How We Detect Black Holes

Although we cannot see black holes, we can detect or guess the presence of one by measuring its effects on objects around it. The following effects may be used:

• Mass estimates from objects orbiting a black hole or spiraling into the core
• Gravitational lens effects
• Emitted radiation

Mass
Many black holes have objects around them, and by looking at the behavior of the objects you can detect the presence of a black hole. You then use measurements of the movement of objects around a suspected black hole to calculate the black hole's mass.

What you look for is a star or a disk of gas that is behaving as though there were a large mass nearby. For example, if a visible star or disk of gas has a "wobbling" motion or spinning AND there is not a visible reason for this motion AND the invisible reason has an effect that appears to be caused by an object with a mass greater than three solar masses (too big to be a neutron star), then it is possible that a black hole is causing the motion. You then estimate the mass of the black hole by looking at the effect it has on the visible object.

For example, in the core of galaxy NGC 4261, there is a brown, spiral-shaped disk that is rotating. The disk is about the size of our solar system, but weighs 1.2 billion times as much as the sun. Such a huge mass for a disk might indicate that a black hole is present within the disk.

Photo courtesy NASA/Space Telescope Science Institute Credit: L. Ferrarese (Johns Hopkins University) and NASA Hubble Space Telescope image of the core of galaxy NGC 4261

Gravity Lens
Einstein's General Theory of Relativity predicted that gravity could bend space. This was later confirmed during a solar eclipse when a star's position was measured before, during and after the eclipse. The star's position shifted because the light from the star was bent by the sun's gravity. Therefore, an object with immense gravity (like a galaxy or black hole) between the Earth and a distant object could bend the light from the distant object into a focus, much like a lens can. This effect can be seen in the image below.

Photo courtesy NASA/Space Telescope Science Institute Credit: NASA and Dave Bennett (University of Notre Dame) These images show the brightening of MACHO-96-BL5 from ground-based telescopes (left) and the Hubble Space Telescope (right).

In the above image, the brightening of MACHO-96-BL5 happened when a gravitational lens passed between it and the Earth. When the Hubble Space Telescope looked at the object, it saw two images of the object close together, which indicated a gravitational lens effect. The intervening object was unseen. Therefore, it was concluded that a black hole had passed between Earth and the object.

Emitted Radiation
When material falls into a black hole from a companion star, it gets heated to millions of degrees Kelvin and accelerated. The superheated materials emit X-rays, which can be detected by X-ray telescopes such as the orbiting Chandra X-ray Observatory.

Photo courtesy CXC/S.Lee Schematic of a black hole in a binary system, showing the accretion disk around the black hole and emission of X-rays

The star Cygnus X-1 is a strong X-ray source and is considered to be a good candidate for a black hole. As pictured above, stellar winds from the companion star, HDE 226868, blow material onto the accretion disk surrounding the black hole. As this material falls into the black hole, it emits X-rays, as seen in this image:

Photo courtesy NASA/CXC X-ray image of Cygnus X-1 taken from orbiting Chandra X-ray Observatory

In addition to X-rays, black holes can also eject materials at high speeds to form jets. Many galaxies have been observed with such jets. Currently, it is thought that these galaxies have supermassive black holes (billions of solar masses) at their centers that produce the jets as well as strong radio emissions. One such example is the galaxy M87 as shown below:

Photo courtesy NASA Schematic diagram of active galactic nucleus with a supermassive black hole at its center

Photo courtesy NASA/Space Telescope Science Institute Credit: NRAO, NSF, Associate Universities, Inc., NASA, and John Biretta (STScI/Johns Hopkins University) The images on the left and bottom are ground-based radiotelescope images of the heart of galaxy M87. The image on the right is a visible image from the Hubble Space Telescope. Note the jet of material coming from M87.

It is important to remember that black holes are not cosmic vacuum cleaners -- they will not consume everything. So although we cannot see black holes, there is indirect evidence that they exist. They have been associated with time travel and worm holes and remain fascinating objects in the universe.

For more information on black holes and other space-related phenomena, check out the links on the next page!