ACCIDENTAL DISCOVERIES
[Sidenote: The Oxford new star found during work on Astrographic Chart.]
In reviewing various types of astronomical discovery I have laid some
stress upon the fact that they are, generally speaking, far from being
accidental in character. A new planet does not "swim into our ken," at any
rate not usually, but is found only after diligent search, and then only
by an investigator of acute vision, or other special qualifications. But
this is, of course, not always the case. Some discoveries are made by the
merest accident, as we have had occasion to remark incidentally in the
case of the minor planets; and for the sake of completeness it is
desirable to include among our types at least one case of such accidental
discovery. As, however, the selection is a little invidious, I may perhaps
be pardoned for taking the instance from my own experience, which happens
to include a case where one of those remarkable objects called "new stars"
walked deliberately into a net spread for totally different objects. There
is the further reason for choosing this instance: that it will afford me
the opportunity of saying something about the special research in which we
were actually engaged, the work of mapping out the heavens by
photography, or, as it has been called, the Astrographic Chart--a great
scheme of international co-operation by which it is hoped to leave as a
legacy for future centuries a record of the state of the sky in our age.
Such a record cannot be complete; for however faint the stars included, we
know that there are fainter stars which might have been included had we
given longer exposures to the plates. Nor can it be in other ways final or
perfect; however large the scale, for instance, on which the map is made,
we can imagine the scale doubled or increased many-fold. But the map will
be a great advance on anything that has hitherto been made, and some
account of it will therefore no doubt be of interest.
[Sidenote: Origin of the chart.]
We may perhaps begin with a brief historical account of the enterprise.
Photographs of the stars were taken many years ago, but only by a few
enthusiasts, and with no serious hope of competing with eye observations
of the sky. The old wet-plate photography was, in fact, somewhat unsuited
to astronomical purposes; to photograph faint objects a long exposure is
necessary, and the wet plate may dry up before the exposure is
concluded--nay, even before it is commenced, if the observer has to wait
for passing clouds--and therefore it may be said that the successful
application of photography to astronomy dates from the time when the dry
plate was invented; when it became possible to expose a plate in the
telescope for hours, or by accumulation even for days. The dry plate
remains sensitive for a long period, and if it is desired to extend an
exposure beyond the limits of one night, it is quite easy to close up
the telescope and return to the operations again on the next fine night;
and so on, if not perhaps indefinitely, at any rate so long as to
transcend the limits of human patience up to the present.
[Illustration:
VII.--GREAT COMET OF NOV. 7TH, 1882
(_From a photograph taken at the Royal Observatory, Cape of Good Hope._)]
[Sidenote: Comet of 1882.]
[Sidenote: Stars shown on the pictures.]
But to consider our particular project. We may assign, perhaps, the date
1882 as that in which it first began to take shape. In that year there was
a magnificent bright comet, the last really large comet which we, in the
Northern Hemisphere, have had the good fortune to see. Some of us, of
course, were not born at that time, and perhaps others who were alive may
nevertheless not have seen that comet; for it kept somewhat uncomfortably
early morning hours, and I can well remember myself feeling rather more
resentment than gratitude to the man who waked me up about four o'clock to
see it. Many observations were of course made of this interesting visitor,
and what specially concerns us is that at the Cape of Good Hope some
enterprising photographers tried to photograph it. They tried in the first
instance with ordinary cameras, and soon found--what any astronomer could
have told them--that the movement of the earth, causing an apparent
movement of the comet and the stars in the opposite direction, frustrated
their efforts. The difficulties of obtaining pictures of moving objects
are familiar to all photographers. A "snap-shot" might have met the
difficulty, but the comet was scarcely bright enough to affect the plate
with a short exposure. Ultimately Dr. David Gill, the astronomer at the
Cape Observatory, invited one of the photographers to strap his camera to
one of the telescopes at the Observatory, a telescope which could be
carried round by clockwork in the usual way, so as to counteract the
earth's motion, and in effect to keep the comet steadily in view, as
though it were at rest. As a consequence, some very beautiful and
successful pictures of the comet were obtained, and on them a large number
of stars were also shown. They were, as I have said, not by any means the
first pictures of stars obtained by photography, but they represented in
facility and in success so great an advance upon what had been formerly
obtained that they attracted considerable attention. They were sent to
Europe and stimulated various workers to further experiments.
[Sidenote: The brothers Henry begin work.]
[Sidenote: Conference of 1887.]
