The Stars

Pliny says that Hipparchus “ventured to count the stars, a work arduous even for the Deity.” But this was quite a mistaken idea. Those visible to the naked eye are comparatively few in number, and the enumeration of those visible in an opera-glass—which of course far exceed those which can be seen by unaided vision—is a matter of no great difficulty. Those visible in a small telescope of 2¾ inches aperture have all been observed and catalogued; and even those shown on photographs taken with large telescopes can be easily counted. The present writer has made an attempt in this direction, and taking an average of a large number of counts in various parts of the sky, as shown on stellar photographs, he finds a total of about 64 millions for the whole sky in both hemispheres.[270] Probably the total number will not exceed 100 millions. But this is a comparatively small number, even when compared with the human population of our little globe.

With reference to the charts made by photography in the International scheme commenced some years ago, it has now been estimated that the charts will probably contain a total of about 9,854,000 stars down to about the 14th magnitude (13·7). The “catalogue plates” (taken with a shorter exposure) will, it is expected, include about 2,676,500 stars down to 11½ magnitude. These numbers may, however, be somewhat increased when the work has been completed.[271] If this estimate proves to be correct, the number of stars visible down to the 14th magnitude will be considerably less than former estimates have made it.

Prof. E. C. Pickering estimates that the total number of stars visible on photographs down to the 16th magnitude (about the faintest visible in the great Lick telescope) will be about 50 millions.[272] In the present writer’s enumeration, above referred to, many stars fainter than the 16th magnitude were included.

Admiral Smyth says, with reference to Sir William Herschel—perhaps the greatest observer that ever lived—“As to Sir William himself, he could unhesitatingly call every star down to the 6th magnitude, by its name, letter, or number.”[273] This shows great powers of observation, and a wonderful memory.

On a photographic plate of the Pleiades taken with the Bruce telescope and an exposure of 6 hours, Prof. Bailey of Harvard has counted “3972 stars within an area 2° square, having Alcyone at its centre.”[274] This would give a total of about 41 millions for the whole sky, if of the same richness.

With an exposure of 16 hours, Prof. H. C. Wilson finds on an area of less that 110' square a total of 4621 stars. He thinks, “That all of these stars belong to the Pleiades group is not at all probable. The great majority of them probably lie at immense distances beyond the group, and simply appear in it by projection.”[274] He adds, “It has been found, however, by very careful measurements made during the last 75 years at the KÖnigsbergh and Yale Observatories, that of the sixty-nine brighter stars, including those down to the 9th magnitude, only eight show any certain movement with reference to Alcyone. Since Alcyone has a proper motion or drift of 6 per century, this means that all the brightest stars except the eight mentioned are drifting with Alcyone and so form a true cluster, at approximately the same distance from the earth. Six of the eight stars which show relative drift are moving in the opposite direction to the movement of Alcyone, and at nearly the same rate, so that their motion is only apparent. They are really stationary, while Alcyone and the rest of the cluster are moving past them.”[275] This tends to show that the faint stars are really behind the cluster, and are unconnected with it.

It is a popular idea with some people that the Pole Star is the nearest of all the stars to the celestial pole. But photographs show that there are many faint stars nearer to the pole than the Pole Star. The Pole Star is at present at a distance of 1° 13' from the real pole of the heavens, but it is slowly approaching it. The minimum distance will be reached in the year 2104. From photographs taken by M. Flammarion at the Juvisy Observatory, he finds that there are at least 128 stars nearer to the pole than the Pole Star! The nearest star to the pole was, in the year 1902, a small star of about 12½ magnitude, which was distant about 4 minutes of arc from the pole.[276] The estimated magnitude shows that the Pole Star is nearly 10,000 times brighter than this faint star!

It has been found that Sirius is bright enough to cast a shadow under favourable conditions. On March 22, 1903, the distinguished French astronomer Touchet succeeded in photographing the shadow of a brooch cast by this brilliant star. The exposure was 1^h 5^m.

Martinus Hortensius seems to have been the first to see stars in daylight, perhaps early in the seventeenth century. He mentions the fact in a letter to Gassendi dated October 12, 1636, but does not give the date of his observation. Schickard saw Arcturus in broad daylight early in 1632. Morin saw the same bright star half an hour after sunset in March, 1635.

Some interesting observations were made by Professors Payne and H. C. Wilson, in the summer of 1904, at Midvale, Montana (U.S.A.), at a height of 4790 feet above sea-level. At this height they found the air very clear and transparent. “Many more stars were visible at a glance, and the familiar stars appeared more brilliant.... In the great bright cloud of the Milky Way, between and ? Cygni, one could count easily sixteen or seventeen stars, besides the bright ones ? and ?,[277] while at Northfield it is difficult to distinctly see eight or nine with the naked eye.” Some nebulÆ and star fields were photographed with good results by the aid of a 2½-inch Darlot lens and 3 hours’ exposure.[278]

Prof. Barnard has taken some good stellar photographs with a lens of only 1½ inches in diameter, and 4 or 5 inches focus belonging to an ordinary “magic lantern”! He says that these “photographs with the small lens show us in the most striking manner how the most valuable and important information may be obtained with the simplest means.”[279]

