THE NATURE OF THE NEBULAE 



By Edwin Hubble 



[With one plate] 



The exploration of space is an achievement of great telescopes. 

 Men first studied the heavens with their naked eyes. They recog- 

 nized and described the solar system — the central sun with its family 

 of planets — and the discovery is one of the great intellectual feats of 

 the race. But beyond the planets lay the stars. They were too 

 distant for reliable investigation, and their nature remained a subject 

 of sheer speculation. 



For the human eye, with all its perfection, is necessarily a small 

 instrument ; the aperture of the fully opened pupil is only a fifth of an 

 inch. Many an astronomer, in those early days, must have dreamed 

 of a giant's eye that would collect more light, penetrate more deeply 

 into the universe around us. And eventually, that incredible dream 

 materialized. Just 330 years ago the telescope, newly invented, was 

 pointed toward the sky, and the way for exploration on the grand 

 scale was suddenly opened. Galileo, as Kepler wrote to him, had 

 scaled the very walls of heaven. 



The centuries that followed were crowded with investigations that 

 led to the formulation of modern astronomy. Telescopes and their 

 accessories developed and, with each advance, new fields were opened. 

 The nature of the stars was established. The sun itself was definitely 

 recognized as a star, and earlier speculation on the subject was thus 

 confirmed. The many millions of stars, it was found, are not scat- 

 tered indefinitely through the universe ; they form an isolated system. 

 This swarm of stars, this stellar system, drifts through space. From 

 our position somewhere within the system, we look out through the 

 swarm of stars, past the borders, into the universe beyond. 



Telescopes have continued to develop until today we are exploring 

 those outer regions. They are inhabited by stellar systems compar- 

 able with our system of the Milky Way. Those other systems are 

 the extragalactic nebulae. We find them scattered thinly through 

 space out as far as telescopes can reach. They are gigantic beacons, 

 permitting us to survey and study a sample of the universe. Even- 



i Public lecture delivered in San Francisco, Monday evening, March 21, 1938, on the occasion of the 

 presentation of the Bruce Gold Medal of the Astronomical Society of the Pacific. Reprinted by permission 

 from The Publications of the Astronomical Society of the Pacific, vol. 50, No. 293, April 1938. 



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