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Evidence of Black Holes - How do we know that black holes exist?

Gravitational Lensing by a Black Hole

Since even light can’t escape a black hole, we can’t see them. So how do scientists say that they exist? Here are a few links that describe the evidence of black holes: -

1. “…Though we cannot ‘see’ a black hole itself (since not even light can escape the hole’s gravitational field), we may see the hole’s effects on nearby matter. For example, if gas from a nearby star were sucked towards the black hole, the intense gravitational energy would heat the gas to millions of degrees. The resulting X-ray emissions could point to the presence of the black hole. Or, if a massive black hole were surrounded by large amounts of orbiting material — gas, dust, even stars — their rapid motion close to the hole could be observable via shifts in the energy of the radiation they emit. Evidence along these lines is mounting, suggesting that black holes may not be that rare in the universe…”

http://www.eso.org/public/outreach/eduoff/cas/cas2002/cas-projects/austria_cygnus_1/

2. “…While the black hole itself is invisible to the eye, the team proved its existence by tracking the motions of 28 stars circling around it. Just as swirling leaves caught in a gust of wind can provide clues about air currents, so the stars’ movements reveal information about forces at work at the galactic center…”

http://www.telegraph.co.uk/science/science-news/3690822/Proof-that-Albert-Einsteins-black-holes-do-exist-claim-scientists.html

3. “…No object except for a black hole can have an event horizon, so evidence for its (event horizon’s) existence offers resounding proof of black holes in space…”

http://science.nasa.gov/science-news/science-at-nasa/2001/ast12jan_1/

4. “Evidence for a Supermassive Black Hole in M87…”

http://csep10.phys.utk.edu/astr162/lect/active/smblack.html

5. “…Scientists verified the existence of the black holes by studying the speed of the clouds of gas orbiting those regions…”

http://hubblesite.org/reference_desk/faq/answer.php.id=64

6. “…So far black holes appear an unfalsifiable conclusion of the General Theory of Relativity: their properties are such that no radiation comes out of them so they cannot be detected from a distance… Doesn’t this contradict the basic requirement that a scientific theory be falsifiable? Well, no, General Theory of Relativity even in this one of its most extreme predictions is falsifiable. The saving circumstance is provided by the matter surrounding the black hole. All such stuff is continuously being dragged into the hole and devoured, but in the process it gets extremely hot and radiates light, ultraviolet radiation and X rays…”

http://physics.ucr.edu/~wudka/Physics7/Notes_www/node90.html

7. “NASA’s Nuclear Spectroscopic Telescope Array, or NuSTAR, set its X-ray eyes on a spiral galaxy and caught the brilliant glow of two black holes lurking inside…”

NuSTAR discovers 2 Black Holes in IC 342

http://www.nustar.caltech.edu/news/99/62/NASA-s-NuSTAR-Catches-Black-Holes-in-Galaxy-Web/d,news-detail

8. How Do We Know Black Holes Exist?

http://www.youtube.com/watch?v=3sBXdiN0dy4