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If Black Holes Do Not Emit Light, How Are They Detected?
Black holes do not emit light; in fact they completely absorb and trap light that gets close enough to them. So, if this is the case how can we possibly observe or detect them? It turns out that while black holes do not emit light or any matter, the effects of black holes are detectable. As matter is pulled into the black hole, it accelerates and heats up. As the temperature of the matter increases, atoms are ionized. Once the atoms reach temperatures of a few million Kelvin, x-rays are emitted . These x-rays can be detected and observed by scientists here on Earth. While there are other possible x-ray sources in the universe other than black holes, black holes have fluctuating x-ray emission intensity since matter is not pulled into the black hole at a uniform, constant rate
Examples of Black Hole Detection:
Since white dwarf stars and neutron stars have masses of less than this, Cygnus X-1 must be a black hole ..
Additional properties of Cygnus X-1 were also found using observable properties of the binary x-ray system. Based on the rate that the x-ray emission is flickering from the black hole, the size of the black hole can be estimated. The x-ray emission was measured to flicker in the region of hundredths of a second. Therefore since “An object cannot flicker faster than the time required for light to travel across the object” it can be calculated that since light travels 3,000 kilometers in one hundredth of a second, the size of the black hole Cygnus X-1 is only about twenty-five percent of the earth’s diameter .
Detection of: 3C273
The visible light image of 3C273. The jet of light in the lower right
hand corner is a stream of “electrons propelled outwards by the
energy generated near the black hole” .
Another example of the discovery of a black hole using x-ray and radio wave emission is a supermassive black hole in the quasar 3C273. This black hole is one billion times more massive than the Earth’s Sun. The jet created by the black hole is more visible in the radio and x-ray wavelengths as seen in the images below. Through the use of radio and x-ray imaging, the mysterious properties of this quasar were explained through the discovery of this black hole.
X-ray image of 2C273 and supermassive black hole
“The region just outside the black hole event horizon shines
very bright in X-rays (colored yellow). The jet is seen as well.” 
Image of Jet Steam from Black Hole
“particles get propelled in a jet from near the
black hole (at white dot). The jet shines in visible light,
radio waves and X-rays.”
Despite the fact that black holes do not emit light, this does not mean that they cannot be observed. Due to the x-ray and radio wave emissions of matter being accelerated towards the black hole, black holes can be observed and directly detected. In theory it would also be possible to detect a black hole by the way it bends lights. If light passes close to a black hole but not close enough to get pulled in, it will bend and make the star field appear distorted . Multiple images and duplicates would also appear . However, this phenomenon is difficult and very rare to observe. Below is what gravitational lensing for a black hole in theory should look like:
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