The late Dr. Common in England, an amateur astronomer, began that
magnificent pioneer work in astronomical photography which soon brought
him the Gold Medal of the Royal Astronomical Society for his photographs
of nebulae. But the most important result for our purpose was produced in
France. There had been started many years before by the French astronomer
Chacornac a series of star maps round the Zodiac similar in intention to
the Berlin maps which figured in the history of the discovery of Neptune.
Chacornac died before his enterprise was very far advanced, and the work
was taken up by two brothers, Paul and Prosper Henry, who followed
Chacornac in adopting for the work the laborious method of eye
observation of each individual star. They proceeded patiently with the
work on these lines; but when they came to the region where the Zodiac is
crossed by the Milky Way, and the number of stars in a given area
increases enormously, they found the labour so great as to be practically
prohibitive, and were in doubt how to deal with the difficulty. It was at
this critical moment that these comet photographs, showing the stars so
beautifully, suggested the alternative of mapping the stars
photographically. They immediately set to work with a trial lens, and
obtained such encouraging results that they proceeded themselves to make a
larger lens of the same type; this again was satisfactory, and the idea
naturally arose of extending to the whole heavens the scheme which they
had hitherto intended only for the Zodiac, a mere belt of the heavens. But
this rendered the enterprise too large for a single observatory. It became
necessary to obtain the co-operation of other observatories, and with this
end in view an International Conference was summoned to meet in Paris in
1887 to consider the whole project. There were delegates from, if not all
nations, at any rate a considerable number:--
France 20
British Empire 8
Germany 6
Russia 3
Holland 3
U.S. America 3
Austria 2
Sweden 2
Denmark 2
Belgium 1
Italy 1
Spain 1
Switzerland 1
Portugal 1
Brazil 1
Argentine Republic 1
[Sidenote: Choice of instrument.]
[Sidenote: Expense of "doublet."]
[Sidenote: Advantages of reflector.]
[Sidenote: Refractor chosen.]
The Conference had a number of very important questions to discuss, for
knowledge of the photographic method and its possibilities was at that
time in its infancy. There was, for instance, the question whether all the
instruments need be of the same pattern, and if so what that pattern
should be. The first of these questions was settled in the affirmative, as
we might expect; in the interests of uniformity it was desirable that the
maps should be as nearly similar as possible. The second question was not
so easy; there were at least three different types of instruments which
might be used. First of all, there was the photographic lens, such as is
familiar to all who have used an ordinary camera, consisting of two lenses
with a space between; though since each of these lenses is itself made up
of two, we should more correctly say four lenses in all. It was with a
lens of this kind that the comet pictures had been taken at the Cape of
Good Hope, and it might seem the safest plan to adopt what had been shown
to be capable of such good work. But there was this difficulty; the
pictures of the comet were on a very small scale, and taken with a small
lens; a much larger lens was required for the scheme now under
contemplation, and when there are four separate lenses to be made, each
with two surfaces to polish, and each requiring a perfectly sound clear
piece of glass, it will be obvious that the difficulties of making a large
compound lens of this kind are much greater, and the expense much more
serious than in the case of a single lens, or even a pair. It was this
question of expense which had led the brothers Henry to experiment with a
different kind of instrument, in which only one pair of lenses was used
instead of two. Their instrument was, in fact, very similar to the
ordinary telescope, excepting that they were bound to make their lenses
somewhat different in shape in order to bring to focus the rays of light
suitable for photography, which are not the same as those suitable for eye
observation with the ordinary telescope. Dr. Common, again, had used a
third kind of instrument, mainly with the view of reducing the necessary
expense still further, or, perhaps, of increasing the size of the
instrument for the same expense. His telescope had no lens at all, but a
curved mirror instead, the mirror being made of glass silvered on the face
(not on the back as in the ordinary looking-glass). In this case there is
only one surface to polish instead of four, as in the Henrys' telescope,
or eight, as in the case of the photographic doublet; and, moreover, since
the rays of light are reflected from the surface of the glass, and do not
pass _through_ it at all, the internal structure of the glass is not so
strictly important as in the other cases. Hence the reflector is a very
cheap instrument, and it is, moreover, quite free from some difficulties
attached to the other instruments. No correction for rays of light of
different colours is required, since all rays of whatever colour come to
the same focus automatically. These advantages of the reflector were so
considerable as to almost outweigh one well-known disadvantage, which is,
however, not very easily expressed in words. The reflector might be
described as an instrument with a temper; sometimes it gives excellent
results, but at others _something_ seems to be wrong, though the worried
observer does not exactly know what. Long experience and patience are
requisite to humour the instrument and get the best results from it, and
it was felt that this uncertainty was sufficient to disqualify the
instrument for the serious piece of routine work contemplated in mapping
the heavens. Accordingly the handier and more amiable instrument with
which the brothers Henry had done such good work was selected as the
pattern to be adopted.
