M. Colleen Gino

Black holes are one of the most intriguing and mysterious of all astrophysical phenomena. While astrophysical theory has long supported the existence of black holes, it has been hard to fathom an object that is so incredibly dense that nothing, not even light itself, can escape its grasp. How does one go about locating an object that can’t be directly observed? Scientists must use methods other than direct observation to locate and study black holes. This can be accomplished by observing the effects that a black hole has on its surroundings, particularly when a black hole is part of a binary star sysem.

When a star with an initial mass of at least ten times the mass of our Sun nears the end of its life, that is, nuclear fusion can no longer continue in its iron core, gravity will cause its outer layers to collapse in upon the core. This instantaneous collapse of the core triggers an explosion called a supernova, and some of the outer material of the star is expelled outward in a violent explosion. Depending upon the amount of mass remaining in the collapsed core, one possible outcome of such an event is a neutron star. In this case, the protons and electrons merge into neutrons, leaving behind an extremely dense and quickly rotating stellar corpse. The average mass of a neutron star ranges from one to three solar masses, and yet the average radius in only about 10km. The gravity of such a densely packed mass is extremely strong.

If the supernova explosion leaves behind a stellar core that is more than three times the mass of the Sun, it is too massive to become a neutron star. Instead, the core continues to collapse completely to a point of infinite density and zero radius called a singularity – better known as a black hole.

According to Einstein’s theory of general relativity, space becomes curved around a massive object, forming a curved "well" in the surface around it. The degree of this curvature and depth of the "well" depends upon the mass of the object. The matter in a black hole is so densely packed, it is considered to have infinite density. This infinitely dense mass curves the space around it to such an extent that an infinitely deep gravity "well" is created, virtually poking a hole in space.

Schematic of an infinitely deep gravity "well" created by a black hole.
Image courtesy Astronomical Society of the Pacific

The gravity of a black hole is so immense that not even the light that the object would emit can escape. This is because the escape velocity, the speed at which an object (in this case photons) must attain to avoid being drawn back by another object’s gravity (in this case the collapsed star), is greater than the speed of light for such a densely packed mass. Since light cannot escape from the star, the star itself seems to disappear. Hence the name, black hole.

No object or radiation of any kind can break free from the gravitational pull of a black hole. If we can’t see light or any other form of radiation emitted from a black hole, how can we detect one?

Supermassive black holes are believed to exist at the center of most galaxies. There is evidence that our own galaxy, the Milky Way, harbors a 2.5 million solar mass black hole at its heart. Tracing these galactic black holes can be difficult, however, due to the density of stars, gasses and dust obscuring our view of the galactic center. In the case of the Milky Way, the presence of gas and dust spread throughout the plane of our galaxy makes the detection process even more difficult. Only within the past few years have our telescopes been able to resolve stars in the center of our galaxy well enough to detect their orbital motions, supplying us with the evidence of our black hole’s existence.

There are less massive black holes than those found in the center of galaxies. Scientists believe that stellar-mass black holes, formed in a supernova event as described above, are common as well. A stellar-mass black hole on its own would be nearly impossible to detect., as our current method of detecting a black hole relies on observing the black hole’s effects on its surroundings. However, stellar-mass black holes are believed to exist in stellar pairs called binary star systems, making the detection of a stellar-mass black hole possible.

Most of the stars in our galaxy exist in groups of two or more stars. We can detect the existence of a black hole in a binary system if we observe a visible star in orbit around an invisible component. Using Kepler’s laws of motion, the mass of the unseen companion can be determined by the visible star’s orbital velocity. If the object is about three times the mass of the Sun or less, it is a neutron star. But if the object is more than three solar masses, physics tells us it must be a black hole.

Star systems whose orbit lies along our line of sight can be detected spectroscopically. A spectroscopic binary system’s nature is revealed by the periodic Doppler shifting of the lines in its spectrum. When the visible star is moving toward us in its orbit, its spectral lines are shifted toward the blue end of the spectrum. When moving away from us, the lines shift toward the red end of the spectrum. The degree of the spectral line shift tells us its orbital speed and the frequency of the change give us its orbital period. With this information we can determine the mass of the unseen member of the system and identify it as a potential black hole candidate.

Another way to detect a black hole in a binary system is by observing x-rays generated around it. Normal stars are not high emitters of x-rays, energetic radiation with wavelengths much shorter than visible light, so some other process must be responsible when x-rays are detected. When a black hole is a member of a binary system, material from the outer atmosphere of the normal star is drawn toward the black hole by its immense gravity. As the gas swirls toward the black hole it forms a rotating disk called an accretion disk, due to the angular momentum of the pair’s orbital motion. The material in the accretion disk is compressed and accelerated to velocities close to the speed of light as it spirals in close to the black hole. Friction causes the disk material to heat up to temperatures of 10 million Kelvin, resulting in radiation in the form of x-rays and gamma-rays being emitted.

Artist conception of a black hole in a binary system.
Image courtesy Tim Kuzniar, Lochness Productions, www.lochness.com

This scenario can be created by a neutron star as well, and in most cases of strong x-ray emission from binary star systems, astronomers suspect neutron stars to be the culprit. As methods of detection have improved over the past decade, however, there is a growing number of cases where astronomers believe a black hole to be responsible for the copious amount of x-ray radiation observed in some binary systems.

Cygnus X-1 is one of the most likely candidates to date. First detected as a source of x-ray radiation in 1965 , Cygnus X-1 consists of the blue supergiant star HDE 226868 and a compact companion with an orbital period of 5.6 days. Based on the orbital motion of the visible star and the Doppler shift of its spectral lines, the unseen companion is estimated to have a mass of five to seven times than of the Sun, strong evidence that it is a black hole.

Schematic of Cygnus X-1.
Image courtesy NASA

In many cases, determining the orbital velocity of the visible star can be very challenging. Unless the binary system is oriented properly to our line of sight, the velocity of the components cannot be accurately measured. This is not the case with the x-ray binary A0620-00. Due to the alignment of their orbit, which is along our line of sight, we have been able to accurately determine the orbital velocities of both members of the system. Observations show this system to consist of a average mass main sequence star orbiting around a three to seven solar mass black hole every seven hours at a distance of about one million miles.

Schematic of an x-ray binary system viewed edge on. The orbital period of both objects can be easily determined as one body passes in front of the other.
Image courtesy http://www.astro.soton.ac.uk/~trm/PH112/notes/notes/node143.html

Once thought to be nothing more than the figment of someone’s overactive imagination, black holes are now commonly accepted as real objects. As evidence mounts for the existence of black holes, scientists continue to discover new methods of detecting and observing these exotic objects. Observing the shift of the spectral lines and the motions of the stars orbiting the black hole is one method, while observing the high energy emission in the disk of material surrounding a black hole in a binary system is another. As our telescopes and instrumentation improve, we will be able to detect more potential black hole candidates, and therefore gain a better understanding of the dynamics involved in the formation of black holes and their effects on their surroundings.

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Ryden, Barbara, Black Holes, http://www-astronomy.mps.ohio-state.edu/~ryden/ast162_5/notes22.html