Astronomical Curiosities Part 13

Burnham found no peculiarity about its telescopic image. A small and very faint nebula was found by Burnham a little following (that is east of) the _nova_.[343]

The following details of the great new star of 1572--the "Pilgrim Star" of Tycho Brahe--are given by Delambre.[344] In November, 1572, it was brighter than Sirius, Vega, and Jupiter, and almost equal to Venus at its brightest. During December it resembled Jupiter in brightness. In January, 1573, it was fainter and only a little brighter than stars of the 1st magnitude. In February and March it was equal to 1st magnitude stars, and in April and May was reduced to the 2nd magnitude. In June and July it was 3rd magnitude; in September of the 4th, and at the end of 1573 it was reduced to the 5th magnitude. In February, 1574, it was 6th magnitude, and in March of the same year it became invisible to the naked eye.

From this account it will be seen that the decrease in light of this curious object was much slower than that of Nova Persei (1901) ("the new star of the new century"). This would suggest that it was a much larger body.

There were also changes in its colour. When it was of the brightness of Venus or Jupiter it shone with a white light. It then became golden, and afterwards reddish like Mars, Aldebaran, or Betelgeuse. It afterwards became of a livid white colour like Saturn, and this it retained as long as it was visible. Tycho Brahe thought that its apparent diameter might have been about 3 minutes of arc, and that it was possibly 361 times smaller than the earth(!) But we now know that these estimates were probably quite erroneous.

Temporary stars were called by the ancient Chinese "Ke-sing," or guest stars.[345]

A temporary star recorded by Ma-tuan-lin (Chinese Annals) in February, 1578, is described as "a star as large as the sun." But its position is not given.[346]

About the middle of September, 1878, Mr. Greely, of Boston (U.S.A.), reported to Mr. E. F. Sawyer (the eminent observer of variable stars) that, about the middle of August of that year, he had seen the famous variable star Mira Ceti of about the 2nd magnitude, although the star did not attain its usual maximum until early in October, 1878. Mr. Greely stated that several nights after he first saw Mira it had faded to the 4th or 5th magnitude. If there was no mistake in this observation (and Sawyer could find none) it was quite an unique phenomenon, as nothing of the sort has been observed before or since in the history of this famous star. It looks as if Mr. Greely had observed a new or "temporary" star near the place of Mira Ceti; but as the spot is far from the Milky Way, which is the usual seat of such phenomena, this hypothesis seems improbable.

In the so-called Cepheid and Geminid variables of short period, the princ.i.p.al characteristics of the light variation are as follows:--

"1. The light varies without pause.

"2. The amount of their light variation is usually about 1 magnitude.

"3. Their periods are short--a few days only.

"4. They are of a spectral type approximately solar; no Orion, Sirian or Arcturian stars having been found among them.

"5. They seem to be found in greater numbers in certain parts of the sky, notably in the Milky Way, but exhibit no tendency to form cl.u.s.ters.

"6. All those stars whose radial velocities have been studied have been found to be binaries whose period of orbital revolution coincides with that of their light change.

"7. The orbits, so far as determined, are all small, _a_ sin _i_ being 2,000,000 kilometres or less.

"8. Their maximum light synchronizes with their maximum velocity of approach, and minimum light with maximum velocity of recession.

"9. No case has been found in which the spectrum of more than one component has been bright enough to be recorded in the spectrograms."[347]

It is very difficult to find an hypothesis which will explain satisfactorily _all_ these characteristics, and attempts in this direction have not proved very successful. Mr. J. C. Duncan suggests the action of an absorbing atmosphere surrounding the component stars.

On March 30, 1612, Scheiner saw a star near Jupiter. It was at first equal in brightness to Jupiter's satellites. It gradually faded, and on April 8 of the same year it was only seen with much difficulty in a very clear sky. "After that date it was never seen again, although carefully looked for under favourable conditions."

An attempted identification of Scheiner's star was made in recent years by Winnecke. He found that its position, as indicated by Scheiner, agrees with that of the Bonn _Durchmusterung_ star 15, 2083 (8 magnitude).

