XPost: sci.physics.relativity   
   From: relativity@paulba.no   
      
   Den 13.09.2025 09:05, skrev Thomas Heger:   
   > Am Freitag000012, 12.09.2025 um 12:54 schrieb Paul B. Andersen:   
   >>   
   >> Please address the following:   
   >> ===============================   
      
   This is the scenario I ask you to consider:   
      
   >> Consider a star 10000 light years away.   
   >> The spectrum of the star will tell us the following properties   
   >> of the star 10000 years ago:   
   >> its mass, temperature, if it was a new born star, a main sequence star,   
   >> or a star approaching the end of its life-   
   >> If it is a main sequence star with spectral class A0V, its lifespan   
   >> will be ~ 100 million years.   
   >> If it is a main sequence star with spectral class G2V, like the sun,   
   >> its lifespan will be ~ 10 billion years.   
   >> If it is a main sequence star with spectral class K9V, its lifespan   
   >> will be ~ 70 billion years.   
   >>   
   >> The point is that if we see that the star was a main sequence star   
   >> 10000 years ago, we can be pretty sure that it still is   
   >> a main sequence star now. The radial speed of the star will be   
   >> known by its Doppler shift, and several measurements of its position   
   >> will reveal its proper motion. That weans that we will know where   
   >> the star is now.   
   >>   
   >> We know that the Sun was a main sequence star 10000 years ago.   
   >>   
   >> Thomas Heger wrote:   
   >> | We see stars in 10000 light years distance as they were and   
   >> | where they had been 10000 years ago, while we see our Sun as   
   >> | the Sun has been 8 minutes ago.   
   >> | That is a HUGE difference.   
   >> |   
   >> | The picture we see is actually 'layered in time', while   
   >> | the universe isn't.   
   >> |   
   >> | Therefore we see things, which are not real and which do   
   >> | not belong to the same time.   
   >>   
   >> We see that the star was real 10000 years ago, and if it was   
   >> a main sequence star then, we know that it still is a real   
   >> main sequence star now.   
   >>   
   >> It is indeed a weird idea that a star that was real 10000 years   
   >> ago is not real now.   
   >>   
   >> What do you think the stars we see in the telescope are?   
   >> Mirages?   
   >>   
   >> We know that the star and the Sun were 10000 light years   
   >> from Each other 10000 years ago, and we know that the star   
   >> and the Sun still are ~1000   
      
   >   
   > Ten-thousand years is actually nothing in cosmology, which deals with   
   > millions of light-years.   
      
   This is not cosmology, it is astronomy.   
      
   I will snip everything which does not address my scenario.   
      
   >> We know that the star and the Sun were 10000 light years   
   >> from Each other 10000 years ago, and we know that the star   
   >> and the Sun still are ~10000 light years away from each other.   
   >> And for million years the two stars have transferred energy   
   >> (light) to each other, and will keep doing so for million   
   >> of years.   
      
   > The main problem isn't that, but that distance is very hard to estimate   
   > in cosmology.   
      
   I ignore everything which does not address my scenario.   
      
   >> The stars are at any time living side by side ~10000 light years   
   >> from each other.   
      
   > That is't necessarily the case, just because you see two stars that way.   
      
   Yes, that is indeed necessarily the case.   
      
   We have one main sequence star 10000 ly from the Sun.   
   We know a main sequence star will exist 10000 years   
   after it was observed, and we know that the Sun existed   
   10000 years ago.   
   It doesn't matter if the star has moved a little relative   
   to Sun.   
   Both stars existed 10000 years ago, and they still exist now.   
      
   >   
   >> That we happen to live close to one of them doesn't make them   
   >> "belong to separate times."   
   >>   
   >> They are both living at the same time.   
      
   > 'living' is not the right phrase for pictures of stars.   
      
   It is a common expression in astronomy-   
   Stars are born, live, and die.   
      
   >   
   > I meant:   
   >   
   > if you have a depth in a picture from the night sky of several thousand   
   > light years, for instance, you need to consider, that background events   
   > belong to a different time than foreground events.   
   Please address my scenario!   
      
   Our two main sequence stars both existed in the same universe   
   10000 years ago, and they still exist in the same universe.   
   During the 10000 years the Sun has received photons from the star,   
   and the star has received photons from from the Sun.   
      
   If there is a planet with intelligent life around the star,   
   they can see the Sun in their telescope, like we can see the star   
   on our telescope.   
      
   Both stars are equally real, and both live at the same time.   
      
   Do you understand this?   
      
   Is it really possible to fail to understand this?   
      
   >   
   > The discrepancy is actually huge and can easily be millions of years.   
      
   Which "discrepancy" can easily be millions of years?   
      
   > Therefore pictures of the backgrond stars and the forground stars do not   
   > belong together, but should be corrected because of different delays.   
      
   In the star catalogues the "position" is the Right Ascension   
   (aka longitude) and Declination (aka latitude) in the equatorial   
   coordinate system, where the star is observed at a particular   
   time like EPOCH J2000. The RA and Dec are as seen from the Sun.   
      
   Astronomers know that to see the star in their telescope, they have   
   to correct the RA and Dec with the proper motion, stellar aberration   
   and parallax.   
      
   Astronomers know that this is not the position of the stars now.   
   And since the spectrum will reveal where in its lifespan a star is,   
   the astronomer will know if the star still exists.   
   For the best known stars, when the proper motion, distance and   
   radial velocity are precisely known it is obviously possible   
   to calculate the position of the star now. I am sure it is done in   
   some special cases, but generally, what would be the point?   
      
   Astronomers must know where to point their telescope!   
      
   What is your problem with this?   
      
   >   
   > Unfortunately we don't know the exact delay and we also don't know,   
   > which stars are actually the background stars and which one belong to   
   > the foreground.   
   >   
   > We only assume, that some theories are valid, which enable us to   
   > estimate the distances.   
      
   Parallax is simple geometry, and is no uncertain "assumption".   
   Hipparcos has measured the distance to more that two million stars   
   in the Milky Way.   
      
   There are other methods as well. The best known is probably Cepheids.   
   The distance to a galaxy can be determined if a Cepheid is observed   
   in it.   
      
   Look it up.   
      
   >   
   > But those theories are most likely wrong.TH   
   >   
      
   Can you please name the methods for determining the distance   
   which are most likely wrong?   
   --   
   Paul   
      
   https://paulba.no/   
      
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