Detection of Black Holes

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If Black Holes Do Not Emit Light, How Are They Detected?


Artist's Rendition of a Black Hole's Effect on Light


The Detection of Black Holes Through Radio and X-Ray Wave Lengths.  Here a black hole is seen in x-ray wavelength.  The black hole is near the center and jet of x-ray emission is also seen


A Binary System of a Star and A Black Hole


Images From [5], [4],and [2] accordingly (also cited below)

  • Since black holes do not emit light, it is impossible to "see" a black hole


  • Still however, several methods of detecting black holes exist


  • In theory, since black holes can bend light, it would be expected that this phenomenon would be observable.  However, scanning the sky and looking for effects of gravitational lensing due to black holes is a cumbersome task


  • Fortunately, black holes can be detected in other wavelengths, such as x-ray and radio-wavelengths


  • When matter is accelerated into a black hole it collides with other matter and heats up as it is pulled by gravity
    • When it gets hot enough, x-rays are emitted


  • It is also particularly useful if a black hole has a nearby companion object, such as a star.


  • This allows scientists to gather much more data on the size and mass of the black hole as the two objects orbit each other.


  • For more details about black hole detection and the specific details of the detection of Cygnus X-1 read on below



For More Details Read Below

          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 [2].  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 [2].

Examples of Black Hole Detection:

Cygnus X-1

“The optical companion of
the black hole candidate Cygnus X-1” [2]

 Image From: [2]


One effective method of detecting black holes and determining their properties is to observe binary x-ray sources [2].  Binary systems first of all provide ample matter to supply the black hole’s x-ray emissions as well as allow for calculations to find the mass of the objects [2].  Once the mass of the binary x-ray objects are known, the type of stellar objects being observed can be determined [2].  Cygnus X-1 is an excellent example of black hole detection through the observation of a binary x-ray source.  Two x-ray sources located near each other, a B0 supergiant star HDE 226868 and Cygnus X-1 were observed and through this observation of the binary x-ray pair, the properties of Cygnus X-1 were found.  Scientist determined that the supergiant star had a mass of 30 times the Sun’s mass and temperatures of 31,000 Kelvin[2].  Using spectroscopy data, it was also observed that “the spectral lines of HDE 22686 oscillate with a period of 5.6 days” [2].  Therefore, the binary x-ray companion of HDE 22686 must have a mass of at least 7 solar mass in order to have the gravitational force necessary to create the shifting of the star’s spectral lines [2].  Since white dwarf stars and neutron stars have masses of less than this, Cygnus X-1 must be a black hole [2]. 

Image from [2] 


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 [2].

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” [4]

Image From: [4]

            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.” [4]

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.”[4]

          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 [5].  Multiple images and duplicates would also appear [5].  However, this phenomenon is difficult and very rare to observe.  Below is what gravitational lensing for a black hole in theory should look like:


Image From: [5]







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