With reference to the rising and setting of the stars due to the earth’s rotation on its axis, the late Sir George B. Airy, Astronomer Royal of England, once said to a schoolmaster, “I should like to know how far your pupils go into the first practical points for which reading is scarcely necessary. Do they know that the stars rise and set? Very few people in England know it. I once had a correspondence with a literary man of the highest rank on a point of Greek astronomy, and found that he did not know it!”[280]

Admiral Smyth says, “I have been struck with the beautiful blue tint of the smallest stars visible in my telescope. This, however, may be attributed to some optical peculiarity.” This bluish colour of small stars agrees with the conclusion arrived at by Prof. Pickering in recent years, that the majority of faint stars in the Milky Way have spectra of the Sirian type and, like that brilliant star, are of a bluish white colour. Sir William Herschel saw many stars of a redder tinge than other observers have noticed. Admiral Smyth says, “This may be owing to the effect of his metallic mirror or to some peculiarity of vision, or perhaps both.”[281]

The ancient astronomers do not mention any coloured stars except white and red. Among the latter they only speak of Arcturus, Aldebaran, Pollux, Antares, and Betelgeuse as of a striking red colour. To these Al-Sufi adds Alphard (a HydrÆ).

Sir William Herschel remarked that no decidedly green or blue star “has ever been noticed unassociated with a companion brighter than itself.” An exception to Herschel’s rule seems to be found in the case of the star LibrÆ, which Admiral Smyth called “pale emerald.” Mr. George Knott observed it on May 19, 1852, as “beautiful pale green” (3·7 inches achromatic, power 80), and on May 9, 1872, as “fine pale green” (5·5 inches achromatic, power 65).

The motion of stars in the line of sight, as shown by the spectroscope—should theoretically alter their brightness in the course of time; those approaching the earth becoming gradually brighter, while those receding should become fainter. But the distance of the stars is so enormous that even with very high velocities the change would not become perceptible for ages. Prof. Oudemans found that to change the brightness of a star by only one-tenth of a magnitude—a quantity barely perceptible to the eye-a number of years would be necessary, which is represented by the formula

for a star approaching the earth, and for a receding star

This is in geographical miles, 1 geographical mile being equal to 4·61 English miles.

Reducing the above to English miles, and taking an average for both approaching and receding stars, we have

where p = parallax in seconds of arc, and m = radial velocity in English miles per second.

Prof. Oudemans found that the only star which could have changed in brightness by one-tenth of a magnitude since the time of Hipparchus is Aldebaran. This is taking its parallax as 0·52. But assuming the more reliable parallax 0·12 found by Dr. Elkin, this period is 4? times longer. For Procyon, the period would be 5500 years.[282] The above calculation shows how absurd it is to suppose that any star could have gained or lost in brightness by motion in the line of sight during historical times. The “secular variation” of stars is quite another thing. This is due to physical changes in the stars themselves.

The famous astronomer Halley, the second Astronomer Royal at Greenwich, says (Phil. Trans., 1796), “Supposing the number of 1st magnitude stars to be 13, at twice the distance from the sun there may be placed four times as many, or 52; which with the same allowance would nearly represent the star we find to be of the 2nd magnitude. So 9 × 13, or 117, for those at three times the distance; and at ten times the distance 100 × 13, or 1300 stars; of which distance may probably diminish the light of any of the stars of the 1st magnitude to that of the 6th, it being but the hundredth part of what, at their present distance, they appear with.” This agrees with the now generally accepted “light ratio” of 2·512 for each magnitude, which makes a first magnitude star 100 times the light of a 6th magnitude.

On the 4th of March, 1796,[283] the famous French astronomer Lalande observed on the meridian a star of small 6th magnitude, the exact position of which he determined. On the 15th of the same month he again observed the star, and the places found for 1800 refer to numbers 16292-3 of the reduced catalogue. In the observation of March 4 he attached the curious remark, “Étoile singuliÈre” (the observation of March 15 is without note). This remark of Lalande has puzzled observers who failed to find any peculiarity about the star. Indeed, “the remark is a strange one for the observer of so many thousands of stars to attach unless there was really something singular in the star’s aspect at the time.” On the evening of April 18, 1887, the star was examined by the present writer, and the following is the record in his observing book, “Lalande’s Étoile singuliÈre (16292-3) about half a magnitude less than ? Cancri. With the binocular I see two streams of small stars branching out from it, north preceding like the tails of comet.” This may perhaps have something to do with Lalande’s curious remark.

