# Astro- modelling a white dwarf via polytrope

1. Sep 6, 2005

### laam

have solved the lane - emden eqn numerically for scaled radius, and the derivative of scaled pressure with respect of scaled radius, are given the eqn of state, n, and mass, how would i go about working out the central density and the radius?
thanks

2. Sep 8, 2005

### pattiecake

Huh? Sorry, but can you restate that a little more clearly? Maybe include the equations you worked with too?

3. Dec 9, 2008

### Widdekind

The Lane-Emden Equation only generates finite "stars" for low Polytropic Indices (0 to 4). For those indices, the maximum Scaled Radius is some constant (eg., Pi or sqrt(6)). Thus, the maximum "actual radius" is just that constant multiple of the Scale Length:
$$S_{n} = \sqrt{ \frac{(n+1) k_{B}}{4 \pi G m} \frac{T_{c}}{\rho_{c}} }$$​
The central density & temperature are Free Parameters*.
* For derivation, see: https://www.physicsforums.com/showthread.php?t=278009 [post #8]