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I've read in some texts that we can't directly observe a black hole in space because not even light can escape from its gravity. Some of the indirect observational methods mentioned are, gravitational lensing and gamma ray outbursts created by swirling matter into the event horizon. My question is, if a black hole is surrounded by glowing matter, it should appear just like a star (except with a different spectral signature), and it should be fairly easy to detect. I don't understand the statement: "If you find an object orbiting around nothing, there's likely to be a black hole at the center". Though a black hole is invisible, the enveloping matter should reveal its identity... What am I missing here? Please answer!

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    $\begingroup$ possible duplicate of Can astronomers directly detect black holes? $\endgroup$ – Gowtham Mar 5 '15 at 9:59
  • $\begingroup$ Thanks for your time, but that doesn't really answer my question. What I meant that isn't it possible to observe a black hole from visible light spectrum? Why would it appear "black"? The surrounding matter would make it glow like an ordinary star, isn't it? $\endgroup$ – Vinit Shandilya Mar 5 '15 at 10:04
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    $\begingroup$ The glow will be also falling in, photons are caught by the black hole too. $\endgroup$ – anna v Mar 5 '15 at 10:05
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    $\begingroup$ Only if the photons dive beyond horizon. What about those who escape from it? Just like Hawking radiation. $\endgroup$ – Vinit Shandilya Mar 5 '15 at 10:08
  • $\begingroup$ Related: physics.stackexchange.com/questions/137837 and physics.stackexchange.com/questions/149976 $\endgroup$ – Kyle Kanos Mar 5 '15 at 13:07
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The reason has to do with time dilation, and specifically, with the resulting red shift.

A black hole forms from a collapsing star, which is of course made of brightly glowing matter. The event horizon forms in the centre and moves outwards while the star-matter falls towards it. Because of gravitational time dilation, the infalling matter never crosses the event horizon from the outside perspective, and thus can technically still be "seen."

However, this time dilation also causes the light the matter emits to be redshifted. Essentially, every photon the matter emits is reduced in frequency due to the time dilation, and the time in between photons also reduces, asymptotically approaching infinity. This means that the black hole very rapidly converges to something that would appear completely black to an outside observer - the matter falling into it can only be seen by someone with the patience to collect many very low frequency photons over many billions of years.

That said, matter that falls into the black hole after it's been formed can certainly glow brightly enough to be detected - very much so in the case of an active galactic nucleus.

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    $\begingroup$ I don't understand this answer at all. It is not the reason that black holes are hard to detect. Indeed many black holes are detected thanks to light emitted by the matter falling into them. That's what a quasar is. $\endgroup$ – Rob Jeffries Mar 5 '15 at 14:12
  • $\begingroup$ @RobJeffries the question mentioned "enveloping matter", which I took to mean the remains of the star that birthed the hole, as described in my answer. Stuff falling into the hole after its formation is another matter - I've added a note to the answer to explain that this can indeed be detected. $\endgroup$ – Nathaniel Mar 5 '15 at 14:23
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This is a curious question. Many black holes are detected and identified due to light emitted from material falling into them.

When black holes accrete matter, conservation of angular momentum would usually lead to the formation of an accretion disk. The release of gravitational potential energy as material falls means that this disk can be hot, and it is radiation from this hot disk that is one of the ways in which black holes are identified.

Examples would include most stellar-sized black hole binary systems - these are among the most powerful X-ray sources in our Galaxy. ((For example: Cygnus X-1).

In terms of super-massive black holes, accretion of matter is responsible for powering active galactic nuclei and quasars.

So yes, if a black hole is surrounded by a significant amount of matter which it is accreting, then it will be observable and may actually be quite bright.

There are examples of "dark" black holes. These would be black holes that are not accreting matter at any significant rate. An example would be the black hole at the center of our Galaxy - whose identity is only revealed by observing the motion of stars around it.

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  • $\begingroup$ Just to clear the picture, the light emitted by the accretion disk formed by swirling matter towards the black hole would also experience gravitational red-shift.. is it correct? $\endgroup$ – Vinit Shandilya Mar 5 '15 at 14:50
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    $\begingroup$ @VinitShandilya Yes, but given the stuff orbits at least a few Schwarzchild radii from the BH, so the redshifts would not be so extreme to prevent it being seen. $\endgroup$ – Rob Jeffries Mar 5 '15 at 15:16
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I think you should be more specific in the type of black hole, because the spinning (and spinning-charged) black holes can make light reach the photonsphere and not be swallowed by the black hole, but enter an orbit around it. Eventually debris that are also in orbit will be heated up and started to glow, and it's the first step for a quasar to be born.

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