William F. Denning.

Telescopic Work for Starlight Evenings online

. (page 27 of 32)
Online LibraryWilliam F. DenningTelescopic Work for Starlight Evenings → online text (page 27 of 32)
Font size
QR-code for this ebook


_New or Temporary Stars._—These stars (sometimes classed with variable
stars) furnish us with rare instances of vast physical changes
occurring among sidereal objects, usually so steadfast and endurable.
The alternating lustre of certain variable stars represents phenomena
of regular recurrence, and is probably to be explained by simple
causes; but the sudden outbursts and rapid decline of temporary stars
are facts of a more startling character, and need a more exceptional
explanation. The first of these objects recorded in history appears
to have been seen in Scorpio 134 years before the Christian era, and
it suggested to Hipparchus of Rhodes the idea of forming a catalogue
of stars, so that in future ages observers might have the means of
recognizing new stars or any other changes in the configuration of the
heavens. Hipparchus completed his catalogue in 128 B.C.; it contained
1025 stars, and forms one of the most valuable memorials we possess of
the labours of the ancient astronomers. Another temporary star is said
to have appeared in 130 A.D., but this and several other objects of
presumably similar character noticed in later years may just possibly
have been comets, and considerable doubt hangs over the descriptions.
It will therefore be safest to confine our remarks to more modern and
better attested instances of these phenomena[53]:—

1572, November 11.—The famous star of Tycho Brahe. He thus described
his first view of it:—“One evening when I was considering, as
usual, the celestial vault, the aspect of which is so familiar to
me, I perceived with indescribable astonishment a bright star of
extraordinary magnitude near the zenith in the constellation of
Cassiopeia.” He adds:—“The new star was destitute of a tail, or of
any appearance of nebulosity; it resembled the other stars in all
respects, only that it twinkled even more than stars of the first
magnitude. In brightness it surpassed Sirius, α Lyræ, or Jupiter. It
could be compared in this respect only to Venus when she is nearest
the earth (when a fourth part of her illuminated surface is turned
towards us). Persons who were gifted with good sight could distinguish
the star in the daytime, even at noon, when the sky was clear.” This
brilliant NOVA began to fade early in Dec. 1572, and in April and May
1573 it resembled a star of the 2nd mag., in July and Aug. one of the
3rd mag., and in Oct. and Nov. one of the 4th mag. In March 1574 the
star completely disappeared (to the naked eye), after a visibility of
about 17 months. It exhibited some curious variations of colour. It
was white when most brilliant; it then became yellow, and afterwards
red, so that its hue in the early part of 1573 was similar to that
of Mars. But in May it again became white, and continued so until it
ceased to be visible. The position of this star (for 1890) is R.A.
0^h 18^m 41^s, Dec. +63° 32′·2. It was supposed to be a reapparition
of the brilliant stars which shone between Cepheus and Cassiopeia in
945 and 1264, and to have possibly been associated with the “Star of
Bethlehem;” but there is no reliable evidence on which this view can
be supported, as the alleged “stars” of 945 and 1264 were undoubtedly
comets, misdescribed in old records. Cornelius Gemma is reputed to have
seen the celebrated star of 1572 a few days before Tycho Brahe, viz.,
on November 8, 1572.

1604, October 10.—Discovered by Brunowski, who announced it to
Kepler. It was brighter than a star of the 1st mag., also than Mars,
Jupiter, or Saturn, which were not far distant at the time. It did
not begin to fade immediately; for a month after its discovery it was
still brighter than Jupiter, and of a white lustre. At the middle of
November it surpassed Antares, but was inferior to Arcturus. In April
1605 it had fallen to the 3rd mag., and went on decreasing until in
October it could scarcely be seen with the naked eye owing to the
twilight resulting from its proximity to the Sun. In March 1606 it was
invisible. The position of this object was about midway between ξ and
58 Ophiuchi, or at R.A. 17^h 24^m, Dec.-21° 207′ (1890).

1670, June 20.—Discovered by the Carthusian Monk Anthelme in R.A. 19^h
43^m 3^s, Dec. +27° 3′ (1890), a few degrees east of β Cygni. It was
of the 3rd mag., and continued in view, with constantly fluctuating
brightness, for nearly two years. At the end of March 1672 it was
6th mag., and has never reappeared. Hind found a small, hazy, and
ill-defined star in the same place, but this is probably not the same
as Anthelme’s star of 1670.

