Astronomical Curiosities Part 20

VOLANS, the Flying Fish, is north of Mensa, and south and west of Argo.

Its brighter stars, with the exception of a and , form an irregular six-sided figure. Its brightest star is (365) according to the Harvard measures. The Cordoba estimates, however, range from 36 to 44, and Gould says its colour is "bright yellow." Williams rated it 38.

CHAMaeLION.--This small constellation lies south of Volans, and north of Mensa and Octans. None of its stars are brighter than the 4th magnitude, its brightest being a (408 Harvard) and ? (410).

ARGO.--This large constellation extends much further south than Al-Sufi could follow it. The most southern star he mentions is e Carinae, but south of this are several bright stars. Carinae is 180 according to the Harvard measures; ? Carinae, 308; ?, 303; ?, 356; and others. A little north-west of ? is the long-period variable R Carinae (9{h} 29{m}7, S. 62 21', 1900). It varies from 45 at maximum to 10 at minimum, and the period is about 3097 days. A little east of R Carinae is another remarkable variable star, _l_ Carinae (R.A. 9{h} 42{m}5, S. 62 3'). It varies from 36 to 50 magnitude, with a period of 35 days from maximum to maximum. All the light changes can be observed with an opera-gla.s.s, or even with the naked eye. It was discovered at Cordoba. The spectrum is of the solar type (G).

MUSCA, the Bee, is a small constellation south of the Southern Cross and Centaurus. Its brightest stars are a (284 Harvard) and (326). These two stars form a fine pair south of a Crucis. Closely south-east of a is the short-period variable R Muscae. It varies from 65 to 76 magnitude, and its period is about 19 hours. All its changes of light may be observed with a good opera-gla.s.s.

APUS, the Bird of Paradise, lies south-east of Musca, and north of Octans.

Its brightest star is a, about the 4th magnitude. Williams calls it "deep yellow." About 3 north-west of a, in the direction of the Southern Cross, is ? Apodis, which was found to be variable at Cordoba from 5 to 6. The spectrum is of the third type, which includes so many variable stars.

TRIANGULUM AUSTRALIS, the Southern Triangle, is a small constellation north of Apus, and south of Norma. A fine triangle, nearly isosceles, is formed by its three bright stars, a, , ?, the brightest a being at the vertex. These three stars form with a Centauri an elongated cross. The stars and ? are about 3rd magnitude. is reddish. e (411, Harvard) is also reddish, and is nearly midway between and ?, and near the centre of the cross above referred to. a is a fine star (188 Harvard) and is one of the brightest stars in the sky--No. 33 in a list of 1500 highest stars given by Pickering. About 1 40' west of e is the short-period variable R Trianguli Australis (R.A. 15{h} 10{m}8, S. 66 8') discovered at Cordoba in 1871. It varies from 67 to 74, and the period is about 3{d} 7{h}2.

Although not visible to ordinary eyesight it is given here, as it is an interesting object and all its light changes may be well seen with an opera-gla.s.s. A little south-east of is another short-period variable, S Trianguli Australis (R.A. 15{h} 52{m}2, S. 63 30'), which varies from 64 to 74, with a period of 63 days; and all its fluctuations of light may also be observed with a good opera-gla.s.s.

CIRCINUS, the Compa.s.s, is a very small constellation lying between Triangulum and Centaurus. Its brightest star, a, is about 3 magnitude, about 4 south of a Centauri.

PAVO, the Peac.o.c.k, lies north of Octans and Apus, and south of Telescopium. Its brightest star is a, which is a fine bright star (212 Harvard). ? is a short-period variable. It varies from 38 to 52, and the period is about 9 days. This is an interesting object, as all the fluctations of light can be observed by the naked eye or an opera-gla.s.s. e Pavonis was measured 410 at Harvard, but the Cordoba estimates vary from 36 to 42. Gould says "it is of a remarkably blue colour."

INDUS.--This constellation lies north of Octans, and south of Sagittarius, Microscopium, and Grus. One of its stars, a, is probably referred to by Al-Sufi in his description of Sagittarius; it lies nearly midway between Sagittarii and a Gruis, and is the brightest star of the constellation.

The star e Indi (474 Harvard) has a remarkably large proper motion of 4"68 per annum. Its parallax is about 0"28, and the proper motion indicates a velocity of about 49 miles a second at right angles to the line of sight.

TOUCAN.--This constellation lies north of Octans, and south of Phnix and Grus, east of Indus, and west of Hydrus. Its brightest star is a, of about the 3rd magnitude.

There are seven "celestial rivers" alluded to by the ancient astronomers:--

1. The Fish River, which flows from the urn of Aquarius.

2. The "River of the Bird," or the Milky Way in Cygnus.

3. The River of the Birds--2, including Aquila.

4. The River of Orion--Erida.n.u.s.

5. The River of the G.o.d Marduk--perhaps the Milky Way in Perseus.

6. The River of Serpents (Serpens, or Hydra).

7. The River of Gan-gal (The High Cloud)--probably the Milky Way as a whole.

There are four serpents represented among the constellations. These are Hydra, Hydrus, Serpens, and Draco.

According to the late Mr. Proctor the date of the building of the Great Pyramid was about 3400 B.C.[452] At this time the Spring Equinox was in Taurus, and this is referred to by Virgil. But this was not so in Virgil's time, when--on account of the precession of the equinoxes--the equinoctial point had already entered Pisces, in which constellation it still remains.

At the date 3400 B.C. the celestial equator ran along the whole length of the constellation Hydra, nearly through Procyon, and a little north of the bright red star Antares.

The star Fomalhaut (a Piscis Australis) is interesting as being the most southern 1st magnitude star visible in England, its meridian alt.i.tude at Greenwich being little more than eight degrees.[453]

With reference to the Greek letters given to the brighter stars by Bayer (in his Atlas published in 1603), and now generally used by astronomers, Mr. Lynn has shown that although "Bayer did uniformly designate the brightest stars in each constellation by the letter a,"[454] it is a mistake to suppose--as has often been stated in popular books on astronomy--that he added the other Greek letters _in order of brightness_.

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 ?