Star at black hole event horizon

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  • #1
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Curious if the time dilation at the edge of an event horizon would have the apparent effect of prolonging the life of the star to an outside observer - so for example a blue hyper giant at the edge of an event horizon with an expected main sequence time of, say, 500 million years, would remain visible to an outside observer for billions of years due to time dilation. If so, how does the apparent contradiction between the energy of radiation emitted by the star over its internal 500M main sequence clock translate to the radiation energy measured by an outside observer over billions of years in their reference frame?
 

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  • #2
Ibix
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Radiation from the star would be extremely redshifted from our perspective. It will rapidly dim and redshift into invisibility as it approaches the horizon, meaning that there's no problem with energy conservation.
 
  • #3
BWV
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Radiation from the star would be extremely redshifted from our perspective. It will rapidly dim and redshift into invisibility as it approaches the horizon, meaning that there's no problem with energy conservation.
Thanks, in general you would have to know the black hole was there using some other method to know if the light was redshifted from a higher frequency or not?
 
  • #4
Ibix
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A redshifted blackbody spectrum is a lower temperature blackbody spectrum, but the redshift of all the spectral lines would clue you in.
 
  • #5
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A star falling head-on into a supermassive black hole is a process with a timescale of several hours. You'll see the star disappear within days at most, redshifting to oblivion quickly in that time.

A more realistic scenario is a close flyby that rips apart the star and produces a lot of hot gas that forms an accretion disk around the black hole afterwards. Material that leaves the inner edge of the accretion disk disappears into the black hole quickly.
 
  • #6
stefan r
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If you have time dilation the number of photons you detect per second drop proportionately.

A star falling head-on into a supermassive black hole is a process with a timescale of several hours. ...

Timescale depends on the size of the black hole.
 
  • #7
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I wanted the black hole to be larger than the star. That doesn't leave that much room for the size and therefore the timescale. If the black hole is much smaller than the star the interaction will be more complicated.
 
  • #8
stefan r
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I wanted the black hole to be larger than the star. That doesn't leave that much room for the size and therefore the timescale. If the black hole is much smaller than the star the interaction will be more complicated.
Wikipedia lists the black hole TON 618 as 1,300 astronomical unit Schwarzschild radius. That is 15,000 times the size of Sagittarius A*. Plenty of room for size/time differences.

We can also switch the question so that it asks what happens to a white dwarf instead of a hypergiant. The surface gravity is higher than the gravity at the event horizon so it could drop in as an intact object and/or orbit a few times.
 
  • #9
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3 km = 10 microseconds per solar mass. Pick your favorite size, get the typical timescale. Doesn't really matter in comparison to the lifetime of a star OP asked about.

18 hours for TON 618, 2 hours for Messier 87 (the one with the image).
 

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