PHOTOGRAPHIC INVESTIGATIONS
OF FAINT NEBULAE
THE UNIVERSITY OF CHICAGO PRESS
CHICAGO, ILLINOIS
THE BAKER & TAYLOR COMPANY
NEW YORK
THE CAMBRIDGE UNIVERSITY PRESS
LONDON
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THE MISSION BOOK COMPANY
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The University of Chicago
Publications of the Yerkes
Observatory
VOLUME IV PART II
PHOTOGRAPHIC INVESTIGATIONS
OF FAINT NEBULAE
BY
EDWIN P. HUBBLE
THE UNIVERSITY OF CHICAGO PRESS
CHICAGO, ILLINOIS
Copyright 1920 By
The University of Chicago
All Rights Reserved
Published January 1920
Composed and Printed By
The University of Chicago Press
Chicago, Illinois. U.S.A.
[Pg 1]
PHOTOGRAPHIC INVESTIGATIONS
OF FAINT NEBULAE[1]
By Edwin P. Hubble
The study of nebulae is essentially a photographic problem for camerasof wide angle and reflectors of large focal ratio. The photographicplate presents a definite and permanent record beside which visualobservations lose most of their significance. Perhaps the one fieldleft for the older method is the measurement of sharp nuclei deeplyenshrouded in nebulosity. New nebulae are now but rarely seen in thesky, although an hour’s exposure made at random with a large reflectorhas more than an even chance of adding several small faint objects tothe rapidly growing list of those already known. About 17,000 havealready been catalogued, and the estimates of those within reach ofexisting instruments, based on the ratio of those previously known tothose new in various fields, lie around 150,000.
Extremely little is known of the nature of nebulae, and no significantclassification has yet been suggested; not even a precise definitionhas been formulated. The essential features are that they are situatedoutside our solar system, that they present sensible surfaces, andthat they should be unresolved into separate stars. Even then anexception must be granted for possible gaseous nebulae which appearstellar in the telescope, but whose true nature is revealed by thespectroscope. It may well be that they differ in kind and do notform a unidirectional sequence of evolution. Some at least of thegreat diffuse nebulosities, connected as they are with even naked-eyestars, lie within our stellar system; while others, the great spirals,with their enormous radial velocities and insensible proper motions,apparently lie outside our system. The planetaries, gaseous but welldefined, are probably within our sidereal system, but at vast distancesfrom the earth.
In addition to these classes are the numberless small, faint nebulae,vague markings on the photographic plate, whose very forms areindistinct. They may give gaseous spectra, or continuous; they maybe planetaries or spirals, or they may belong to a different classentirely. They may even be clusters and not nebulae at all. Thesequestions await their answers for ins