Star at black hole event horizon

In summary: Plenty of time for a star to drop in and orbit a few times.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. Pick your favorite size, get the typical timescale. Doesn't really matter in comparison to the lifetime of a star OP asked about.
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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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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.
 
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Ibix said:
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?
 
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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.
 
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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.
 
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If you have time dilation the number of photons you detect per second drop proportionately.

mfb said:
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.
 
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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.
 
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mfb said:
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.
 
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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).
 

Related to Star at black hole event horizon

1. What is a star at the black hole event horizon?

A star at the black hole event horizon refers to a star that is located at the edge of a black hole, where the gravitational pull is so strong that even light cannot escape. This point is known as the event horizon and marks the boundary of the black hole.

2. How does a star end up at the black hole event horizon?

A star can end up at the black hole event horizon if it gets too close to a black hole and is pulled in by its strong gravitational force. This can happen if the star is in a binary system with a black hole or if it is on a collision course with a black hole.

3. What happens to a star at the black hole event horizon?

Once a star reaches the black hole event horizon, it will be stretched and torn apart by the intense gravitational forces. This process is known as spaghettification. The star's material will then be pulled into the black hole, adding to its mass.

4. Can a star survive at the black hole event horizon?

No, a star cannot survive at the black hole event horizon. The gravitational forces at this point are too strong and will ultimately lead to the star's destruction. However, some of the star's material may be ejected from the black hole in the form of jets.

5. Can we observe a star at the black hole event horizon?

It is not possible to directly observe a star at the black hole event horizon because light cannot escape from this point. However, we can indirectly observe the effects of a star's presence at the event horizon, such as changes in the surrounding environment or gravitational waves.

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