Astronomical Curiosities Part 10

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 Ga.s.sendi 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 nebulae and star fields were photographed with good results by the aid of a 2-inch Darlot lens and 3 hours' exposure.[278]

Prof. Barnard has taken some good stellar photographs with a lens of only 1 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 Hydrae).

Sir William Herschel remarked that no decidedly green or blue star "has ever been noticed una.s.sociated with a companion brighter than itself." An exception to Herschel's rule seems to be found in the case of the star Librae, which Admiral Smyth called "pale emerald." Mr. George Knott observed it on May 19, 1852, as "beautiful pale green" (37 inches achromatic, power 80), and on May 9, 1872, as "fine pale green" (55 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 quant.i.ty barely perceptible to the eye-a number of years would be necessary, which is represented by the formula

5916 years ----------------- parallax motion

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

6195 years ---------- p m

This is in geographical miles, 1 geographical mile being equal to 461 English miles.

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

27,660 years ------------ p m

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 a.s.suming 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 2512 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, "etoile singuliere" (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 etoile singuliere (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 Ura.n.u.s.[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 8 south of the bright star Regulus (a Leonis) is a faint nebula (H I, 4 s.e.xtantis). 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 Veronicae 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-G.o.ds 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 Brahe'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 cla.s.sification of the stars, the order of _increasing_ temperature is represented by the following, beginning with those in the earliest stage of stellar evolution:--Nebulae, 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 cla.s.sification. Antares and Aldebaran are by some authorities considered to be _cooling_ suns.

According to Ritter's views of the Const.i.tution of the Celestial Bodies, if we "divide the stars into three cla.s.ses according to age corresponding to these three stages of development, we shall a.s.sign to the first cla.s.s, A, those stars still in the nebular phase of development; to the second cla.s.s, B, those in the transient stage of greatest brilliancy; and to the cla.s.s C, those stars which have already entered into the long period of slow extinction. It should be noted in this cla.s.sification that we refer to relative and not absolute age, since a star of slight ma.s.s pa.s.ses through the successive phases of its development more rapidly than the star of greater ma.s.s."[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."

My investigations on "The Secular Variation of Starlight" (_Studies in Astronomy_, chap. 17, and _Astronomical Essays_, chap. 12) based on a comparison of Al-Sufi's star magnitudes (tenth century) with modern estimates and measures, tend strongly to confirm the above views.

With regard to the 3rd-type stars, such as Betelgeuse and Mira Ceti, Schuster says, "It has been already mentioned that observers differ as to whether their position is anterior to the hydrogen or posterior to the solar stars, and there are valid arguments on both sides."

Scheiner, however, shows, from the behaviour of the lines of magnesium, that stars of type I. (Sirian) are the hottest, and type III. the coolest, and he says, we have "for the first time a direct proof of the correctness of the physical interpretation of Vogel's spectral cla.s.ses, according to which cla.s.s II. is developed by cooling from I., and III. by a further process of cooling from II."[296]

Prof. Hale says that "the resemblance between the spectra of sun-spots and of 3rd-type stars is so close as to indicate that the same cause is controlling the relative intensities of many lines in both instances. This cause, as the laboratory work indicates, is to be regarded as reduced temperature."[297]

According to Prof. Schuster, "a spectrum of bright lines may be given by a ma.s.s of luminous gas, even if the gas is of great thickness. There is, therefore, no difficulty in explaining the existence of stars giving bright lines." He thinks that the difference between "bright line" stars and those showing dark lines depends upon the rate of increase of the temperature from the surface towards the centre. If this rate is slow, bright lines will be seen. If the rate of increase is rapid, the dark-line spectrum shown by the majority of the stars will appear. This rate, he thinks, is regulated by the gravitational force. So that in the early stages of condensation bright lines are more likely to occur. "If the light is not fully absorbed," both bright and dark lines of the same element may be visible in the same star. Schuster considers it quite possible that if we could remove the outer layers of the Sun's atmosphere, we should obtain a spectrum of bright lines.[298]

M. Stratonoff finds that stars having spectra of the Orion and Sirian types--supposed to represent an early stage in stellar evolution--tend to congregate in or near the Milky Way. Star cl.u.s.ters in general show a similar tendency, "but to this law the globular cl.u.s.ters form an exception."[299] We may add that the spiral nebulae--which seem to be scattered indifferently over all parts of the sky--also seem to form an exception; for the spectra of these wonderful objects seem to show that they are really star cl.u.s.ters, in which the components are probably relatively small; that is, small in comparison with our sun.

If we accept the hypothesis that suns and systems were evolved from nebulae, and if we consider the comparatively small number of nebulae hitherto discovered in the largest telescopes--about half a million; and if we further consider the very small number of red stars, or those having spectra of the third and fourth types--usually considered to be dying-out suns--we seem led to the conclusion that our sidereal system is now at about the zenith of its life-history; comparatively few nebulae being left to consolidate into stars, and comparatively few stars having gone far on the road to the final extinction of their light.

Prof. Boss of Albany (U.S.A.) finds that about forty stars of magnitudes from 3 to 7 in the constellation Taurus are apparently drifting together towards one point. These stars are included between about R.A.

3{h} 47{m} to 5{h} 4{m}, and Declination + 5 to + 23 (that is, in the region surrounding the Hyades). These motions apparently converge to a point near R.A. 6{h}, Declination + 7 (near Betelgeuse). Prof. Boss has computed the velocity of the stars in this group to be 456 kilometres (about 28 miles) a second towards the "vanishing point," and he estimated the average parallax of the group to be 0"025--about 130 years' journey for light. Although the motions are apparently converging to a point, it does not follow that the stars in question will, in the course of ages, meet at the "vanishing point." On the contrary, the observed motions show that the stars are moving in parallel lines through s.p.a.ce. About 15 kilometres of the observed speed is due to the sun's motion through s.p.a.ce in the opposite direction. Prof. Campbell finds from spectroscopic measures that of these forty stars, nine are receding from the earth with velocities varying from 12 to 60 kilometres a second, and twenty-three others with less velocities than 38 kilometres.[300] It will be obvious that, as there is a "vanishing point," the motion in the line of sight must be one of _recession_ from the earth.

It has been found that on an average the parallax of a star is about one-seventh of its "proper motion."[301]

Adopting Prof. Newcomb's parallax of 0"14 for the famous star 1830 Groombridge, the velocity perpendicular to the line of sight is about 150 miles a second. The velocity _in_ the line of sight--as shown by the spectroscope--is 59 miles a second approaching the earth. Compounding these two velocities we find a velocity through s.p.a.ce of about 161 miles a second!

An eminent American writer puts into the mouth of one of his characters, a young astronomer, the following:--

"I read the page Where every letter is a glittering sun."

From an examination of the heat radiated by some bright stars, made by Dr. E. F. Nicholls in America with a very sensitive radiometer of his own construction, he finds that "we do not receive from Arcturus more heat than we should from a candle at a distance of 5 or 6 miles."

With reference to the progressive motion of light, and the different times taken by light to reach the earth from different stars, Humboldt says, "The aspect of the starry heavens presents to us objects of _unequal date_. Much has long ceased to exist before the knowledge of its presence reaches us; much has been otherwise arranged."[302]