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Re: Question about calculation of G

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On 31/12/2016 9:12 AM, Peter Riedt wrote:
> Question about calculation of G
>
> We can use the equation G = gstar*rstar^2/Mstar to calculate the value of G in respect of the sun and the three Alpha Centauri stars using elements that are officially accepted. A problem is that gstar and Mstar are calculated with G and the equation can be called circular.
>
> However, in the case of the sun the answer confirms the Codata value of G whereas in the case of the other three stars the answers do not. Why is it so?
>
> 	log g	   gstar	   rstar	 Mstar      calc G
> Sun		  273.97	696,000,000  1.98855E+30  6.674E-11
> ACa	4.30	  199.53	853,992,000  2.18741E+30  6.652E-11
> ACb	4.37	  234.42	602,040,000  1.80361E+30  4.711E-11
> ACc	5.20	1,584.89	 98,136,000  2.44592E+29  6.240E-11
>
> log g = surface acceleration in log
> gstar = surface acceleration in m/sec2
> rstar = radius of star in m
> Mstar = mass of the star
>

The surface gravity cannot be measured directly, since we can't go 
there. It has to be inferred from the mass of the star, its radius, and 
the value of G, taken to be that measured in the solar system, since we 
cannot, as yet, measure it anywhere else.

The radius of a star is not a well-defined quantity (since it doesn't 
have a well-defined surface), so if you get the radius from one source, 
the surface gravity from another, and the mass from a third source, 
you're likely to find inconsistencies.

If you found all the quantities from one source, and they imply a 
different value for G, then the source has simply done its math wrong.

It has no more significance than that, and, in particular, says nothing 
about the value of G.

Sylvia.

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Question about calculation of G Peter Riedt <riedt1@yahoo.co.uk> - 2016-12-30 14:12 -0800
  Re: Question about calculation of G Dirk Van de moortel <dirkvandemoortel@hotspam.not> - 2016-12-30 23:20 +0100
  Re: Question about calculation of G Peter Riedt <riedt1@yahoo.co.uk> - 2016-12-31 00:05 -0800
  Re: Question about calculation of G Sylvia Else <sylvia@not.at.this.address> - 2016-12-31 19:17 +1100
    Re: Question about calculation of G Peter Riedt <riedt1@yahoo.co.uk> - 2016-12-31 01:20 -0800

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