Way of a Galaxy

Viewed through the far-sighted optical telescopes of modern astronomy, the great spiral galaxies that dot the depths of space look as stable as anything in the universe. But the view may be a cosmic illusion. Astrophysicists Fred Hoyle and William A. Fowler, from Caltech. told the American Physical Society that galaxies often explode with improbable energy. Even the Milky Way Galaxy, of which the earth and the entire solar system are only a tiny part, may have blown up many times already--and could pop off again.

Hoyle and Fowler base their theory on the mysterious double sources of radio energy that radio telescopes have found in the sky. At first those distant transmitters of energy seemed to be associated with nothing at all--at least nothing that could be photographed with optical telescopes. Then Caltech's giant interferometer in Owens Valley (two 90-ft. radio telescopes working in unison) mapped many double sources with unprecedented accuracy. When the new radio map of the sky was superimposed on photographs taken with the 200-in. Palomar telescope, a galaxy was often neatly bracketed between paired spots of radio energy.

The amount of radio energy from any source is fairly easy to measure, and when Caltech Radio Astronomers P. Maltby. T. A. Matthews and A. T. Moffet com pared this figure with the distance of the associated galaxy,* they got something of a shock. Some of the stronger pairs were emitting 4.4 times 10 ^44 ergs* per second in radio energy, an astonishing figure that represents more than 100 billion times the heat and light energy emitted by the sun.

Astronomers believe that this vast flow of radio energy, which they call synchrotron radiation, comes from high-speed electrons moving through clouds of turbulent gas that have been expelled from the galaxy. Best estimates are that when those clouds were freshly expelled from the strongest radio galaxies, they contained 5 times 10^60 ergs. This jolt equals the amount of energy that would be released if all the matter in 2,500,000 suns were totally converted into energy. Such total conversion is theoretically possible, but astrophysicists do not know of any way that it could take place in the practical universe.

Where, then, can this vast amount of energy come from? Hoyle and Fowler rule out hydrogen fusion and other nuclear reactions that go on in the sun. Such reactions do not take place suddenly enough, or provide enough energy. The only reasonable remaining source of energy is gravitation, which can grow incredibly strong when a very large amount of matter is gathered together. This energy is released when something happens that permits a large mass of material to fall toward a center of gravitational attraction.

Relativistic Limit. A galactic explosion starts, say Hoyle and Fowler, when a large mass of gas accumulates in the center of a galaxy's nucleus. The gas behaves like a very large star, equal in mass to 100 million suns. For a while it burns hydrogen and changes it to helium just as normal stars do, and the temperature in its center rises to 70 million degrees. Then the star burns its helium, forming heavier elements. Its central temperature rises to 500 million degrees, while its powerful gravitation causes it to shrink toward its superheated middle. In spite of its enormous mass, the great star is now only about one-third the size of the earth's orbit.

A crisis comes swiftly. The temperature inside the star rises suddenly above one billion degrees, and strange reactions take place in its tortured center. Vast numbers of tiny particles called neutrinos escape into space, and powerful gamma rays turn energy into matter. Both these processes sop up so much energy that the great star suddenly collapses, exploding inward like a shattered TV picture tube. And as the great star's material starts to fall toward its center, gravitation pulls the stuff along so strongly that its speed quickly approaches the speed of light, which astronomers call respectfully "the relativistic limit," that cannot be exceeded.

According to Hoyle and Fowler's figures, an exploding galactic star is always somewhat irregular, and the part of it that collapses to a small dense core expends most of its gravitational energy. The rest of the star, absorbing this same energy, is blasted away at nearly the speed of light. Some of its material slows down as it tangles with the flat disk of stars and gas that make up the galaxy. It is the parts that move out from the top and bottom of the galaxy that escape to form turbulent clouds that shine as powerful radio sources for hundreds of thousands of years.

Chance for Survival. Hoyle and Fowler believe that these gigantic explosions are normal events in the life of galaxies. They happen over and over, and their violent nuclear reactions create the heavier elements that are found in stars and planets. For all their fierceness, though, they do not destroy galaxies or apparently damage them much. Some galaxies still seem in normal shape although the radio clouds from two explosions are speeding away from them.

The solar system lies in the dense disk of the Milky Way Galaxy and at the safe distance of 30,000 light-years from the center. If a giant star were even now exploding at the galactic center, the earth and its inhabitants would have a good chance to survive the effects of the blast, which they would not know about for at least 30,000 years.

* According to the exploding-universe theory, the galaxies are receding from one another, and the greater the distance between them the faster they recede. Since their speed away from the earth can be measured by the shift of their light toward the red end of the spectrum, their distance, which is proportionate to speed, can be measured too. * Units of energy. One foot-pound equals 13,558,200 ergs.

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