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His general method (in practice, enormously difficult) was to determine the intrinsic brightness of objects in a nebula and then gauge its distance (the fainter it is, the farther away). Variable stars called Cepheids, discovered at Harvard, told him that the bright nebula called Messier 31 is 680,000 light-years away. Messier 31 was therefore no mere part of the Milky Way galaxy (the star-congregation in which our sun is a fourth-rate star), but an isolated star-system far out in space and as big as our entire galaxy. Other, longer-ranged measuring sticks carried him farther on his march into space. By this time most of the world's astronomers were looking over his broad shoulders, or helping him with their own observations, as he trudged out into the universe with astronomical seven-league boots.
There seemed to be no end to the swarming nebulae. The most distant showed as tiny, dim blobs. By a complex statistical method Hubble proved, after years of work, that these dimmest glimmers were so far away that their light, speeding at 186,000 miles per second, took 500,000,000 years to reach Mount Wilson.
His success in measuring the distance of dimmer & dimmer nebulae led to his crowning discovery—the theory of the expanding universe.
Universe in Flight. Astronomers have a speedometer to clock the motions of skittish heavenly bodies. They take spectrographs: photographs of the body's light spread out by a prism into a band of colors. If the band is "shifted toward the red" (i.e., if it is redder than normal), it shows that the body is moving away from the earth.
Working with short, amiable spectrograph expert Milton Humason, Hubble studied the light of the distant nebulae. In every case he found a "red shift."* The farther off a nebula was, the faster it appeared to be rushing away, and the enormous speeds (thousands of miles per second) were new, strange and startling to astronomers.
While Humason's spectrographs gradually improved, Hubble theorized until he came to a momentous conclusion: that the speed of recession of the nebulae is directly proportionate to their distance. This meant that each of the large units of matter in the universe (nebulae) is moving away from every other unit. The Milky Way galaxy (the earth's local nebula) is not the only center of the explosion. Every other nebula is equally an explosion center.
Casting around for a layman's analogy, Hubble compared the exploding universe to a rubber balloon with small dots (representing nebulae) spaced equally far apart on its surface. When the balloon is blown up larger, each dot becomes farther from every other dot. Place an observer on any dot, and he will see the same picture. Every other dot-nebula will be moving away from him.
Hot Genesis. When Hubble finished his "velocity-distance" law, he might have sat back (but probably did not) to marvel at his accomplishment. He had formulated the very first rule, buttressed with observation, that explained the behavior of every major unit in the visible universe. Other men (including Einstein) had theorized about the universe, using their minds as telescopes. Hubble had evolved his theory by looking at the universe itself.