Identifier: adolfostahllectu00astruoft (find matches)
Title: The Adolfo Stahl lectures in astronomy, delivered in San Francisco, California, in 1916-17 and 1917-18, under the auspices of the Astronomical Society of the Pacific
Year: 1919 (1910s)
Authors: Astronomical Society of the Pacific Aitken, Robert Grant, 1864-1951
Subjects: Astronomy
Publisher: San Francisco Stanford University Press
Contributing Library: Gerstein - University of Toronto
Digitizing Sponsor: MSN

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hen, Theta Orionis is awonderful object; a group of suns forming a single physicalsystem of a size so vast that our solar system, in comparison,shrinks to insignificance. But far more wonderful is thecloud-like mass of greenish-white light enveloping these stars.Just visible to the naked eye as a hazy patch, the brighter partis readily seen with a pair of opera glasses; but to get anadequate idea of its beauty, its extent, and the bewilderingcomplexity of its details it is necessary to view it with a power-ful telescope, or to study a photograph taken with a largemodern reflector. It is hopeless to attempt description, justas many able astronomers have found it hopeless to try toportray all of its features by even the most careful drawings. I have called it a nebula, but that term is applied to at leastthree different classes of objects, the spirals, the planetaries.and the irregular gaseous nebulae. Our object belongs to thethird category, for the spectroscope long ago showed that it
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PLATE XLII. Vacant Lanes and Nebula in Taurus. Plwfographed by E. E. Barnard zvifh the \0-incIi Bnicc telescope, Jan. 9.1907, 5j/2 hours exposure. News fro.m the Stars 161 consists of gases shining by inherent Hght, but whether thisHght is due to intense heat or to some other cause it has untilrecently been quite impossible to say. Even now we are notprepared to assert that the question has been definitely settled.The great difficulty about believing it to be due to heat is thealmost incredible extent and tenuity of the nebula. On thephotographs taken with the Crossley reflector both the northand south, and the east and west diameters exceed 40 minutesof arc. To translate this value into linear measure, milesor kilometers, it is necessary to know how far away the objectis. This we do not know, but it is possible to set a minimumvalue for the distance. The parallax is certainly less than 0.01second of arc; that is, a line 93,000,000 miles long (the dis-tance from the Earth to the Sun),

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