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Newsgroups: sci.physics.relativity
Subject: Re: Gravitational red-shifting in the biggest star. What are the real colors?
From: nospam@de-ster.demon.nl (J. J. Lodder)
Reply-To: jjlxa31@xs4all.nl (J. J. Lodder)
Date: Sun, 29 Sep 2024 15:57:22 +0200
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Paul.B.Andersen <relativity@paulba.no> wrote:

> Den 28.09.2024 04:34, skrev rhertz:
> > This link illustrates a bit:
> > 
> > https://en.wikipedia.org/wiki/Gravitational_redshift
> > 
> > Using the most common formula from that link: "To first approximation,
> > gravitational redshift is proportional to the difference in
> > gravitational potential divided by the speed of light squared"
> > 
> > ?f/f = ??/? = z = GM/c? (1/R - 1/r) = ?(R)/c? - ?(r)/c?
> 
>   ??/? = GM/Rc?  observed at infinity  (r -> ∞)
> 
> https://www.space.com/41290-biggest-star.html
> > 
> > G = 6.6743E?11 m^3 kg^?1 s^?2
> > M = 5E+09 x 1.989E+30 Kg = 9.945E+39 Kg
> > R = 1,700 x 634,000 Km = 1,077,800,000,000 m
> > 
> > 
> > ?(R)/c? = 6,842,736.59
> 
>  From whence did you get the idiotic idea that the mass
> of UY Scuti was 5 billion solar masses? :-D
> 
> M = 30 solar masses =  5.967e31 kg
> R = 696340e3?1700 m = 57868e6 m
> c = 299792458 m/s
> 
> ??/? = GM/Rc? = 7.65e-7
> 
> Which is less than the red shift from the Sun.
> 
> > 
> > In comparison, ?(RSun)/c? = 0.000002327
> 
> M = 1.989E+30 kg
> R = 696340e3 m
> 
> ??/? = GM/Rc? = 2.12e-6
> 
> 
> 
> > 
> > WHAT IS WRONG WITH MY CALCULATIONS, BASED ON THE WIKI LINK?
> 
> Now you know.

He is looking for trouble where none exists.
Even for neutron stars you get a nice finite gravitational redshift,
which is 'easily' observable. For example:
<https://pubmed.ncbi.nlm.nih.gov/12422210/>

They find a redshift value of z = 0.35 ,
which is consistent with standard neutron star models,
(but not with some exotic ones)

Jan

(such a large gravitational redshift dwarfs any possible Doppler shift)