This star is not a known variable. Winnecke watched it for 17 years, but found no variation of light. From Scheiner's recorded observations his star seems to have reached the 6th magnitude, which is considerably brighter than the _Durchmusterung_ star watched by Winnecke.[348]

With reference to the colours of the stars, the supposed change of colour in Sirius from red to white is well known, and will be considered in the chapter on the Constellations. The bright star Arcturus has also been suspected of variation in colour. About the middle of the nineteenth century Dr. Julius Schmidt, of Athens, the well-known observer of variable stars, thought it one of the reddest stars in the sky, especially in the year 1841, when he found its colour comparable with that of the planet Mars.[349] In 1852, however, he was surprised to find it yellow and devoid of any reddish tinge; in colour it was lighter than that of Capella. In 1863, Mr. Jacob Ennis found it "decidedly orange." Ptolemy and Al-Sufi called it red.

Mr. Ennis speaks of Capella as "blue" (cla.s.sing it with Rigel), and comparing its colour with that of Vega![350] But the present writer has never seen it of this colour. To his eye it seems yellowish or orange. It was called red by Ptolemy, El Fergani, and Riccioli; but Al-Sufi says nothing about its colour.

Of Ursae Minoris, Heis, the eminent German astronomer said, "I have had frequent opportunities of convincing myself that the colour of this star is not always equally red; at times it is more or less yellow, at others most decidedly red."[351]

Among double stars there are many cases in which variation of colour has been suspected. In some of these the difference in the recorded colour may possibly be due to "colour blindness" in some of the observers; but in others there seems to be good evidence in favour of a change. The following may be mentioned:--

? Ca.s.siopeiae. Magnitudes of the components about 4 and 7. Recorded as red and green by Sir John Herschel and South; but yellow and orange by Sestini.

? Trianguli. Magnitudes 5 and 7. Secchi estimated them as white or yellow and blue; but Webb called them yellow and green (1862).

? Leonis, 2 and 3. Sir William Herschel noted them white and reddish white; but Webb, light orange and greenish yellow.

12 Canum Venaticorum, 2 and 6. White and red, Sir William Herschel; but Sir John Herschel says in 1830, "With all attention I could perceive no contrast of colours in the two stars." Struve found them both white in 1830, thus agreeing with Sir John Herschel. Sestini saw them yellow and blue in 1844; Smyth, in 1855, pale reddish white and lilac; Dembowski, in 1856, white and pale olive blue; and Webb, in 1862, flushed white and pale lilac.

On October 13, 1907, Nova Persei, the great new star of 1901, was estimated to be only 1144 magnitude, or about 11. When at its brightest this famous star was about zero magnitude; so that it has in about 6 years faded about 11 magnitudes in brightness; in other words, it has been reduced to 1/40000 of its greatest brilliancy!

CHAPTER XVII

Nebulae and Cl.u.s.ters

In his interesting and valuable work on "The Stars," the late Prof.

Newcomb said--

"Great numbers of the nebulae are therefore thousands of times the dimensions of the earth's...o...b..t, and most of them are thousands of times the dimensions of the whole solar system. That they should be completely transparent through such enormous dimensions shows their extreme tenuity. Were our solar system placed in the midst of one of them it is probable that we should not be able to find any evidence of its existence"!

Prof. Perrine thinks that the total number of the nebulae will ultimately be found to exceed a million.[352]

Dr. Max Wolf has discovered a number of small nebulae in the regions near Algol and Nova Persei (the great "new star" of 1901). He says, "They mostly lie in two bands," and are especially numerous where the two bands meet, a region of 12 minutes of arc square containing no less than 148 of them. They are usually "round with central condensation," and form of Andromeda nebula.[353]

Some small nebulae have been found in the vicinity of the globular cl.u.s.ters. They are described by Prof. Perrine as very small and like an "out of focus" image of a small star. "They appear to be most numerous about cl.u.s.ters which are farthest from the galaxy." Prof. Perrine says, "Practically all the small nebulae about the globular cl.u.s.ters are elliptical or circular. Those large enough to show structure are spirals.