1848, April 28.—During the long interval of 178 years separating 1670
from 1848 not a single new star appears to have revealed itself.
Observers had multiplied, astronomical instruments had been much
improved, star-catalogues were plentiful, and yet the sidereal heavens
gave no intimation of a stellar outburst. No better proof than this
could be afforded as to the great rarity of temporary stars. At length,
in the spring of 1848, the spell was broken, and Mr. Hind announced
that a new star of a reddish-yellow colour had appeared in Ophiuchus,
R.A. 16^h 53^m 20^s, Dec.-12° 43′ (1890). He expressed himself as
certain that no star brighter than the 9th mag. had been there previous
to April 5. After rising to the 4th mag. it soon faded, and in 1851
could only be observed in large instruments. In 1875 it was still
visible as a very minute star.

1860, May 21.—M. Auwers, of Konigsberg, noticed a star of the 7th mag.
near the centre of the bright resolvable nebula (M. 80), lying between
α and β Scorpii, R.A. 16^h 10^m 29^s, Dec.-22° 42′ (1890). On May 18
the star was not there, and it disappeared altogether in three weeks.
It was independently seen by Pogson on May 28, and to him it seemed
that “the nebula had been _replaced_ by a star, so entirely were its
dim rays overpowered by the concentrated blaze in their midst.”

1866, May 12.—Discovered by Birmingham at Tuam. It was of the 2nd mag.,
and situated in Corona, R.A. 15^h 54^m 54^s, Dec. +26° 14′ (1890).
The outburst must have been very sudden, as Schmidt, at Athens, was
observing this region three hours before the new star was detected, and
is certain it was then fainter than the 4th mag. The star was found
to be identical with one on Argelander’s charts estimated as 9½ mag.
It faded from the 2nd to the 6th mag. by May 20, and was thereafter
invisible to the naked eye.

1876, Nov. 24.—A yellow star of the 3rd mag. was seen by the ever
vigilant Schmidt at Athens near ρ Cygni, and where no such star existed
on Nov. 20. The position of the object was R.A. 21^h 37^m 23^s, Dec.
+42° 20′ (1890). It soon grew fainter, so that on Dec. 13 it was of the
6th mag. and devoid of colour. In the spectroscope it presented much
the same lines as Birmingham’s star of May 1866. In addition to the
continuous spectrum it showed bright lines of hydrogen.

1885, August 31.—Dr. Hartwig announced the appearance of a star-like
nucleus in the great nebula (M. 31) of Andromeda, R.A. 0^h 36^m 43^s,
Dec. +40° 40′ (1890). Other observers soon corroborated the discovery.
The star appears to have been first seen on Aug. 19; it was not visible
on the preceding night. On Sept. 1 its mag. was 6·5, on Sept. 2, 7·3,
on Sept. 3, 7·2, Sept. 4, 8·0, Sept. 18, 9·2, &c. On Feb. 7, 1886,
it had dwindled down to the 16th mag., according to an estimate made
by Prof. Hall with the great Washington refractor. The spectrum was
continuous, and Proctor and Gore considered “that the evidence of the
spectroscope showed that the new star was situated _in_ the nebula.”

The phenomena presented by the temporary stars alluded to are so
different to those of ordinary variables that it is very questionable
whether they ought to be classed together. Our knowledge of the former
would no doubt progress more rapidly were they specially looked for
and more instances discovered. Those who have acquired a familiar
acquaintance with the naked-eye stars should examine them as often as
possible with this end in view. Some of these objects lose light so
quickly that unless they are caught near the maximum they are likely to
escape altogether, and this shows the necessity of being constantly on
the alert for their appearance. I have frequently, while watching for
meteors, reviewed the different constellations in the hope of picking
up a new object, but have never succeeded in doing so.