[Sidenote: Doublet would have been better.]
It is curious that at the Conference of 1887 nothing at all was said about
the type of instrument first mentioned (the "doublet lens"), although a
letter was written in its favour by Professor Pickering of Harvard College
Observatory. Since that time we have learnt much of its advantages, and it
is probable that if the Conference were to meet now they might arrive at a
different decision; but at that time they were, to put it briefly,
somewhat afraid of an instrument which seemed to promise, if anything, too
well, especially in one respect. With the reflector and the refractor it
had been found that the field of good images was strictly limited. The
Henrys' telescope would not photograph an area of the sky greater in
extent than 2 deg. in diameter at any one time, and the reflector was more
limited still; within this area the images of the stars were good, and it
had been found that their places were accurately represented. Now the
"doublet" seemed to be able to show much larger areas than this with
accuracy, but no one had been able to test the accuracy to see whether it
was sufficient for astronomical purposes; and although no such feeling was
openly expressed or is on record, I think there is no doubt that a feeling
existed of general mistrust of an instrument which seemed to offer such
specious promises. Whatever the reason, its claims were passed over in
silence at the Conference, and the safer line (as it was then thought) of
adopting as the type the Henrys' instrument, was taken.
[Sidenote: The eighteen observatories.]
This was perhaps the most important question settled at the Conference,
and the answers to many of the others naturally followed. The size of the
plates, for instance, was settled automatically. The question down to what
degree of faintness should stars be included, resolved itself into the
equivalent question, What should be the length of time during which the
plates were exposed? Then, again, the question, What observatories should
take part in the work? became simply this: What observatories could
afford to acquire the instruments of this new pattern and get other funds
for carrying out the work specified? It was ultimately found that eighteen
observatories were able to obtain the apparatus and funds, though
unfortunately three of the eighteen have since found it impossible to
proceed. The following is the original list, and in brackets are added the
names of three other observatories which in 1900 undertook to fill the
places of the defaulters.
OBSERVATORIES CO-OPERATING FOR THE ASTROGRAPHIC CHART.
+----------------------+------------+----------+
| Observatory. | Zones of | Number |
| |Declination.|of Plates.|
+----------------------+------------+----------+
|Greenwich |+90 deg. to +65 deg.| 1149 |
|Rome |+64 deg. " +55 deg.| 1140 |
|Catania |+54 deg. " +47 deg.| 1008 |
|Helsingfors |+46 deg. " +40 deg.| 1008 |
|Potsdam |+39 deg. " +32 deg.| 1232 |
|Oxford |+31 deg. " +25 deg.| 1180 |
|Paris |+24 deg. " +18 deg.| 1260 |
|Bordeaux |+17 deg. " +11 deg.| 1260 |
|Toulouse |+10 deg. " + 5 deg.| 1080 |
|Algiers |+ 4 deg. " - 2 deg.| 1260 |
|San Fernando |- 3 deg. " - 9 deg.| 1260 |
|Tacubaya |-10 deg. " -16 deg.| 1260 |
|Santiago (Monte Video)|-17 deg. " -23 deg.| 1260 |
|La Plata (Cordoba) |-24 deg. " -31 deg.| 1360 |
|Rio (Perth, Australia)|-32 deg. " -40 deg.| 1376 |
|Cape of Good Hope |-41 deg. " -51 deg.| 1512 |
|Sydney |-52 deg. " -64 deg.| 1400 |
|Melbourne |-65 deg. " -90 deg.| 1149 |
+----------------------+------------+----------+
[Sidenote: Sky covered twice.]
In the list is also shown the total number of plates that were to be taken
by each observatory. When once the size of the plates had been settled,
it was a straightforward matter to divide up the sky into the proper
number of regions necessary to cover it completely, not only without gaps
between the plates, but with actually a small overlap of contiguous
plates. And more than this, it was decided that the whole sky should be
completely covered _twice over_. It was conceivable that a question might
arise whether an apparent star image on a plate was, on the one hand, a
dust speck, or, on the other hand, a planet, or perhaps a variable or new
star. By taking two different plates at slightly different times,
questions of this kind could be settled; and to make the check more
independent it was decided that the plates should not be exactly repeated
on the same portion of sky, but that in the second series the centre of a
plate should occupy the point assigned to the corner of a plate in the
first series.