Doubtless the majority of these are spirals."[354] This seems further evidence in favour of the "spiral nebular hypothesis" of Chamberlin and Moulton.

A great photographic nebula in Orion was discovered by Prof. Barnard in 1894. In a drawing he gives of the nebula,[355] it forms a long streak beginning a little south of ? Orionis (Bellatrix), pa.s.sing through the star 38 Orionis north of 51 and south of 56 and 60 Orionis. Then turning south it sweeps round a little north of ? Orionis; then over 29 Orionis, and ends a little to the west of ? Orionis. There is an outside patch west of Rigel. Barnard thinks that the whole forms a vast spiral structure; probably connected with the "great nebula" in the "sword of Orion," which it surrounds.

From calculations of the brightness of surface ("intrinsic brightness") of several "planetary" nebulae made by the present writer in the year 1905, he finds that the luminosity is very small compared with that of the moon.

The brightest of those examined (_h_ 3365, in the southern hemisphere, near the Southern Cross) has a surface luminosity of only 1/400 of that of the moon.[356] The great nebulae in Orion and Andromeda seem to have "still smaller intrinsic brightness."

Arago says--

"The s.p.a.ces which precede or which follow simple nebulae, and _a fortiori_ groups of nebulae, contain generally few stars. Herschel found this rule to be invariable. Thus every time that, during a short interval, no star appeared, in virtue of the diurnal motion, to place itself in the field of his motionless telescope, he was accustomed to say to the secretary who a.s.sisted him (Miss Caroline Herschel), 'Prepare to write; nebulae are about to arrive.'"[357]

Commenting on this remark of Arago, the late Herbert Spencer says--

"How does this fact consist with the hypothesis that nebulae are remote galaxies? If there were but one nebula, it would be a curious coincidence were this one nebula so placed in the distant regions of s.p.a.ce as to agree in direction with a starless spot in our sidereal system! If there were but two nebulae, and both were so placed, the coincidence would be excessively strange. What shall we say on finding that they are habitually so placed? (the last five words replace some that are possibly a little too strong).... When to the fact that the general ma.s.s of nebulae are ant.i.thetical in position to the general ma.s.s of the stars, we add the fact that local regions of nebulae are regions where stars are scarce, and the further fact that single nebulae are habitually found in comparatively starless spots, does not the proof of a physical connection become overwhelming?"[358]

With reference to the small elongated nebula discovered by Miss Caroline Herschel in 1783 near the great nebula in Andromeda, Admiral Smyth says, "It lies between two sets of stars, consisting of four each, and each disposed like the figure 7, the preceding group being the smallest."[359]

Speaking of the "nebula" Messier 3--a globular cl.u.s.ter in Canes Venatici--Admiral Smyth says, "This ma.s.s is one of those b.a.l.l.s of compact and wedged stars whose laws of aggregation it is so impossible to a.s.sign; but the rotundity of the figure gives full indication of some general attractive bond of union."[360] The terms "compact and wedged" are, however, too strong, for we know that in the globular cl.u.s.ters the component stars must be separated from each other by millions of miles!

Prof. Chamberlin suggests that the secondary nebula (as it is called) in the great spiral in Canes Venatici (Messier 51) may possibly represent the body which collided with the other (the chief nucleus) in a grazing collision, and is now escaping. He considers this secondary body to have been "a dead sun"--that is, a dark body.[361] This would be very interesting if it could be proved. But it seems to me more probable that the secondary nucleus is simply a larger portion of the ejected matter, which is now being gradually detached from the parent ma.s.s.

Scheiner says "the previous suspicion that the spiral nebulae are star cl.u.s.ters is now raised to a certainty," and that the spectrum of the Andromeda nebula is very similar to that of the sun. He says there is "a surprising agreement of the two, even in respect to the relative intensity of the separate spectral regions."[362]