The star numbered 1647 in Baily’s Flamsteed Catalogue is now known to have been an observation of the planet Uranus.[284]

Prof. Pickering states that the fainter stars photographed with the 8-inch telescope at Cambridge (U.S.A.) are invisible to the eye in the 15-inch telescope.[285]

Sir Norman Lockyer finds that the lines of sulphur are present in the spectrum of the bright star Rigel ( Orionis).[286]

About 8½° south of the bright star Regulus (a Leonis) is a faint nebula (H I, 4 Sextantis). On or near this spot the Capuchin monk De Rheita fancied he saw, in the year 1643, a group of stars representing the napkin of S. Veronica—“sudarium VeronicÆ sive faciem Domini maxima similitudina in astris expressum.” And he gave a picture of the napkin and star group. But all subsequent observers have failed to find any trace of the star group referred to by De Rheita![287]

The Bible story of the star of the Magi is also told in connection with the birth of the sun-gods Osiris, Horus, Mithra, Serapis, etc.[288] The present writer has also heard it suggested that the phenomenon may have been an apparition of Halley’s comet! But as this famous comet is known to have appeared in the year B.C. 11, and as the date of the Nativity was probably not earlier than B.C. 5, the hypothesis seems for this (and other reasons) to be inadmissible. It has also been suggested that the phenomenon might have been an appearance of Tycho BrahÉ’s temporary star of 1572, known as the “Pilgrim star”; but there seems to be no real foundation for such an hypothesis. There is no reason to think that “temporary” or new stars ever appear a second time.

Admiral Smyth has well said, “It checks one’s pride to recollect that if our sun with the whole system of planets, asteroids, and moons, and comets were to be removed from the spectator to the distance of the nearest fixed star, not one of them would be visible, except the sun, which would then appear but as a star of perhaps the 2nd magnitude. Nay, more, were the whole system of which our globe forms an insignificant member, with its central luminary, suddenly annihilated, no effect would be produced on those unconnected and remote bodies; and the only annunciation of such a catastrophe in the Sidereal “Times” would be that a small star once seen in a distant quarter of the sky had ceased to shine.”[289]

Prof. George C. Comstock finds that the average parallax of 67 selected stars ranging in brightness between the 9th and the 12th magnitude, is of the value of 0·0051.[290] This gives a distance representing a journey for light of about 639 years!

Mr. Henry Norris Russell thinks that nearly all the bright stars in the constellation of Orion are practically at the same distance from the earth. His reasons for this opinion are: (1) the stars are similar in their spectra and proper motions, (2) their proper motions are small, which suggests a small parallax, and therefore a great distance from the earth. Mr. Russell thinks that the average parallax of these stars may perhaps be 0·005, which gives a distance of about 650 “light years.”[291]According to Sir Norman Lockyer’s classification of the stars, the order of increasing temperature is represented by the following, beginning with those in the earliest stage of stellar evolution:—NebulÆ, Antares, Aldebaran, Polaris, a Cygni, Rigel, e Tauri, Crucis. Then we have the hottest stars represented by e Puppis, ? Argus, and Alnitam (e Orionis). Decreasing temperature is represented by (in order), Achernar, Algol, Markab, Sirius, Procyon, Arcturus, 19 Piscium, and the “Dark Stars.”[292] But other astronomers do not agree with this classification. Antares and Aldebaran are by some authorities considered to be cooling suns.

According to Ritter’s views of the Constitution of the Celestial Bodies, if we “divide the stars into three classes according to age corresponding to these three stages of development, we shall assign to the first class, A, those stars still in the nebular phase of development; to the second class, B, those in the transient stage of greatest brilliancy; and to the class C, those stars which have already entered into the long period of slow extinction. It should be noted in this classification that we refer to relative and not absolute age, since a star of slight mass passes through the successive phases of its development more rapidly than the star of greater mass.”[293] Ritter comes to the conclusion that “the duration of the period in which the sun as a star had a greater brightness than at present was very short in comparison with the period in which it had and will continue to have a brightness differing only slightly from its present value.”[294]

In a valuable and interesting paper on “The Evolution of Solar Stars,”[295] Prof. Schuster says that “measurements by E. F. Nichols on the heat of Vega and Arcturus indicated a lower temperature for Arcturus, and confirms the conclusion arrived at on other grounds, that the hydrogen stars have a higher temperature than the solar stars.” “An inspection of the ultraviolet region of the spectrum gives the same result. These different lines of argument, all leading to the same result, justify us in saying that the surface temperature of the hydrogen stars is higher than that of the solar stars. An extension of the same reasoning leads to the belief that the helium stars have a temperature which is higher still.” Hence we have Schuster, Hale, and Sir William Huggins in agreement that the Sirian stars are hotter than the solar stars; and personally I agree with these high authorities. The late Dr. W. E. Wilson, however, held the opinion that the sun is hotter that Sirius!

Schuster thinks that Lane’s law does not apply to the temperature of the photosphere and the absorbing layers of the sun and stars, but only to the portions between the photosphere and the centre, which probably act like a perfect gas. On this view he says the interior might become “hotter and hotter until the condensation had reached a point at which the laws of gaseous condensation no longer hold.”

With reference to the stars having spectra of the 3rd and 4th type (usually orange and red in colour), Schuster says—

“The remaining types of spectra belong to lower temperature still, as in place of metallic lines, or in addition to them, certain bands appear which experiments show us invariably belong to lower temperature than the lines of the same element.

“If an evolutionary process has been going on, which is similar for all stars, there is little doubt that from the bright-line stars down to the solar stars the order has been (1) helium or Orion stars, (2) hydrogen or Sirian stars, (3) calcium or Procyon stars, (4) solar or Capellan stars.”