# Determining the radius of a star

1. Jul 24, 2010

### travwg33

I'm working on trying to determine a formula that can model the density of the sun.

density:
$$\rho$$ = m/v

the volume of a three dimensional figure is b*h
therefore: v of cylinder = Area of a circle * h (delta r = h)
v=$$\pi$$R2$$\Delta$$r

therefore: $$\rho$$ = ($$\Delta$$m)/($$\pi$$R2$$\Delta$$r)

If this equation is wrong please comment.
Well in order for me to model this I need to figure out the radius of the sun; I know I can easily search for the value on the internet, but how is it actually determined?

Last edited: Jul 24, 2010
2. Jul 25, 2010

Staff Emeritus
This is a solved problem: google Lane-Emden equation for one of the early models.

Why are you modeling cylinders if the sun is a sphere?

We know the distance to the sun and the sun's angular size. Geometry tells us the rest.

3. Jul 26, 2010

### travwg33

You are right, I have no idea why I said cylinder, but thanks for the other suggestions.

Just last week I went to an astronomy camp and during one of the lecture, the professor introduced the class to rudimentary stellar modeling, namely modelling our own sun using excel. During the lesson we derived the equation for hydrostatic equilibrium. During the prosess we made an educated guess as to the equation for density in order to save time which we just modeled as a simple line with negative slope.
After doing so we modeled various parts of the star. This sparked my interest in stellar modelling. Are there any decent websites that can help me learn more about this?

4. Jul 27, 2010

### Chronos

Calculating the volume of a distant star is not terribly difficult if you know the distance [not difficult]. The spectrum gives you the average surface brightness and its magnitude tells you how much surface is required. Once you have the mass [a bit more difficult], problem solved. There are more than enough binary systems to give us good mass estimates.

5. Jul 27, 2010

### FawkesCa

i think what Travwg33 was asking, and what everyone seems to be missing, is his equation correct. he was deviving his thoughts, not just asking FOR the answer. i think he wants to learn how to devive equations on his own, a noble persuit. if that is your question... i have no frakking clue. if the others answered your question however, feel free to tell me to shut it. i probably should anyway. =3