The Universe in the Classroom

Black Holes

How can you "see'' a black hole?

You might wonder how a black hole could be found if nothing including light can escape from it. Black holes have mass, which causes a gravitational force, which effects objects near them. This gravitational force would be very strong near the black hole, and could have noticeable effects on its environment. Material falling into the black hole would gain energy from the gravitational field, and would be crushed and heated as it tried to squeeze into the black hole's tiny throat, causing it to emit x-rays. The first example of a black hole was discovered by just such a gravitational effect on a companion star.

CYGNUS X-1 SYSTEM
An artist's conception of the Cygnus X-1 system. HDE 226868 is a massive blue supergiant star; its companion is believed to be a black hole, surrounded by an accretion disc of gases from HDE 226868 which are spiraling into the black hole. The star and the black hole are in orbit around each other. The black hole's existence was deduced from the orbital motion of the star, and from the X-rays produced by the gas in the accretion disc which is heated as it falls toward the black hole. (Courtesy William J. Kaufmann III, Universe, W.H. Freeman & Company, 1991. Used with permission.)

Cygnus X-1 was the name given to a source of x-rays in the constellation Cygnus, discovered in 1962 with a primitive x-ray telescope flown on a rocket. By 1971, the location of the x-ray source in the sky had been measured more precisely, using rocket and satellite observations. A key breakthrough came in March 1971, when a new source of radio waves was discovered in Cygnus, near the position of the x-ray source. The radio signal varied at the exact same time when the strength of the x-rays changed strong evidence that the radio and x-ray sources were the same object.

A faint star called HDE 226868 appears at the position of this radio source. Astronomers studying the light of HDE 226868 have found two important facts: (1) HDE 226868 is a blue supergiant star -- a massive, normal star near the end of its life; and (2) the star is orbiting another massive object in a 5.6-day orbit.

From the gravitational force needed to keep HDE 226868 in orbit, the mass of the companion can be determined -- it is about 10 solar masses. But there is no sign of any visible light from the companion -- and something in the object produces x-rays. The explanation or "model'' which best fits these facts is that the companion is a black hole of about 10 solar masses -- the corpse of a massive star which was once the companion of HDE 226868. The x-rays are produced as gas from the atmosphere of the blue supergiant star falls into the collapsed object and is heated. The collapsed object cannot be a white dwarf or neutron star, because these objects can't have masses greater than 1.44 and three solar masses, respectively. We may never be able to "prove'' this theory of Cygnus X-1 by "seeing'' the black hole, but the circumstantial evidence is strong. Three other objects -- LMC X-3 in the Large Magellanic Cloud galaxy, and A0620-00 and V404 Cygni in our galaxy -- are also believed to have black holes as one of their components.

Supermassive black holes

A quarter of a century ago, astronomers discovered distant objects rare, distant objects which were producing extraordinarily large power in an extraordinarily small volume -- the power of a trillion Suns in a volume not much larger than the solar system. They called these objects quasi-stellar radio sources -- quasars, for short -- because they looked like stars, and produced large amounts of radio waves as well as light. Astronomers also realized that, although quasars were rare, there were many other objects -- apparently galaxies of stars -- which showed less extreme versions of the same phenomenon: very large power from a very small volume. These objects shared another remarkable property: jets of high-energy particles emitted from their cores. These properties were so difficult to explain, using the physical knowledge of the time, that some astronomers even questioned whether that knowledge was correct!

In the years since, astronomers realized that there is an explanation for these active galactic nuclei which is consistent with observations and with theory -- even though this explanation boggles the mind: at the core of these galaxies is a supermassive black hole, with the mass of millions or billions of Suns. The size of its event horizon would be about the same as the size of the solar system. The observed power output could be explained if only one solar mass of material were to fall into the black hole each year -- an amount of material which could easily come from the "winds'' of gas produced by stars near the core. The jets of particles in active galactic nuclei are produced by material spiraling into a disk around the black hole, and being squashed out the top and bottom of the disk as it tries to enter the black hole. This explanation for the "engine'' in an active galactic nucleus has been strongly supported by images obtained by the Hubble Space Telescope (see image below).

How does a supermassive black hole form? Some theories hold that the first generation of stars included a large proportion of very massive stars, all of which formed black holes which somehow merged. Other theories hold that a single "seed'' black hole accreted stars and gas, growing more and more massive with time.

There is evidence that these supermassive black holes exist in many galaxies, including our own Milky Way, and our nearest neighbor galaxy, the Andromeda galaxy (also known as M31). There is also evidence that they form early in the life of the galaxy: we see quasars so far away that their light, traveling at 300,000 kilometers/sec, must have left these objects shortly after they formed. Supermassive black holes may therefore be a normal part of the process of birth and evolution of galaxies.
NGC 4261
A composite image of the active galaxy NGC 4261, showing jets of radio-emitting particles spurting from the core of the galaxy. On the right: a false-color image from the Hubble Space Telescope, showing a dark, doughnut-shaped structure surrounding a possible supermassive black hole, believed to be the "central engine" of the galaxy. (Photo credit: Walter Jaffe, Leiden Observatory; Holland Ford, STScI, NASA)

Black holes and science fiction

A concept as bizarre as a black hole naturally attracts the interest and creativity of science fiction writers. A favorite theme is the use of a black hole as a path to elsewhere or else when in the universe. Mathematically, a pair of black holes could form a "bridge'' between two locations in the universe, but it is not clear how such a bridge could form or survive. A black hole, such as one formed as a stellar corpse, would be rather inconvenient for space travel, because the matter falling into it would be crushed and incinerated by tidal forces as it entered the black hole. A supermassive black hole would have less extreme tidal forces, but the nearest one is thought to be at the center of our galaxy -- and therefore inconveniently distant! A rotating black hole has more interesting possibilities, because there exists a region called the ergosphere, just outside the event horizon, which has the following property -- objects can enter and exit from the ergosphere (if they could stand the tidal forces). A spaceship full of trash could enter the ergosphere, dump its load into the black hole, and come out with more energy than it entered with -- thus solving the energy crisis arid the pollution problem simultaneously (at least in theory)!

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