[Sidenote: Times of exposure.]
Then there came the important question of time of exposure, which involved
a long debate between those who desired the most modest programme possible
consistent with efficiency, and those enthusiasts who were anxious to
strain the programme to the utmost limits attainable. Ultimately it was
resolved to take two series of plates; one series of long exposure which
was set in the first instance at 10 minutes, then became 15, then 30, then
40, and has by some enterprising observers been extended to 1-1/2 hours;
the other a series of short exposures which have been generally fixed at
6 minutes. Thus instead of covering the sky twice, it was decided to cover
it in all four times, and the number of plates assigned to each
observatory in the above list must be regarded as doubled by this new
decision. And further still, on the series of short-exposure plates it was
decided to add to the exposure of six minutes another one of three
minutes, having slightly shifted the telescope between the two so that
they should not be superimposed; and later still, a third exposure of
twenty seconds was added to these. It would take too long to explain here
the reasons for these details, but it will be clear that the general
result of the discussion was to extend the original programme
considerably, and render the work even more laborious than it had appeared
at the outset.
[Sidenote: Measurement of plates.]
[Sidenote: The reseau.]
[Sidenote: The microscope.]
[Sidenote: Reversal of plates.]
[Sidenote: Personal equation.]
When all these plates have been taken, the work is by no means finished;
indeed, it is only just commencing. There remains the task of measuring
accurately on each of the short-exposure plates the positions of the stars
which it represents, numbering on the average some 300 or 400; so that for
instance at Oxford the total number of stars measured on the twelve
hundred plates is nearly half a million. These are not all separate stars;
for the sky is represented twice over, and there is also the slight
overlap of contiguous plates; but the number of actual separate stars
measured at this one observatory is not far short of a quarter of a
million, and it has taken nearly ten years to make the measurements, with
the help of three or four measurers trained for the purpose. To render the
measures easy, a network or reseau of cross lines is photographed on each
plate by artificial light after it has been exposed to the stars, so that
on development these cross lines and the stars both appear. We can see at
a glance the approximate position of a star by counting the number of the
space from left to right and from top to bottom in which it occurs; and we
can also estimate the fraction of a space in addition to the whole number;
but it is necessary for astronomical purposes to estimate this fraction
with the greatest exactness. The whole numbers are already given with
great exactness by the careful ruling of the cross lines, which can be
spaced with extraordinary perfection. To measure the fraction, we place
the plate under a microscope in the eye-piece of which there is a finally
divided cross scale; the centre of the cross is placed over a star image,
and then it is noted where the lines of the reseau cut the cross scale. In
this way the position of the image of a star is read off with accuracy,
and after a little practice with considerable rapidity. It has been found
at Oxford that under favourable conditions the places of nearly 200 stars
per hour can be recorded in this way by a single measurer, if he has some
one to write down for him the numbers he calls out. This is only one form
of measuring apparatus; there are others in which, instead of a scale in
the eye-piece, micrometer screws are used to measure the fractions; but
the general principle in all these instruments is much the same, and the
rate of work is not very different; while to the minor advantages and
disadvantages of the different types there seems no need here to refer.
One particular point, however, is worth noting. After a plate has been
measured, it is turned round completely, so that left is now right, and
top is now bottom, and the measurements are repeated. This repetition has
the advantage first of all of checking any mistakes. When a long piece of
measuring or numerical work of any kind is undertaken there are invariably
moments when the attention seems to wander, and some small error is the
result. But there are also certain errors of a systematic character
similar to those denoted by the term "personal equation," which has found
its way into other walks of life. In the operation of placing a cross
exactly over the image of a star, different observers would show slight
differences of habit; one might place it a little more to the right than
another. But when the plate is turned round the effect of this habit on
the measure is exactly reversed, and hence if we take the mean of the two
measures any personal habit of this kind is eliminated. It has been found
by experience that such personal habits are much smaller for measures of
this kind than for those to which we have long been accustomed in
observations made by eye on the stars themselves. The troubles from
"personal equation" have been much diminished by the photographic method,
and certain peculiarities of the former method have been clearly exhibited
by the comparison. For instance, it has gradually become clear that with
eye observations personal equation is not a constant quantity, but is
different for stars of different brightness. When observing the transit of
a bright star the observer apparently records an instant definitely
earlier than in recording the transit of a faint one; and this peculiarity
seems to be common to the large majority of observers, which is perhaps
the reason why it was not noticed earlier. But when positions of the stars
determined in this way are compared with their positions measured on the
photographic plates, the peculiarity is made clearly manifest. For
example, at Oxford, our first business after making measurements is to
compare them with visual observations on a limited number of the brighter
stars made at Cambridge about twenty years ago. (About 14,000 stars were
observed at Cambridge, and we are dealing with ten times that number.) The
comparison shows that the Cambridge observations are affected with the
following systematic errors:--
If stars of magnitude 10 are observed correctly,
then " " 9 " 0.10 secs. too early
" " 8 " 0.16 "
" " 7 " 0.19 "
" " 6 " 0.21 "
" " 5 " 0.23 "
[Sidenote: Main object of the work.]
