Participants acknowledge the variability in predictions regarding the radius increase of red giants, indicating that multiple competing views remain on the exact factor of growth and the complexity of stellar evolution.

Contextual Notes

The discussion reflects limitations in determining a precise formula for radius increase due to dependencies on various factors such as mass and chemical composition, as well as the sensitivity of stellar evolution processes.

Madi Araly
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Hi there,
I have a question regarding the life cycle of a star. I know that when entering the red giant phase of a star's life, its radius/overall size will increase dramatically, but I was wondering if there's a basic way to determine the factor it will grow by during this process.

I've seen predictions for our Sun's expansion range from 100x its current radius to 200x its current radius, but the considerable difference between these two measurements left me skeptical.

Is there a formula to determine (even an approximation of) what a star's radius as a red giant may be?
 
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You might want to start here:
https://en.wikipedia.org/wiki/Stellar_evolution

The precise evolutionary track of a star depends quite sensitively on its mass and chemical composition (which in turn determine what types of nuclear reactions occur in the star (which in turn determine the energy output of a star (which in turn determines the star's hydrostatic equilibrium (which in turn determine the size of the star)))). So it's not a terribly straightforward problem. Maybe there's a simple formula for certain cases, but I don't know it.
 
The http://mesa.sourceforge.net/assets/movies/1M_pre_ms_to_wd.mov is a great resource for looking at how stars of different masses and compositions evolve. http://mesa.sourceforge.net/assets/movies/1M_pre_ms_to_wd.mov has a movie of the evolution of a star like the sun. The radius changes a lot over time, which probably explains the different numbers you are seeing.

According to that MESA movie, most of the the red giant phase has a log_R = 2.24, which is about 175 times the current solar radius.
 
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phyzguy said:
The http://mesa.sourceforge.net/assets/movies/1M_pre_ms_to_wd.mov is a great resource for looking at how stars of different masses and compositions evolve. http://mesa.sourceforge.net/assets/movies/1M_pre_ms_to_wd.mov has a movie of the evolution of a star like the sun. The radius changes a lot over time, which probably explains the different numbers you are seeing.

According to that MESA movie, most of the the red giant phase has a log_R = 2.24, which is about 175 times the current solar radius.

This is just what I was looking for, thank you!
 

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