The Search for Black Holes

The Search

By the late 1970s relativity theory had given black holes a natural place at the end of the progression: normal star, white dwarf, neutron star (see the page "A Short History of Black Holes"). The problem now was to show that there really are black holes out there. For this job, general relativity gave way to astrophysics and space technology. The primary tools were not just optical telescopes, but also x-ray detectors, radio telescopes, and spectrographs.


Black Holes in Binary Pairs

Since black holes themselves can't be seen it's natural to look for their effects on neighboring matter. The simplest case (mentioned in the page "Overview...") was to search for a visible binary star whose companion is invisible. Two considerations helped. Matter drawn from the visible star would be powerfully attracted by the black hole and the collision of torrents of particles around the black hole should result in strong emission of x-rays. Many x-rays sources were known, so the problem became to find one paired with a visible star. Many such pairs were found. The possibility that the invisible companion is just a neutron star--also an good x-ray source--can be ruled out if it is too massive, since a neutron star of more than two or three times the mass of our sun must collapse further. (This mass is easily estimated.) By the 1980s evidence for the existence of black had become overwhelming.


Giant Black Holes

In addition to the "standard" black hole resulting from of collapse of a single star, two speculative types were mentioned in "A Short History of Black Holes." The first of these--small black holes created by the big bang--had to be abandoned: If there ever were any, it was predicted that they would quantum-evaporate toward a terminal explosion.

But the prospect of giant black holes in the core of galaxies was more promising. The development of radio telescopes allowed a closer examination of known radio galaxies. Surprisingly, the radio waves often came, not from the galaxy itself, but from two gigantic lobes on opposite sides of the galaxy--looking something like a two-bladed propellor. This radiation could be millions of times greater than that produced by our galaxy.

Figure1. The galaxy NGC 4261 viewed by both optical and radio telecopes. The white disk is a ordinary elliptical galaxy viewed optically--billions of stars in roughly spherical array. The huge orange lobes, many times larger than the galaxy itself, are visible only to radio telescopes.

A giant black hole was the only reasonable candidate for the engine powering this enormous display. It would provide power because it was spinning, grinding up nearby stars of the galaxy and jetting them out along its axis of rotation. It was predicted that there would be a huge disk around the central black hole--something like the rings around Saturn--with its innermost rings fueling the jets. But there was no way to see into the core of any galaxy, even our own.


The Hubble Space Telescope

The greatest advance in astronomy since Galileo's telescope of 1609 was expected in 1990 when the Hubble Space Telescope was lifted into orbit by the Space Shuttle. It was a bitter disappointment when its lens turned out to be defective, severely limiting its use.

In 1993 a Shuttle mission to the Hubble telescope brought a corrective lens and a variety of other lesser improvements. With the HST bolted onto the Shuttle, the new instrumentation was installed.

Figure 2. Two astronauts working on the Hubble telescope. Below is the northwest coast of Australia.

Operating at its full potential, the Hubble telescope began a flow of thousands of magnificent photographs, unmatched by anything seen from Earth. (It isn't easy to refute NASA's claim that the Hubble Space Telescope is "the greatest scientific instrument of all time." When the HST looked at the radio galaxy NGC 4261--the one shown in Figure 1--it found just what had been predicted

Figure 3. A Hubble photograph of the core of NGC 4261, matched against the earth-based picture of the galaxy and its jets. In the "close-up" picture, the central hub conceals the presumed black hole, which is destroying the inner ring of the surrounding disk to feed the jets.

Quasars

The two greatest challenges in astronomy in the last fifty years had opposite status. Black holes came from theory, and the problem was to see if there really are any. By contrast, quasars were plainly visible--the problem was to find theory to account for their awesome properties. Often 10 billion lightyears away (thus in the early age of the universe) they could be billions of times brighter than our galaxy, though no larger.

Once the existence of giant black holes in the cores of galaxies began to win acceptance, it became reasonable to suppose that quasars are simply those whose enormous brightness dominates the ordinary stars of their galaxy.

It is now an accepted fact that galaxies have black holes at their core--with mass proportional to that of the galaxy. These burn fiercely in youth (quasars), diminishing in time to radio galaxies, and finally dim to the level of the one in our galaxy.


Figure 4. A Prevision from 1889.


References

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