This may serve as an illustration of various incidental results which are
already flowing from the enormous and laborious piece of work which, as
far as the University Observatory at Oxford is concerned, we have just
completed, though some of the other colleagues are not so far advanced.
But the main results will not appear just yet. The work must be repeated,
and the positions of the stars just obtained must be compared with those
which they will be found to occupy at some future date, in order to see
what kind of changes are going on in the heavens. Whether this future date
shall be one hundred years hence, or fifty, or ten, or whether we should
begin immediately to repeat what has been done, is a matter not yet
decided, and one which requires some little consideration.
[Sidenote: The concluding year.]
I have said perhaps enough to give you a general idea of the work on which
we have been engaged at Oxford for the last ten years. Ten years ago it
seemed to stretch out in front of us rather hopelessly; the pace we were
able to make seemed so slow in view of the distance to be covered. We felt
rather like the schoolboy who has just returned to school and sees the
next holidays as a very remote prospect, and we solaced ourselves much in
the same way as he does, by making a diagram representing the total number
of plates to be dealt with and crossing off each one as it was finished,
just as he sometimes crosses off the days still remaining between him and
the prospective holidays. It was pleasant to watch the growth of the
number of crosses on this diagram, and by the end of the year 1902 we had
the satisfaction of seeing very little blank space remaining. Now, up to
this point it had not much mattered whether any particular plate was
secured in any particular year, or in a subsequent year, so long as there
were always sufficient plates to keep us occupied in measuring them. But
it then became a matter of importance to secure each plate at the proper
time of year; for the sun, as we know, travels round the Zodiac among the
stars, obliterating by his radiance a large section of the sky for a
period of some months, and in this way a particular region of the heavens
is apt to "run into daylight," as the observatory phrase goes, and ceases
to be available for photography during several months, until the sun is
again far enough away to allow of the particular region being seen at
night.
[Sidenote: A disappointment.]
[Sidenote: A curious plate.]
[Sidenote: A strange object.]
[Sidenote: A new star?]
Roughly speaking then, if a plate which should be taken in February is not
secured in this month owing to bad weather, the proper time for taking it
will not occur again until the following February; and when there was a
fair prospect of finishing our work in 1903, it became important to secure
each plate at the proper time in that year. Hence we were making special
efforts to utilise to the full any fine night that Providence sent in our
way, and on such occasions it is clearly an economy, if not exactly to
"make hay while the sun shines," at any rate to take plates vigorously
while the sun is _not_ shining and the night is fine; leaving the
development of them until the daytime. There is, of course, the risk that
the whole night's work may in this way be lost owing to some fault in the
plates, which might have been detected if some of them were immediately
developed. Perhaps in the early days of our work it would have been
reckless or foolish to neglect this little precaution; but we had for
years been accustomed to rely upon the excellence of the plates without
finding our trust betrayed; and the sensitiveness of the plates had
increased rather than diminished as time went on. Hence it will be readily
understood that when one fatal morning we developed a series of some
thirty plates, and found that owing to some unexplained lack of
sensitiveness they were all unsuitable for our purpose, it came as a most
unwelcome and startling surprise. It was, of course, necessary to make
certain that there was no oversight, that the developer was not at fault,
and that the weather had not been treacherous. All such possibilities were
carefully considered before communication with the makers of the plates,
but it ultimately became clear that there had been some unfortunate
failure in sensitiveness, and that it would be necessary to repeat the
work with opportunities restricted by the intervening lapse of time.
