# [mesa-users] hydrostatic equilibrium calculation with mesa data

Ehsan Moravveji e.moravveji at gmail.com
Wed Oct 5 06:19:44 EDT 2016

```Dear Noam,

Regarding how the hydrostatic equlibrium is satisfied in your mesa model, you can follow the following page (exercises for master students at KU Leuven), and perhaps that walks you through the necessary steps to compute dP/dr.
http://www.ster.kuleuven.be/~ehsan/sse/exercise05/exercise05.html <http://www.ster.kuleuven.be/~ehsan/sse/exercise05/exercise05.html>

Regarding the meshing in MESA, please dive into the first instrument paper (Paxton et al. 2011). You will see quite some light there.

Best,
Ehsan.

> On 05 Oct 2016, at 09:43, Noam Chai <noamchai at gmail.com> wrote:
>
> Hi,
>
> I'm having a trouble calculating the hydrostatic equilibrium for a simple star with the mass of the sun. I use matlab 2014a to calculate the gradient of dpdr, by using two methods:
> 1. the inner function 'gradient'.
> 2. with a for loop - (p(i+1)-p(i-1))/(r(i+1)-r(i-1))
>
> The plot from both methods have lots of significant spikes that looks like noise in it. Did anyone struggled with this problem? why does it happend?
>
> Another question:
> How does mesa decides the intervals of the radius? it seems that the intervals are not constant. How can I change the length of the vector r and how can I make it an evenly spaced vector?
>
> Thank you all,
>
> Noam
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