STARS 



689 



combined with the fact that the stars are to us 

 mere luminous points. It is magnified by the tele- 

 scope, and becomes often then sufficient to trans- 

 form the star into something like a tangled mass 

 of worsted. The various points on the surface 

 of a planet, the sun, or the moon are similarly 

 affected, causing an indistinctness of telescopic 

 vision, which when the air is much disturbed 

 Tenders accurate observation impossible. But the 

 star, being one point of light, twinkles a* a whole, 

 while the planet, sun, or moon, being even in the 

 smallest case many points, does not twinkle as a 

 whole, and the average of its rays gives a steady 

 impression to the eye. Scintillation is most observ- 

 able on nights when the atmosphere is disturbed, 

 and always greatest at the horizon, being least at 

 the zenith and on quiet nights. 



Stars are classed in magnitudes according to their 

 brilliance. The 1st magnitude includes the bright 

 st class, only some twenty-four in number, the 

 2d magnitude the next brightest class, and so on, 

 descending in order of brightness to the 15th or 

 even the 17th magnitude, where we encounter the 

 present limit of telescopic vision. At first this 

 classification was most arbitrary, astronomers 

 differing as to the magnitude to which many stars 

 should belong. Hence a decimal notation was 

 introduced, and stars were classed as 1*5 magni- 

 tude, 2 '3, 415, &c. This must not, however, be 

 confounded with an obsolete notation found in 

 some old books, in which I -2 mag. meant a star 

 between 1st mag. and 2d, but nearer 1st than 2d, 

 and 2'1 meant the same, but nearer 2d than 1st. 

 The regular instrumental measurement of star- 

 magnitudes is an entirely modern work, proceed- 

 ing rapidly still. Professor Pickering of Har- 

 vard hag published ( 1891 ) two catalogues, together 

 including more than 25,000 stars whose magnitudes 

 are inatrumentally determined, and other astrono- 

 mers are at work in the same field. A rough guide 

 to ordinary estimates is furnished by the fact that 

 stars barely visible to a good eye are classed in 

 mag. 6, while telescopic stars range from mag. 7 

 to mag. 15 or 16. Arcturns is nearly 1 "0 mag. 

 Pollux and Hegulns are an average 2d mag. fi 

 Arietis is a shade above 3d mag. 



Three explanations may be given for this great 

 variety in brilliance among the stars : ( 1 ) that 

 they are all at nearly the same distance from us, 

 *na are in themselves different in size or bright- 

 ness ; (2) that they are of nearly equal brightness, 

 but ranged at very varied distances ; (3) that they 

 vary both in lustre and distance. The first view 

 has long lieen obsolete, lielonging to the infancy of 

 astronomy ; the second, though equally erroneous, 

 is still to be found treated in many text-books as 

 if it had some foundation, no doubt because it 

 forms a theoretical basis for the star-gauging carried 

 out by the Herschels (q.v.). The third view is 

 unquestionably the correct one, as will more clearly 

 appear when we examine the measured distances 

 of the stars. There must also be considered 

 the probability that space is not perfectly trans- 

 parent, and may entirely absorb the light of a 

 star, if its distance from the observer be great 

 enough. If this were the case it would help to 

 account for the observed variety in stellar light. It 

 is at least remarkable that the number of faint 

 stars is so much greater than of bright ones, the 

 former exceeding the latter by millions. 



Besides this variety between separate stars there 

 are remarkable changes in the light of some stars 

 from time to time. These are known as Variables. 

 The number known is continually enlarged by 

 fresh discoveries, and is now several hundreds. This 

 variability is almost always periodic ; but the 

 length and form of the periods are strikingly 

 diflerent for different stars, o Ceti, or Mira, as it 

 410 



is called, requires 331 days 8 Ijours to accomplish 

 its changes. For about a fortnight it is nearly mag. 

 2, decreasing for three months it becomes invisible, 

 remains so for five months, and then gradually 

 increases for the rest of its period. Algol, or 

 jS Persei, has a period of 2 d. 20 h. 49 m. ; but its 

 actual change from mag. 2 to 4 is accomplished in 

 3^ h. 7) Argus, again, varies from mag. 1 to 6 in 

 seventy years. The so-called ' new ' stars appear- 

 ing from time to time are possibly only extreme 

 instances of this variability. A noted example is 

 the 'Nova' (or new star) of 1572, which reached 

 such brightness as to be visible at mid-day in 

 November of that year, but immediately began 

 to diminish, entirely disappearing by March 1574. 

 It is probably not identical with the temporary 

 stars of 945 and 1 264 A. D. Several similar appear- 

 ances are known. In August 1885 a 'Nova' 

 appeared in the nucleus of the great Andromeda 

 nebula. About the 6th mag. when first seen, it 

 steadily faded, until by February 7, 1886, it was 

 only of mag. 16. On February 1, 1892, another 

 nova of mag. 6 was pointed out in Auriga by Mr 

 T. D. Anderson of Edinburgh. Its place is 'K.A. 

 5 h. 25 m. 3 s., dec. + 30 21'. Within two 

 months it had grown very faint, decreasing since 

 March 9 by half a mag. per day. Its spectrum 

 shows many bright lines, including those of hydro- 



fen and sodium. As to the cause of these out- 

 ursts and the other minor fluctuations of the 

 variables there has been much speculation. The 

 passage of attendant meteor-swarms in front of 

 the star, the changes in its atmosphere, positive 

 collision of stars or meteor-swarms, passage of 

 the star behind the edge of a nebula owing to 

 its parallax these and other explanations have all 

 been advanced ; but as yet no satisfactory theory 

 has been given. The spectroscope has revealed 

 in some ' Novas ' outbursts of glowing gas ; but 

 the cause of such outbursts remains as yet un- 

 revealed. The analogy of the solar spot period 

 ( see SUN ) would seem to show that solar physics 

 holds the key to this strange problem, our sun 

 being really a slightly variable star. 



Variety in colour is as great among the stars as 

 variety in light. Comparing Sirius with Betelgeuse 

 this is easily seen. The investigation into this 

 subject is as yet in its infancy, and we may here 

 only note that very red stars are never bright, and 

 that the two constituents of a binary star usually 

 are of different colours. Blue or green stars of 

 marked colour are also never found alone, but 

 always as members of a close double star. 



The measurement of the distance of the stars, 

 one of the most difficult problems presented to the 

 human mind, has been also one of its greatest 

 triumphs. It was one of the strongest objections 

 to the Copernican system that if the earth moved 

 as Copernicus said, then the stars would appear to 

 move also, as trees and houses appear to move 

 when seen from a train in motion, unless indeed 

 the stars were at an inconceivable and impossible 

 distance. The evidences for the motion of the 

 earth were too strong to be long disputed, 

 and the alternative of the inconceivable distance 

 of the stars had to be admitted. Thus men 

 became familiarised with the thought of immense 

 distances, and soon began efforts to measure them. 

 The movement of the earth transfers it every 

 six months from one extremity to the other 

 of a line 180,000,000 miles in length. So enor- 

 mous is the distance of the stars that this change 

 of place in our observatories has hardly any effect 

 on the direction in winch we view them. If a 

 star is directly overhead on the meridian when 

 we are at one end of this immense base-line it 

 is still overhead when we have arrived at the other 

 end. Some minute change, however, there must 