However, disappointments from this or similar causes are not unknown in
astronomical work; and we set about this repetition with as little loss of
time and cheerfulness as was possible. Under the circumstances, however,
it seemed desirable to examine carefully whether anything could be saved
from the wreck--whether any of the plates could be admitted as _just_
coming up to the minimum requirements. And I devoted a morning to this
inquiry. In the course of it I came across one plate which certainly
seemed worth an inclusion among our series from the point of view of the
number of stars shown upon it. It seemed quite rich in stars, perhaps even
a little richer than might have been expected. On inquiry I was told that
this was not one of the originally condemned plates, but one which had
been taken since the failure in sensitiveness of the plates had been
detected; was from a new and specially sensitive batch with which the
courteous makers had supplied us; but though there were certainly a
sufficient number of stars upon the plate, owing to some unexplained cause
the telescope had been erroneously pointed, and the region taken did not
correspond to the region required. To investigate the cause of the
discrepancy I thereupon took down from our store of plates the other one
of the same region which had been rejected for insufficiency of stars, and
on comparing the two it was at once evident that there was a strange
object on the plate taken later of the two, a bright star or other
heavenly body, which was not on the former plate. I have explained that by
repeating the exposure more than once, it is easily possible to recognise
whether a mark upon the plate is really a celestial body or is an
accidental blot or dust speck, and there was no doubt that this was the
image of some strange celestial body. It might, of course, be a new
planet, or even an old one which had wandered into the region; but a few
measures soon showed that it was not in movement. The measures consisted
in comparing the separation of the three exposures with the separation of
the corresponding exposures of obvious stars, for the exposures were not,
of course, simultaneous, and if the body were a planet and had moved in
the interval between them, this would be made manifest on measuring the
separations. No such movements could be detected; and the possibilities
were thus restricted to two. So far as we knew the object was a star, but
might be either a star of the class known as _variable_ or of that known
as _new_. In the former case it would become bright and faint at more or
less regular intervals, and might possibly have been already catalogued;
for the number of these bodies already known amounts to some hundreds.
Search being made in the catalogues, no entry of it was found, though it
still might be one of this class which had hitherto escaped detection. Or
it might be a "new star," one of those curious bodies which blaze up quite
suddenly to brightness and then die away gradually until they become
practically invisible. The most famous perhaps of these is the star which
appeared in 1572, and was so carefully observed by Tycho Brahe; but such
apparitions are rare, and altogether we have not records as yet of a score
altogether; so that in this latter case the discovery would be of much
greater interest than in the former. In either event it was desirable to
inform other observers as soon as possible of the existence of a strange
body; already some time had elapsed since the plate had been taken, March
16th, for the examination of which I have spoken was not made until March
24th. Accordingly, a telegram was at once despatched to the Central Office
at Kiel, which undertakes to distribute such information all over the
world, and a few post-cards were sent to observers close at hand who might
be able to observe the star the same night. Certain observations with the
spectroscope soon made it clear that the object was really a "new star."
[Sidenote: The discovery accidental.]
[Sidenote: Mrs. Fleming's discoveries.]
This, therefore, is the discovery which we made at Oxford: as you will
see, in an entirely accidental manner, during the course of a piece of
work in which it was certainly never contemplated. Its purely accidental
nature is sufficiently illustrated by the fact that if the plates
originally supplied by the makers had been of the proper quality, the
plate which led to the discovery would never have been taken. If the
plates exposed in February had been satisfactory, we should have been
content, and should not have repeated the exposure on March 16th. Again I
can testify personally how purely accidental it was that the examination
was made on March 24th to see whether anything could be saved, as I have
said, from the wreck. The idea came casually into my mind as I was walking
through the room and saw the neat pile of rejected plates; and one may
fairly call it an accidental impulse. This new star is not, however, the
first of such objects to have been discovered "accidentally"; many of the
others were found just as much by chance, though a notable exception must
be made of those discovered at the Harvard Observatory, which are the
result of a deliberate search for such bodies by the careful examination
of photographic plates. Mrs. Fleming, who spends her life in such work,
has had the good fortune to detect no less than six of these wonderful
objects as the reward of her laborious scrutiny; and she is the _only_
person who has thus found new stars by photography until this accidental
discovery at Oxford. The following is a complete list of new stars
discovered to date:--
LIST OF NEW STARS.
