# B Question about Hertzsprung-Russell-diagram

1. Mar 27, 2017

### Meerio

Why can you read out the radii of the stars by knowing their temperature and luminosity?
Can't make sense out of it.

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2. Mar 27, 2017

### phyzguy

The Stefan-Boltzmann equation tells you the amount of radiation emitted from a unit area of the star as a function of temperature. The total luminosity is then given by just summing this over the surface area of the star, so:

$$L = \sigma T^4 * 4 \pi R^2$$

So if you know L and T, you can calculate R.

3. Mar 27, 2017

### Meerio

wow that was easy thanks!