Question regarding the ability to detect a star that is moving away from you.

Simplex0
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Topic 193119

If a star moves towards an observer with the speed of light the observer will not be able to se the star until it actually reaches him but what happens if it moves away from the observer?

I understand that a star that moves away from a observer will have a red shifted spectra, the faster the relative speed is the greater the red shift will be.
My question is... Is there a relative speed above where the star can´t be detected at all?

Nereid
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Question regarding the ability to detect a star that is moving a

Quote:

If a star moves towards an observer with the speed of light the observer will not be able to se the star until it actually reaches him but what happens if it moves away from the observer?

I understand that a star that moves away from a observer will have a red shifted spectra, the faster the relative speed is the greater the red shift will be.
My question is... Is there a relative speed above where the star can´t be detected at all?


Answering only the very last part ... I think, in the real universe, there is ...

As even the emptiest of parts of the universe does still contain a thin soup of free electrons (and ions), there is a plasma frequency associated with it. We will see photons from the fast-moving star (which is an emitter of photons) redshifted; at a sufficiently high redshift, even the star's most energetic photons will be absorbed by the inter-stellar (or inter-galactic) medium, as they will be below the plasma frequency.

Odysseus
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RE: If a star moves towards

Quote:
If a star moves towards an observer with the speed of light the observer will not be able to se the star until it actually reaches him but what happens if it moves away from the observer?


Well, it can’t be approaching quite that fast! Taking a speed of 0.99c for example, the star would indeed be observable, but its light would be blue-shifted: most of it would reach us as ultraviolet rays with about one-tenth the wavelength they had WRT the star. As the speed approaches closer to c, the radiation will be further blue-shifted, becoming X-rays or even ‘harder’.

Quote:
I understand that a star that moves away from a observer will have a red shifted spectra, the faster the relative speed is the greater the red shift will be.
My question is... Is there a relative speed above where the star can´t be detected at all?


It depends on the frequency-response of the detector. Receding at 0.99c, a typical star could only be picked up in infrared, its spectrum peaking somewhere in the area of 5– to 10-micron wavelengths and dropping off well below the frequencies that we can see (and to which the atmosphere is transparent). The Spitzer Space Telescope could see it just fine. Were the star’s speed increased to 0.9999999c a radiotelescope would be needed in order to detect it. At speeds where the star’s apparent blackbody temperature approaches that of the CMBR, I guess it would get lost in the ‘noise’.

Simplex0
Simplex0
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Thank you for your

Thank you for your answers.

But Odysseus, to mee is seams more like "it can’t be measured to approaching quite that fast!"
Which is not the same as how fast it actually moves.

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