_Star Colours_ form another interesting department of sidereal
astronomy. It is obviously desirable to record the hues presented,
not only by double stars and binary systems, but by isolated stars
also, as changes of tint have been strongly suspected. Cicero, Seneca,
Ptolemy, and others speak of Sirius as a red star, whereas it is
now an intense white; and if we rely on ancient descriptions similar
changes appear to have affected some other prominent stars. But the
old records cannot be implicitly trusted, owing to the errors of
transcribers and translators; and Mr. Lynn (‘Observatory’, vol. ix. p.
104) quotes facts tending to disprove the idea that Sirius was formerly
a red star. In the majority of cases double stars are of the same
colour, but there are many pairs in which the complementary colours
are very decided. Chambers remarks that the brighter star is usually
of a ruddy or orange hue, and the smaller one blue or green. “Single
stars of a fiery red or deep orange are not uncommon, but isolated
blue or green stars are very rare. Amongst conspicuous stars β Libræ
(green) appears to be the only instance.” As an example of fiery-red
stars Antares may be mentioned; Aldebaran is deep reddish orange, and
Betelgeuse reddish orange. Amongst the more prominent stars Capella,
Rigel, and Procyon may be mentioned as showing a bluish tinge, Altair
and Vega are greenish, Arcturus is yellow, while Sirius, Deneb,
Polaris, Fomalhaut, and Regulus are white. Mr. Birmingham published a
catalogue of “The Red Stars” in the ‘Transactions of the Royal Irish
Academy’, for August 1877, and Mr. Chambers has a _working-catalogue_
of 719 such objects in the ‘Monthly Notices,’ vol. xlvii. pp. 348-387.
The region of Cygnus appears to be especially prolific in red stars,
and many of these objects are variable. In a paper read at a recent
meeting of the Astronomical Society of the Pacific Mr. Pierson stated
that in binary systems where the stars are of equal magnitude the
colours are invariably the same, while those differing in magnitude
differ also in colour and the larger star is always nearer the red end
of the spectrum than its secondary. In the estimation of star-colours
reflecting-telescopes are very reliable owing to their perfect
achromatism.

_Groups of Stars._—Great dissimilarity is apparent in the clustering of
stars. The heavens furnish us with all gradations—from the loose, open
groups like that in Coma Berenices, in the Pleiades, or in Præsepe, to
the compact globular clusters, in which some thousands of stars are so
densely congregated that considerable optical power is required to
disintegrate them. Some, it is true, yield more easily than others.
The great cluster (Messier 13) in Hercules readily displays the swarms
of stars of which it is composed; but others are so difficult that it
is only in the largest instruments they are resolved into star-dust.
Further references to these wonderful objects will be made in the next
chapter, and some of the principal examples described; our purpose
here is to allude to a few of the more scattered groups, and to some
noteworthy instances of multiple stars.

_Coma Berenices._ A naked-eye cluster, consisting of many stars,
chiefly from the 5th to 6th mags. A telescope adds a number of smaller
stars. Nebulæ may be often swept up hereabouts, as it is not far north
of the rich nebulous region of Virgo.

_The Pleiades._ Six stars are usually distinguished by the naked eye,
and a seventh is occasionally remarked. Möstlin (the instructor of
Kepler) counted fourteen, Miss Airy has drawn twelve, and Carrington,
like Möstlin, saw fourteen. In 1877 I distinctly made out fourteen
stars in this group. The telescope reveals a considerable number
of small stars and Tempel’s large nebula near Merope. Kepler saw
thirty-two stars with a telescope, and Hooke seventy-eight; but Wolf,
at Paris, after three years of unremitting labour with a 4-foot
reflector, catalogued 671 stars in the group. A photograph, however,
with a 12-inch refractor showed 1421 stars; and a more recent negative
includes no less than 2326. There is an interesting little triangle
close to the brightest star, Alcyone; and several of the leading stars
are involved in nebulosity, discovered by means of photography.

_Præsepe._ A fine group of small stars, divisible by the unaided eye
on a clear night. Chambers says the components are not visible without
a telescope; while Webb notes that the group is just resolvable by the
naked eye. Thirty-six stars were glimpsed with Galilei’s telescope;
but modern instruments show many more. Marth, using Lassell’s 4-foot
reflector at Malta, discovered several faint nebulæ and nebulous stars
in this cluster.

χ _Persei._ Perceptible to the eye as a patch of hazy material lying
between the constellations Cassiopeia and Persei. In a telescope it
forms a double cluster, and is one of the richest and most beautiful
objects that the sky affords. The tyro who first beholds it is
astonished at the marvellous profusion of stars. It can be fairly well
seen in a good field-glass, but its chief beauties only come out in
a telescope, and the larger the aperture the more striking will they
appear. It is on groups of this character that the advantage of large
instruments is fully realized. The power should be very low, so that
the whole of the two clusters may be seen in the field. An eyepiece of
40, field 65′, on my 10-inch reflector, presents this object in its
most imposing form.

