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This question already has an answer here:

Good morning, I know some similar questions have been asked but I don't find a convincing answer in these. Excuse me if I have missed the answer.

My question is:

I have learned that some black holes are formed by contraction of stard ( the exact processus does not matter here ). The problem is that the time dilatation due to gravitation is increasing with the contraction being infinite when all the matter pass inside the horizon.

But for an external observer it will take an infinite amount of time so that the black holes we see today are nearly perfect black holes but not totally since all the matter have not passed the horizon. Am I wrong here?

Furthermore, these black holes could not be totally black since all the matter of the original stars do not pass the horizon. So it would be possible for them to emit a bit of light no?

But this is not what I read so if someone could explain me, I would be grateful to him ;)

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marked as duplicate by Javier, Qmechanic Dec 9 '18 at 16:03

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    $\begingroup$ Possible duplicate of Can black holes form in a finite amount of time? $\endgroup$ – Javier Dec 9 '18 at 15:53
  • $\begingroup$ Your question does not seem to distinguish between "black hole" as a theoretical object, and "black hole" as an astronomical phenomenon. A black hole is almost completely black (It should, in theory, emit a relatively tiny amount of Hawking radiation), but any actual black hole is likely to be part of a larger astronomical formation in which other parts of the formation (e.g., the black hole's accretion disk) do emit light and other radiations. $\endgroup$ – Solomon Slow Dec 9 '18 at 17:12
  • $\begingroup$ "the time dilatation due to gravitation" - It is the opposite actually. The gravitational attraction is caused by the time dilation, but not vice versa. $\endgroup$ – safesphere Dec 10 '18 at 6:12
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Furthermore, these black holes could not be totally black since all the matter of the original stars do not pass the horizon. So it would be possible for them to emit a bit of light no?

Have a look at this:Visible Light from a Black Hole Spotted by Telescope, a First

For the first time, astronomers have seen dim flickers of visible light from near a black hole, researchers with an international science team said. In fact, the light could be visible to anyone with a moderate-size telescope.

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black hole

Anything falling into black holes cannot escape, not even light, earning black holes their name. However, as disks of gas and dust fall or accrete onto black holes — say, as black holes rip apart nearby stars — friction within these accretion disks can superheat them to 18 million degrees Fahrenheit (10 million degrees Celsius) or more, making them glow extraordinarily brightly.

Scientists discovered accreting black holes in the Milky Way more than 40 years ago. Previous research suggested that the accretion disks of black holes can have dramatic effects on galaxies. For instance, streams of plasma known as relativistic jets that spew out from accreting black holes at near the speed of light can travel across an entire galaxy, potentially shaping its evolution. However, much remains unknown about how accretion works, since matter can behave in very complex ways as it spirals into black holes, said study lead author Mariko Kimura, an astronomer at Kyoto University in Japan, and her colleagues.

So in the process of being consumed by the black hole,electromagnetic radiation is emitted and as seen above also visible light . In the process of falling the accelerations induce electromagnetic emmissions.

For electromagnetic emissions from the horizon Hawking radiation should be mentioned, but as the peak energy is related inversely to the mass, not in the visible.

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  • $\begingroup$ This describes light coming from the accretion disk, not light coming from the black hole itself. $\endgroup$ – probably_someone Dec 9 '18 at 20:42
  • $\begingroup$ @probably_someone of course, but it answers the OP "furthermore, these black holes could not be totally black since all the matter of the original stars do not pass the horizon. So it would be possible for them to emit a bit of light no? " I do read the content of a question after all. $\endgroup$ – anna v Dec 10 '18 at 4:23
  • $\begingroup$ But then you're defining the accretion disk as a part of the black hole, as opposed to a separate object, which is debatable. It doesn't have to share any of the properties of the black hole (it doesn't even have to have the same angular momentum, for instance) - it just happens to be near the event horizon. $\endgroup$ – probably_someone Dec 10 '18 at 12:58

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