New answers tagged black-hole
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Let's try a crude back of the envelope calculation to get orders of magntitude:
The tidal force near the event horizon is of the order of $G M/r^3$ which is something like $10^{18} g/m$. So a probe with a mass of 1 gram and diameter of 1cm would experience about $10^{12} N$ of force trying to "spaghettify" it by accelerating ends of it at about $10^{16} g$ ...
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Black holes are black. They are only observed directly by telescopes if they are accreting matter. Any radiation observed comes from the matter surrounding the black hole. Generally speaking, the smaller the black hole, the hotter the accreted material becomes. For something of planetary mass, one might expect X-rays and Gamma rays from accreting material.
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Yes, when during the 1980s, scientist observe that there was a massive star that explode into a supernova, releasing large amounts of matter into space. Then they use signals to detect remnants from the explosions but couldn't find any.
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“We think that ‘The Cow’ is the formation of an accreting black hole or neutron star,” said Northwestern’s Raffaella Margutti, who led the research. “We know from theory that black holes and neutron stars form when a star dies, but we’ve never seen them right after they are born. Never.” ... Another team of astronomers, led by Paul Kuin from, an ...
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M87 was actually the easiest black hole for the Event Horizon Telescope (EHT) to attempt, and was thus sensibly its first target. For the EHT to work you need 1) an active BH accreting matter such that it is a strong radio source, and 2) that it be close and massive enough to be angularly large. While M87 is some 20x further away than, say M31 or M33, it ...
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The mass (equivalently energy) is radiated away as Hawking radiation.
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From The Unknown Universe: What We Don't Know About Time and Space in Ten Chapters by Stuart Clark:
By 1998, Hawking was ready to concede the bet. He thought that although the specific evidence for Cygnus X-1 had not changed, there was now so much evidence for other black holes dotted around the galaxy an the wider Universe that there was no more room ...
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M33 does not appear to contain a supermassive black hole: in fact there's no evidence that it contains a central black hole at all. The upper limit on the mass of a central black hole based on the dynamics of the core region is a few thousand solar masses.
Merritt et al. (2001) "No Supermassive Black Hole in M33?" derive an upper limit of 3000 solar masses ...
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Applying a numerical density to a black hole isn't possible. The material inside the event horizon will fall to a "singularity" (or some other ultrahigh density state that we currently have no adequate theory to describe) on a relatively short timescale.
What you can do, is exactly what you have done, which is divide the gravitational mass of a black hole ...
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Since this question was asked, we have observed the shadow of a black hole and its accretion disk, and it behaves the way our models predicted.
As such, it would seem that black holes and their event horizons do exist.
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The amplitude of a detected gravitational wave depends on a number of factors - the luminosity of the source (which in turn depends on the masses and orbital period of the merging binary system), the orientation of the binary system with respect to the line of sight (since gravitational waves are emitted highly anisotropically, the inclination of the binary ...
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From the Wikipedia black hole article
This is a picture of a real black hole.
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As explained in this answer, dimensional analysis indicates that this radius depends on $GM$ and on a velocity squared. However, the fact that the two quantities are exactly equal, rather than there being a constant factor between the two is a pure coincidence.
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