Green Light from Palomar

After 20 months of operation, the great 200-inch Hale telescope on Palomar Mountain yielded its most significant discovery. Palomar's Dr. Milton La Salle Humason, a diffident, self-effacing expert whose own colleagues know almost nothing about him except his birthplace (Dodge Center, Minn.), last week announced that he had photographed the spectra of nebulae 360 million lightyears* away. He found that their light showed the mysterious "red-shift," indicating that they are moving away from the earth at 38,000 m.p.s.--one-fifth of the speed of light.

Astral Speedometer. The redshift was first discovered by Edwin P. Hubble, most famous of the Palomar astronomers, and on it he based his startling theory of "the expanding universe." The spectrum of an astronomical object (a star or nebula) shows numerous bright or dark lines, each representing light of a certain wave length. If the object is stationary in relation to the earth, the lines are in the same places as in the spectrum of the sun. But if it is moving away from the earth, the lines shift toward the red end of the spectrum, because the receding motion "pulls out" the light waves and makes them more like the red (long) waves. The faster the object is receding, the more its light shifts toward the red. So the redshift can be used as a speedometer to measure how fast the nebulae are moving away from the earth.

Hubble found that all distant nebulae are moving away from the earth at spectacular speeds, and that the more distant they are, the faster they move. Using more delicate techniques, his colleague Humason continued his work. With the 100-inch Mt. Wilson telescope, Humason photographed nebulae whose red-shifts indicate that they are receding at 25,000 m.p.s.

Blue to Green. The 200-inch Palomar telescope was built primarily for studying more distant nebulae. It can photograph them as faint blurs at distances something like one billion lightyears, but getting their spectra is more difficult. The light from the nebula is concentrated by the telescope's great mirror upon a prism, which spreads it into a spectrum one-tenth of an inch long. So dim is the image on the photographic plate that four to six hours of exposure are needed to make the picture.

In the most distant nebulae studied so far, the bright "H" and "K" lines of glowing calcium, which are normally blue, are shifted into the green band of the spectrum. If they were bright enough to be seen in color, human eyes would actually see them as green instead of blue. This means that the motion of the nebula has lengthened the wave length of its blue light by more than 800 angstroms (.000003 in.). "It's a tremendous shift," says Dr. Hubble. "In our own stellar system, the average shift is only a fraction of one angstrom."

Tired Light. The spectroscopic limit of the Palomar telescope has not yet been reached. Humason believes that in time he can measure the redshift of nebulae 500 million light-years away. But without other parallel advances, even that study will not clear up the mystery of the expanding universe. No one yet is sure why it is expanding, how long it has done so, or how long it will continue.

Some skeptical cosmologists do not admit that the redshift necessarily means that the nebulae are moving. Perhaps, they say, their light "gets tired," losing some of its energy during its tremendous journey through space. Since loss of energy would lengthen the wave length of light, a sufficient amount of fatigue would account for the shift toward the red in the spectrum.

Another theory was developed by Britain's late Mathematician Edward A. Milne, who died last year. The light from the distant nebulae, said Milne, is "fossil light." It started its journey several hundred million years ago, and light in those ancient days may have been different from light today, just as dinosaurs are different from modern animals. The glowing calcium atoms that now give blue light, for instance, may have given green light then. When the fossil green light reaches the earth, Milne said, it fools astronomers into thinking that the nebulae it came from are moving away from the earth.

Drs. Hubble and Humason leave such explanations to cosmologists. "We are observers," they say proudly. "We report what we see."

* A light-year is the distance that light, traveling at 186,000 m.p.s., covers in one year: 5,865,696,000,000 miles.

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