CLUSTERS AND NEBULAE. 



Bv F. W. HENKEL, B.A., F.R.A.S. 



Moke than two thousand years ago the Greek 

 philosophers, Democritus and Anaxagoras assured 

 theircountrvmen that the luminous circle of the Milky 

 ^^'a^■ consisted of nothing but dense masses of 

 stars too small and too close together to be seen 

 separately. The former, sometimes known as the 

 "Laughing Philosopher"" of Greece, also entertained 

 some equally remarkable vie\\s on other subjects : 

 the atomic theory, the nature of the sun and stars, 

 and so on, many of which have been \-eritied by 

 later investigation. In addition to the Milky Way 

 itself there are to be found, scattered all over the 

 sky, luminous patches of cloud-like material, stellar 

 groups or " nebulae," as some of them are called. 

 Many of these are known to consist, like the Milky 

 Way itself, of numbers of stars too near together to 

 be separately visible to the unaided eye except in a 

 few cases. The well-known Pleiades group in 

 Taurus is an example. There are to be seen by 

 the eye from seven to twelve stars close together, 

 the number varying according to the keenness of 

 sight of the observer and the state of the sky. With 

 the help of the smallest telescope this number is 

 increased to about one hundred, whilst the large 

 telescopes and photographic ap[)liances of modern 

 astronomy have enabled us to detect the presence 

 of a vet greater number of stars as well as streaks 

 and wisps of nebulosity surrounding and enveloping 

 most of the region. The Hyades cluster is situated 

 close bv in the sky, and not far off is the "Praesepe"' 

 in Cancer, which seen as a cloudy patch, becomes 

 resolved into a swarm of stars, whence the name 

 Praesepe (Beehive). 



Other well-known clusters are those known as 

 13 M Herculis, not very far from the " apex "" of the 

 Sun's wa\-. often considered the finest in the 

 Northern Hemisphere, the cluster surrounding 

 the star (o Centauri (see plate), the group in 

 the "sword handle"' of Perseus, the beautiful 

 cluster M Canum Venaticorum (the Hunting 

 Dogs), the cluster M 14 Ophiuchi. and so on. 

 Some of these clusters are shown to be such 

 by the help of very moderate telescopes, though they 

 are better seen with larger instruments, others 

 require more powerful optical aid, whilst yet others 

 have never been resolved into stars, though on 

 other grounds we have reasons for suspecting their 

 " resolvabilitv." as distinct from what are now 

 known as '" true "" nebulae. Since, as we have said, 

 most clusters, if visible at all to the unaided eye, 

 appear to it as cloud-like mist\- objects or "nebulae," 

 and are resolved into separate stars by a greater or 

 less application of telescopic power, it was at one 

 time thought that all such objects were of a similar 

 nature, the difference between different clusters arising 



from the var\ing si/e of their members ortheirdiffering 

 distances from our system. ^^' henever a new or more 

 powerful telescope was applied numbers of these 

 nebulae or cloud-like objects (Latin— «»/je.s, a cloud) 

 were " resolved "" into separate stars, though some, 

 such as the nebula of Orion and the great nebula of 

 Andromeda, showed no signs of such resolvability. 

 However, the great reflector of Lord Rosse being 

 applied to the former, it was reported that this 

 instrument had at last resolved the nebula, but this 

 was a mistake, for as we shall see, no telescope ever 

 can or will resolve it, since it is not composed of stars, 

 but is of an entirely distinct and different nature. 

 Fiftv or sixty years ago, however, this idea of the re- 

 solvabilitv of all nebulae into clusters was generally 

 entertained. It was thought that they were remote 

 clusters or " island universes,"" lying far beyond the 

 SN'stem to which our own Sun belongs (the great 

 Milky Wa\', or Galaxv), so inconceivably remote that 

 light, though travelling at the enormous speed of 

 over 186,000 miles per second, requires many 

 thousands of 3-ears to reach us from them, and many 

 mav have ceased to exist ages ago, and yet it \s ill be 

 many ages hence before we become aware of their 

 extinction. The work of Herschel and later investi- 

 gators, showing the intimate connection between the 

 distribution of the clusters and nebulae and the 

 position of the Milky Way generally, has thrown 

 doubt upon the validity of such views, though many 

 popular writers have made these ideas generally 

 known to the non-scientific public. 



The memorable and systematic exploration of the 

 heavens, with instruments constructed by himself 

 and of far greater power than any previously used 

 bv astronomers, has immortalized the name of Sir 

 \Villiam Herschel, and it is this, perhaps, more 

 than the discovery of a new planet in the Solar 

 system, which was his greatest contribution to our 

 Science. Previously to his day the number of 

 clusters and nebulae known did not exceed about 

 one hundred and fifty or so, many of which were 

 detected by the famous French astronomer Messier,* 

 known as the " comet ferret " from his diligent 

 search for those bodies, whose ill-defined and often 

 mist\- appearance is not unlike that of a nebula, and 

 indeed a comet has been not inaptly termed " a 

 wandering nebula " for more than one reason. Sir 

 John Herschel extended his father's work into the 

 Southern hemisphere, staying for several years at 

 Feldhausen, near Cape Town, and completely sur- 

 veying the whole sky. In 1864, he published a 

 catalogue of five thousand and seventy - nine 

 nebulae and clusters, a great part of which had 

 not been previoush" located hv other observers. 

 A supplement containing afiout a thousand more 



The letter M before the name of a nebula or cluster indicates that it is one uiiich was included in iMessier's catalogue. 



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