Part 20 (2/2)

That this is an error clearly appears from Bayer's own ”Explicatio” to his Atlas, and was long since pointed out by Argelander (1832), and by Dr.

Gould in his _Uranometria Argentina_. Gould says, ”For the stars of each order, the sequence of the letters in no manner represents that of their brightness, but depended upon the positions of the stars in the figure, beginning usually at the head, and following its course until all the stars of that order of magnitude were exhausted.” Mr. Lynn says, ”Perhaps one of the most remarkable instances in which the lettering is seen at a glance not to follow the order of the letters is that of the three brightest stars in Aquila [Al-Sufi's 'three famous stars'], ? being evidently brighter than . But there is no occasion to conjecture from this that any change of relative brightness has taken place. Bayer reckoned both of these two of the third magnitude, and appears to have arranged before ?, according to his usual custom, simply because is in the neck of the supposed eagle, and ? at the root of one of the wings.”[455] Another good example is found in the stars of the ”Plough,”

in which the stars are evidently arranged in the order of the figure and not in the order of relative brightness. In fact, Bayer is no guide at all with reference to star magnitudes. How different Al-Sufi was in this respect!

The stars Aldebaran, Regulus, Antares, and Fomalhaut were called royal stars by the ancients. The reason of this was that they lie roughly about 90 apart, that is 6 hours of Right Ascension. So, if through the north and south poles of the heavens and each of these stars we draw great circles of the sphere, these circles will divide the sphere into four nearly equal parts, and the ancients supposed that each of these stars ruled over a quarter of the sphere, an idea probably connected with astrology. As the position of Aldebaran is R.A. 4{h} 30{m}, Declination North 16 19', and that of Antares is R.A. 16{h} 15{m}, Declination South 25 2', these two stars lie at nearly opposite points of the celestial sphere. From this it follows that our sun seen from Aldebaran would lie not very far from Antares, and seen from Antares it would appear not far from Aldebaran.

The following may be considered as representative stars of different magnitudes. For those of first magnitude and fainter I have only given those for which all the best observers in ancient and modern times agree, and which have been confirmed by modern photometric measures. The Harvard measures are given:--

Brighter than ”zero magnitude” Sirius (-158); Canopus (-086)

Zero magnitude a Centauri (006)

0 to 04 magnitude Vega (014); Capella (021); Arcturus (024); Rigel (034)

05 magnitude Procyon (048)

1st ” Aldebaran (106)

2nd ” a Persei (190); Aurigae (207)

3rd ” ? Bootis (308); ? Capricorni (298)

4th ” ? Leonis (385); ? Scorpii (416); ? Crateris(414); ? Herculis (414)

5th ” ? Pegasi (485); Capricorni (510)

CHAPTER XX

The Visible Universe

Some researches on the distribution of stars in the sky have recently been made at the Harvard Observatory (U.S.A.). The princ.i.p.al results are:--(1) The number of stars on any ”given area of the Milky Way is about twice as great as in an equal area of any other region.” (2) This ratio does not increase for faint stars down to the 12th magnitude. (3) ”The Milky Way covers about one-third of the sky and contains about half of the stars.”

(4) There are about 10,000 stars of magnitude 66 or brighter, 100,000 down to magnitude 87, one million to magnitude 11, and two millions to magnitude 119. It is estimated that there are about 18 millions of stars down to the 15th magnitude visible in a telescope of 15 inches aperture.[456]

According to Prof. Kapteyn's researches on stellar distribution, he finds that going out from the earth into s.p.a.ce, the ”star density”--that is, the number of stars per unit volume of s.p.a.ce--is fairly constant until we reach a distance of about 200 ”light years.” From this point the density gradually diminishes out to a distance of 2500 ”light years,” at which distance it is reduced to about one-fifth of the density in the sun's vicinity.[457]

In a letter to the late Mr. Proctor (_Knowledge_, November, 1885, p. 21), Sir John Herschel suggested that our Galaxy (or stellar system) ”contained within itself miniatures of itself.” This beautiful idea is probably true.

In his account of the greater ”Magellanic cloud,” Sir John Herschel describes one of the numerous objects it contains as follows:--

”Very bright, very large; oval; very gradually pretty, much brighter in the middle; a beautiful nebula; it has very much the resemblance to the Nubecula Major itself as seen with the naked eye, but it is far brighter and more impressive in its general aspect as if it were doubled in intensity. Note--July 29, 1837. I well remember this observation, it was the result of repeated comparisons between the object seen in the telescope and the actual nubecula as seen high in the sky on the meridian, and no vague estimate carelessly set down.

And who can say whether in this object, magnified and a.n.a.lysed by telescopes infinitely superior to what we now possess, there may not exist all the complexity of detail that the nubecula itself presents to our examination?”[458]

The late Lord Kelvin, in a remarkable address delivered before the Physical Science Section of the British a.s.sociation at its meeting at Glasgow in 1901, considered the probable quant.i.ty of matter contained in our Visible Universe. He takes a sphere of radius represented by the distance of a star having a parallax of one-thousandth of a second (or about 3000 years' journey for light), and he supposes that uniformly distributed within this sphere there exists a ma.s.s of matter equal to 1000 million times the sun's ma.s.s. With these data he finds that a body placed originally at the surface of the sphere would in 5 million years acquire by gravitational force a velocity of about 12 miles a second, and after 25 million of years a velocity of about 67 miles a second. As these velocities are of the same order as the observed velocities among the stars, Lord Kelvin concludes that there _is_ probably as much matter in our universe as would be represented by a thousand million suns. If we a.s.sumed a ma.s.s of ten thousand suns the velocities would be much too high.

The most probable estimate of the total number of the visible stars is about 100 millions; so that if Lord Kelvin's calculations are correct we seem bound to a.s.sume that s.p.a.ce contains a number of dark bodies. The nebulae, however, probably contain vast ma.s.ses of matter, and this may perhaps account--partially, at least--for the large amount of matter estimated by Lord Kelvin. (See Chapter on ”Nebulae.”)

In some notes on photographs of the Milky Way, Prof. Barnard says with reference to the great nebula near ? Ophiuchi, ”The peculiarity of this region has suggested to me the idea that the apparently small stars forming the ground work of the Milky Way here, are really very small bodies compared with our own sun”; and again, referring to the region near Cygni, ”One is specially struck with the apparent extreme smallness of the general ma.s.s of the stars in this region.” Again, with reference to ?

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