50 



KNOWLEDGE 



[March 1, 1898. 



us from a much greater surface than the continuous hght, 

 and might malie up in quantity what it wanted in intensity. 

 Such a star would not, I think, be variable in the ordinary 

 sense, but as time rolled on the continuous sjjectrum would 

 become more powerful, the bright gas-lines would die out, 

 and the dark absorption lines would become less numerous 

 and less marked. The radius of the nucleus would become 

 more nearly equal to that of the star, and the depth of the 

 absorbing atmosphere would slowly diminish. 



Cloud formations would produce different phenomena, 

 for clouds could not be expected to remain constant, but 

 to form, dissipate, and reform, though slowly increasing on 

 the whole. They would probably not form at the outer 

 surface of the nebula (since only the lighter and finer 

 gases, such as hydrogen, would be found there), but at 

 some depth below. Absorption-lines, particularly of the 

 lighter gases, might therefore be exj)ected, though not so 

 numerous or so well marked as when the light came from 

 a central nucleus. Bright lines would probably ap25ear at 

 the earlier stages also, for the clouds would not cover the 

 entire surface. The rotation of the star on its axis would 

 probably produce variations of a periodic kind, for it is 

 not likely that the clouds would be equally developed on 

 both hemispheres of the star until they had formed to 

 such an extent as to cover almost the entire surface. 

 Gradually the variations would cease and bright lines die 

 out, the entire surface becoming clouded over. 



These deductions partly agree with and partly differ 

 from the results of observation. Stars presenting the 

 phenomena indicative of a central nucleus seem more 

 inclined to variation than those whose phenomena are 

 suggestive of clouds. Perhaps, instead of a central nucleus, 

 clouds are formed at a great depth in such cases. Again, 

 in the cloud-stars the bright lines should be most con- 

 spicuous when there is least cloud, i.e., at the minimum, 

 but they seem to be most conspicuous at the maximum. 

 Of this I see no satisfactory explanation. An outburst of 

 highly-heated gases through a break in the cloud might 

 perhaps produce this effect, but if sunspots are of this 

 character they are always darker than the surrounding 

 portion of the surface. Messrs. Carriugtou and Hodgson, 

 however, observed a phenomenon of a different kind on 

 one well-known occasion. Truly yours, 



W. H. S. MoNCK. 



THE ri ARGUS REGION OF THE MILKY WAY. 



By A. C. Ranyaki). 



WE are indebted to Mr. H. C. Eussell, Director 

 of the Sydney Observatory, New South Wales, 

 for the two photographs given in this month's 

 number of Knowledge. They represent, on 

 different scales, the remarkable region of the 

 southern Milky Way ai-ound the variable star ij Argiis. At 

 first sight it is not easy to compare them or to recognize 

 what part of the one picture is represented on a larger 

 scale in the other. 



The left-hand picture was taken with a short focused 

 instrument, which threw an intensely bright but small 

 image on the plate, while the right-hand picture was taken 

 with a 13-inch refracting telescope of about eleven feet 

 focal length, used at the Sydney Observatory for the inter- 

 national photographic survey of the heavens. The image 

 thrown by it upon the sensitive plate was larger and 

 fainter, but more sharply defined, than the image thrown 

 by the smaller instrument, consequently the right-hand 

 picture does not show the fainter outlying parts of the 

 nebula, which are shown in the other picture, but it shows 



the nebulous structure on a larger scale. la fact the whole 

 region shown in the right-hand picture is comprised within 

 an area of about two inches by three, near to the centre of 

 the left-hand plate. 



This region of the Milky Way shows some dark channels 

 and dark structures, which are such striking features when 

 seen in the telescope that they attracted the attention of 

 Sir John Herschel, and are minutely described by him in 

 the " Cape Observations." He also recognized the con- 

 nection between the dark regions and the lines of stars 

 which border them. The facts disclosed point to such 

 important conclusions that I propose to reproduce Sir 

 John Herschel's drawing in another number of Knowledoe, 

 together with an additional large photograph of this region, 

 and will postpone any remarks with regard to the dark 

 structures till all the photographs are before my readers 

 for comparison. 



In the meantime those who are interested in the subject 

 will find a photograph on a small scale of the i; Argus region 

 and surrounding clusters in the June number of Knowledge 

 for 1891. It shows the exceptional character of this region 

 of the Milky Way. As Herschel remarked, it is coarser 

 grained than other parts of the galaxy, and is rich in 

 loosely-packed clusters of stars. The ■>} Argus nebula is a 

 star cluster as well as a nebula, and the question, What 

 is a nebula ? seems to be intimately associated with the 

 equally difBcult question. What is a star cluster '? 



The variable star tj Argus, which gives its name to this 

 nebulous cluster, is a reddish star now too small to be visible 

 to the naked eye. It is immersed in the nebulosity on the 

 following side of the dark structure, or hole in the nebula, 

 which Herschel named the " key-hole," though it looks 

 very little like a key-hole in these photographs, i; Argiis 

 is in the ujDper or northern nebulous mass surrounded by a 

 number of smaller stars. It was first observed by Halley, 

 at St. Helena, in 1677, and was rated by him as of the 4th 

 magnitude. According to Winnecke, it was next seen by 

 Pere Noel, a Jesuit missionary, in China, about ten years 

 later, who rated it as a 2nd magnitude star, as it also 

 appeared to Lacaille sixty-four years later, in 1751. Sir 

 John Herschel mentions that in the subsequent catalogues 

 of Fallows (1822), Brisbane (1826), Johnson (1832). and 

 Taylor, it was ranked as a star of the 2nd magnitude, 

 but that it was observed in February, 1827, by Mr. 

 W. J. Burchell, at San Paulo, in Brazil, to be as 

 brilliant as a Crucis. Another vigorous outburst of its 

 hght was observed by Sir John Herschel at the Cape, 

 on the 16th December, 1837. It suddenly increased from 

 the 2ud magnitude and became as bright as Rigel. 

 It continued to increase in brightness till the 2nd of 

 January, 1838, when it was as bright as a Centauri. 

 It then declined in brightness and again increased, then 

 decreased and increased again, till, in April, 1843, Sirius 

 alone amongst the fixed stars slightly outshone it. This 

 intense brightness was maintained for some ten years, 

 when it was observed by GilUss at Santiago in 1850 as very 

 little inferior to Canopus in light, and in colour to be more 

 deeply tinged with red than Mars. According to Miss 

 Clerke, who gives a curve representing its changes of bright- 

 ness (see her Si/stcm of the Stars, p. 118), it was still of 

 the first magnitude in 1856, it fell to the second in 1858, 

 to the third in 1850, and ceased to be visible to the naked 

 eye in 1868. Twenty years later its magnitude was rated 

 by Mr. Finlay at the Cape as 7'6. Since that time it has 

 slightly increased in brightness, and its tint has changed 

 from "dull scarlet" to "bright orange." Evidently 

 changes are still going on ; the observations seem to 

 indicate that it is an irregular variable, rather than a star 

 having a regular period of change. 



