230 
ASTRONOMY: E. P. HUBBLE 
CHANGES IN THE FORM OF THE NEBULA N. G. C. 2261 
By Edwin p. Hubble 
YERKES OBSERVATORY. UNIVERSITY OF CHICAGO 
Received by the Academy, March 9. 1916 
A comparison of photographs has established changes in structural 
detail in the nebula N.G.C. 2261 (h 399) R.A. 6^ 35-, Dec. +8°50'. 
This nebula is the finest example of a cometary nebula in the northern 
skies. The nucleus is known as the variable star R Monocerotis, said 
to range from magnitudes 9 to 13.5, with an irregular period. Lassell 
states that the nucleus is not a star, and Professor Barnard confirms this 
opinion from observations with the forty-inch refractor. 
Plates obtained by me with the twenty-four inch reflector during the 
last six months were compared in the Blink-Mikroskop with an excellent 
plate taken with the same telescope in March, 1908, by Mr. F. C. 
Jordan. In the interval, the following edge of the nebula has bulged 
out to a greater convexity; a bright portion of the nebulosity just north 
of the nucleus has shifted about 5'' toward the east; the north preced- 
ing part has moved toward the south following. In the center, however, 
is a sharply defined brighter wedge-shaped portion pointing to the 
east, which shows no motion. 
In compHance with the request of the Director of this Observatory, 
Mme. Dorothea Roberts had the great kindness to prepare and send us 
both positive and negative copies of the plate of this nebula taken by 
the late Dr. Isaac Roberts at Starfield on January 27, 1900, and shown in 
Knowledge, vol. 24, p. 181. These amply confirm the reahty of the 
phenomena and further estabHsh that the motion is progressive both in 
direction and amount. The first impression is that the nebula is turn- 
ing about its own axis after the manner of a top, and there is some indi- 
cation of a helical motion toward the nucleus. The observed shifts 
seem to be rather of mass than of illumination and are independent of the 
variabiUty of the nucleus. 
Such changes are so novel that a question at once arose as to whether 
they might not be due to differences in exposure times or other photo- 
graphic conditions rather than in the nebula itself. Fifteen plates ob- 
tained during the last six months mider widely var3dng conditions of 
steadiness, transparency, and exposure time, agree perfectly in detail 
when compared among themselves. Of these, three were taken on the 
same night, with half, full, and double the normal exposure time, and 
show no differences other than the symmetrical building up of the image. 
Further, Director Frost and Professor Barnard have examined the plates 