+----------------------------------------------+
|Ref. No.| Constellation. | Year.| Discoverer. |
+----------------------------------------------+
| 1 | Cassiopeia | 1572 | Tycho Brahe.|
| 2 | Cygnus | 1600 | Janson. |
| 3 | Ophiuchus | 1604 | Kepler. |
| 4 | Vulpecula | 1670 | Anthelm. |
| 5 | Ophiuchus | 1848 | Hind. |
| 6 | Scorpio | 1860 | Auwers. |
| 7 | Corona Borealis| 1866 | Birmingham. |
| 8 | Cygnus | 1876 | Schmidt. |
| 9 | Andromeda | 1885 | Hartwig. |
| 10 | Perseus | 1887 | Fleming. |
| 11 | Auriga | 1891 | Anderson. |
| 12 | Norma | 1893 | Fleming. |
| 13 | Carina | 1895 | Fleming. |
| 14 | Centaurus | 1895 | Fleming. |
| 15 | Sagittarius | 1898 | Fleming. |
| 16 | Aquila | 1899 | Fleming. |
| 17 | Perseus | 1901 | Anderson. |
| 18 | Gemini | 1903 | At Oxford. |
+----------------------------------------------+
[Illustration:
MARCH 1, 1903 MARCH 14, 1903
VIII.--THE OXFORD NEW STAR.
A PAIR OF PHOTOGRAPHS TAKEN AT THE HARVARD COLLEGE OBSERVATORY BEFORE
AND AFTER ITS APPEARANCE
(_The arrow indicates the place of the new star. It will be seen that
the left-hand picture though it shews fainter stars than the other, has
not a trace of the new star._)]
[Sidenote: Dr. Anderson.]
[Sidenote: Nova Persei.]
Generally these stars have been noted by eye observation, as in the case
of the two found by Dr. Anderson of Edinburgh. In these cases also we may
say that deliberate search was rewarded; for Dr. Anderson is probably the
most assiduous "watcher of the skies" living, though he seldom uses a
telescope; sometimes he uses an opera-glass, but usually the naked eye. He
describes himself as an "Astrophil" rather than as an astronomer. "I love
the stars," he says; "and whenever they are shining, I must be looking."
And so on every fine night he stands or sits at his open study window
gazing at the heavens. I believe he was just about to leave them for his
bed, near 3 A.M. on the night of February 21, 1901, when, throwing a last
glance upward, he suddenly saw a brilliant star in the constellation
Perseus. His first feeling was actually one of disappointment, for he felt
sure that this object must have been there for some time past without his
knowing of it, and he grudged the time lost when he might have been
regarding it. More in a spirit of complaint than of inquiry, he made his
way to the Royal Observatory at Edinburgh next day to hear what they had
to say about it, though he found it difficult to approach the subject. He
first talked about the weather, and the crops, and similar topics of
general interest; and only after some time dared he venture a casual
reference to the "new portent in the heavens." Seeing his interlocutor
look somewhat blank, he ventured a little farther, and made a direct
reference to the new star in Perseus; and then found to his astonishment,
as also to his great delight, that he was the first to bring news of it.
The news was soon communicated to other observers; all the telescopes of
the world were soon trained upon it; and this wonderful "new star of the
new century" has taught us more of the nature of these extraordinary
bodies than all we knew before.
[Sidenote: Records previous to discovery.]
[Sidenote: Was Nova Geminorum previously shining faintly?]
[Sidenote: The suspicion negatived.]
Perhaps I may add a few remarks on one or two features of these bodies.
Firstly, let us note that Professor Pickering of Harvard is now able to
make a most important contribution to the _former_ history of these
objects--that is to say, their history preceding their actual detection.
We remember that, after Uranus had been discovered, it was found that
several observers had long before recorded its place unknowingly; and
similarly Professor Pickering and his staff have usually photographed
other new objects unknowingly. There are on the shelves at Harvard vast
stores of photographs, so many that they are unable to examine them when
they have been taken; but once any object of interest has been discovered,
it is easy to turn over the store and examine the particular plates which
may possibly show it at an earlier date. In this way it was found that Dr.