κ _Crucis_. Sir J. Herschel’s observations at the Cape have made this
object familiar to northern observers. It is composed of more than 100
stars, from the 7th mag. downwards; and some of the brighter ones are
highly coloured, so that the general effect is greatly enhanced and
fully justifies Herschel’s statement that the group may be likened to
“a superb piece of fancy jewelry.”

ζ _Ursæ Majoris_ (_Mizar_). This group is interesting both as a
naked-eye and as a telescopic object. There is a 5th mag. star, named
Alcor, about 11½′ distant from Mizar, and the former was considered
a good test-object for unaided vision by the Arabian astronomers.
But the star has probably brightened; for it can now be easily seen,
and certainly offers no criterion of good vision. Mizar is a fine
telescopic double, the companion being 4th mag. and distant 14½″. Any
small telescope will show it, and there is another 8th mag. star very
near.

σ _Orionis_. This appears as a double-quadruple star, with several
others in the same field. A 3-inch will reveal most of them, though
some of the fainter stars in the group will be beyond its reach.

θ _Orionis_. In the midst of the great nebula of Orion there is a
tolerably conspicuous quadruple star, the components of which form
a trapezium. This is visible in a 2-inch refractor. In 1826 Struve
discovered a fifth star, and in 1830 Sir J. Herschel found a sixth;
these were both situated a little outside the trapezium. All these
stars have been seen in a 3-inch telescope. The great 36-inch
equatoreal at Mount Hamilton has added several others; one was
detected by Alvan G. Clark (the maker of the object-glass) and another
by Barnard. These were excessively minute, and placed within the
trapezium. Barnard[54] has also glimpsed an extremely minute double
star exterior to the trapezium, and forming a triangle with the stars A
and C on the following diagram:—

[Illustration: Fig. 63.

The Trapezium in Orion, as seen with the 36-inch refractor.]

Several observers, including Huggins, Salter, and others, had
previously drawn faint stars in the interior of the trapezium; but
these could not be seen by Hall and Burnham in the large refractors at
Washington and Chicago, and were thus proved to have no real existence.
The new stars observed in the 36-inch telescope are only just within
the limits of its capacity, and therefore cannot be identical with
stars alleged to have been previously seen in small instruments.
The fifth and sixth stars in the trapezium have been supposed to be
variable, and not without reason; possibly the others are equally
liable to change, but this is only conjecture. Sir J. Herschel says
that to perceive the fifth and sixth stars “is one of the severest
tests that can be applied to a telescope” (‘Outlines,’ 11th edit. p.
610); yet Burnham saw them both readily in a 6-inch a few minutes
before sunrise on Mount Hamilton in September 1879.

β and ε Lyræ also form multiple groups, which will well repay
observation either with large or small telescopes.

_Further Observations._—Anyone who attempts to indicate with tolerable
fulness the methods and requirements of observation in the stellar
department of astronomy will find a heavy task lies before him; and
it is one to which he will be unable to do justice in a small space,
owing to the variety of matters to be referred to and the necessity
of being particular in regard to each one. In what follows I shall
merely make very brief allusions, as it is hoped the description
already given of past work will be a sufficient guide for the future.
Moreover, those who take up a special branch of inquiry will hardly
rest content with the meagre information usually conveyed in a general
work on astronomy, but will consult those authorities who deal more
exclusively with that branch. Double and binary stars may be said to
form one department, variable and temporary stars another, the colours
of stars a third, while many others may be signified—such as the
determination of star-magnitudes, positions, grouping, and distances;
also the proper motions and number of stars, besides photographic and
spectroscopic work,—each and all of which comprise a field of useful
and extensive inquiry. The amateur will of course choose his own sphere
of labour, consistently with his inclination and the character of his
appliances. In connection with double stars, valuable work yet remains
to be done, though the Herschels and the Struves gathered in the
bulk of the harvest and Burnham has gleaned much that was left. With
regard to bright stars, it may be assumed that very few, if any, close
companions, visible in moderately small glasses, now await discovery,
unless, indeed, in cases where the star forms part of a binary system
of long period, and the epoch of periastron has fallen in recent years.
But with telescopic stars there must be many interesting doubles, some
of them binaries, still unknown. These should be swept up and submitted
to measurement. A great desideratum in this branch is a new general
catalogue of double stars; for such a work would greatly facilitate
reference, and save the trouble of searching through different lists
in order to identify an object. Burnham has given some practical hints
on double-star work in the ‘Sidereal Messenger,’ and his remarks are
reproduced in that excellent work ‘Astronomy for Amateurs.’

