# Increase in Radius when a Star becomes a Red Giant

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1. Feb 23, 2016

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?

2. Feb 24, 2016

### TeethWhitener

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.

3. Feb 24, 2016

### phyzguy

The MESA stellar evolution program is a great resource for looking at how stars of different masses and compositions evolve. This link 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.

4. Feb 24, 2016