As to variable stars, some of these permit of naked-eye estimation,
others need a field-glass, and there are some which require to be
followed in a good telescope. The observer who enters this department
may either desire to find new objects or to obtain further data with
regard to old ones. If the former, he cannot do better than watch
some of the suspected variables in Gore’s Catalogue of 736 objects,
published by the Royal Irish Academy. Whether suspected or known
variables are put under surveillance, the plan of comparison will be
the same. Several stars near the variable in position, and nearly equal
in light, should be compared with it, and the differences in lustre,
in tenths of a magnitude, recorded as frequently as possible. The
extent and period of the variation will become manifest by a discussion
of the results. The comparison-stars should of course be constant
in light, and, if naked-eye stars, they may be selected from the
_Uranometria Nova Oxoniensis_ or ‘Harvard Photometry.’ If telescopic
stars are required, then recourse must be had to comprehensive charts
such as Argelander’s _Durchmusterung_, which includes stars up to 9½
mag. Variable stars of the Algol type are especially likely to escape
recognition, as they retain a normal brilliancy except during the few
hours near the time of a minimum.

As to star-colours, it must be admitted that our knowledge is in
an unsatisfactory condition. The results of past observation show
discordances which are difficult to account for. When, however, all the
circumstances are considered, we need feel no surprise at this want of
unanimity. In certain cases it is probable that actual and periodical
changes occur in the colours of stars, though absolute proof is still
required. Atmospheric variations unquestionably affect the tints of
stars, and some alterations depend upon altitude, for a celestial
object seen through the dense lower air-strata near the horizon will
hardly preserve the same apparent hues when on the meridian. Telescopes
are also liable to induce false impressions of colour, and especially
by the employment of different eyepieces not equally achromatic. And
the observer’s judgment is sometimes at fault through physiological
influences, or he may have a systematic preference for certain hues
which little impress another observer. Those engaged in this branch
feel the want of a reliable and ready means of comparison, and several
have been tried; but there are objections to their use, and it seems
that the best objects are furnished by the stars themselves. Let the
observer study the colours of well-known stars, and familiarize his
eye with the distinctions in various cases (also with the differences
due to meteorological effects &c.); he will then gradually acquire
confidence, and may use these objects as standards. The difficulty will
be that they cannot be directly compared, in the same field, with other
stars; but relative differences may be noted by turning the telescope
from one object to the other. This will be better than forming
estimates on the basis of an artificial method, which will sure to be
troublesome to arrange, and probably erroneous in practice. In some
stars the colour is so curious as to be attributed with difficulty, and
with regard to faint stars colour-estimates are often unreliable; so
that it is not desirable to go below the 9th mag. unless a very large
instrument is employed.

The necessity of being constantly on the look-out for temporary
stars has been already mentioned. There is also the need for further
observations of such of these objects as still exist. They are,
however, very minute, and the observer will have to be careful as
to their identity. Though no great revival in brilliancy is to be
expected, these objects exhibited some singular fluctuations during
their decline, and it is important to keep them under view as long as
possible.

Many other departments of sidereal work are best left to the
professional astronomer. The derivation of accurate star-places, proper
motions, distances, &c. requires instruments of great refinement and
trained hands to use them. Researches such as these do not come within
the scope of ordinary amateurs. But a vast field is open to them in
respect to double and variable stars; and the physical relations of
many of the former greatly intensify the interest in this branch, and
make it necessary to secure frequent observations.


FOOTNOTES:

[47] There are several forms of this instrument: for particulars of
construction and use the reader is referred to Thornthwaite’s ‘Hints on
Telescopes,’ and Chambers’s ‘Astronomy,’ 4th ed. vol. ii.

[48] Mr. George Knott, of Cuckfield, mentions that the radius of the
first bright diffraction-ring of a stellar image, for a 7-1/3-inch
aperture, is 1″·01, and for one of 2 inches 3″·70 (‘Observatory,’
vol. vi. p. 19; see also vol. i. pp. 107 and 145). Mr. Dawes is quoted
as giving 1″·25 for a 7-inch, 1″·61 for a 5½-inch, and 3″·57 for a
2·4-inch. These figures exceed the theoretical values, if the latter
are adopted from Sir G. B. Airy’s ‘Undulatory Theory of Optics,’ where,



Online LibraryWilliam F. DenningTelescopic Work for Starlight Evenings → online text (page 27 of 32)