Anderson's new star had been visible only for a few days before its
discovery, there being no trace of it on earlier plates. Similarly, in
the case of the new star found at Oxford, plates taken on March 1st and
6th, fifteen days and ten days respectively before the discovery-plate of
March 16th, showed the star. But, in this particular instance, greater
interest attaches to two still earlier plates taken elsewhere, and with
exposures much longer than any available at Harvard. One had been obtained
at Heidelberg by Dr. Max Wolf, and another at the Yerkes Observatory of
Chicago University, by Mr. Parkhurst; and on both there appeared to be a
faint star of about the fourteenth or fifteenth magnitude, in the place
subsequently occupied by the Nova; and the question naturally arose, Was
this the object which ultimately blazed up and became the new star? To
settle this point, it was necessary to measure its position, with
reference to neighbouring stars, with extreme precision; and here it was
unfortunate that the photographs did not help us as much as they might,
for they were scarcely capable of being measured with the requisite
precision. The point was an important one, because if the identity of the
Nova with this faint star could be established, it would be the second
instance of the kind; but so far as they went, measurements of the
photographs were distinctly against the identity. Such was the conclusion
of Mr. Parkhurst from his photograph alone; and it was confirmed by
measures made at Oxford on copies of both plates, which were kindly sent
there for the purpose. The conclusion seemed to be that there was a faint
star _very near_, but _not at_, the place of the new star; and it was
therefore probable that, although this faint star was temporarily
invisible from the brightness of the adjacent Nova, as the latter became
fainter (in the way with which we have become familiar in the case of new
stars), it might be possible to see the two stars alongside each other.
This critical observation was ultimately made by the sharp eyes of
Professor Barnard, aided by the giant telescope of the Yerkes Observatory;
and it seems clear therefore that the object which blazed up to become the
Nova of 1903 could not have previously been so bright as a faint star of
the fourteenth magnitude. Although this is merely a negative conclusion,
it is an important one in the history of these bodies.
[Sidenote: Nebula round Nova Persei.]
[Sidenote: Its changes.]
[Sidenote: Due to travelling illumination.]
The second point to which I will draw your attention is from the history
of the other Nova just mentioned--Dr. Anderson's New Star of 1901. In this
instance it is not the history previous to discovery, but what followed
many months after discovery, that was of engrossing interest; and again
Yerkes Observatory, with its magnificent equipment, played an important
part in the drama. When, with its giant reflecting telescope, photographs
were taken of the region of Nova Persei after it had become comparatively
faint, it was found that there was an extraordinarily faint nebulosity
surrounding the star. Repeating the photographs at intervals, it was
found that this nebulosity was rapidly changing in shape. "Rapidly" is, of
course, a relative term, and a casual inspection of two of the photographs
might not convey any impression of rapidity; it is only when we come to
consider the enormous distance at which the movements, or apparent
movements, of the nebulae must be taking place that it becomes clear how
rapid the changes must be. It was not possible to determine this distance
with any exactness, but limits to it could be set, and it seemed probable
that the velocity of the movement was comparable with that of light. The
conclusion suggested itself that the velocity might actually be identical
with that of light, in which case what we saw was not the movement of
actual matter, but merely that of illumination, travelling from point to
point of matter already existing.
[Illustration:
SEPT. 20, 1901 NOV. 13, 1901
IX--NEBULOSITY ROUND NOVA PERSEI
(_From photographs taken at the Yerkes Observatory by G. W. Ritchey._)]
[Sidenote: When did it all happen?]
An analogy from the familiar case of sound may make clearer what is meant.
If a loud noise is made in a large hall, we hear echoes from the walls.
The sound travels with a velocity of about 1100 feet per second, reaches
the walls, is reflected back from them, and returns to us with the same
velocity. From the interval occupied in going and returning we could
calculate the distance of the walls. The velocity of light is so enormous
compared with that of sound that we are usually quite unable to observe
any similar phenomenon in the case of light. If we strike a match in the
largest hall, all parts of it are illuminated so immediately that we
cannot possibly realise that there was really an interval between the
striking of the match, the travelling of the light to the walls, and its
return to our eyes. The scale of our terrestrial phenomenon is far too
small to render this interval perceptible. But those who accept the theory
above mentioned regarding the appearances round Nova Persei (although
there are some who discredit it) believe that we have in this case an
illustration of just this phenomenon of light echoes, on a scale large
enough to be easily visible. They think that, surrounding the central star
which blazed up so brightly in February 1901, there was a vast dark
nebula, of which we had no previous knowledge, because it was not shining
with any light of its own. When the star blazed up, the illumination
travelled from point to point of this dark nebula and lighted it up; but
the size of the nebula was so vast that, although the light was travelling
with the enormous velocity of 200,000 miles per second, it was not until
months afterwards that it reached different portions of this nebula; and
we accordingly got news of the existence of this nebula some months after
the news reached us of the central conflagration, whatever it was. Remark
that all we can say is that the news of the nebula reached us _some months
later_ than that of the outburst. The actual date when either of the
actual things happened, we have as yet no means of knowing; it may have
been hundreds or even thousands of years ago that the conflagration
actually occurred of which we got news in February 1901, the light having
taken all that time to reach us from that distant part of space; and the
light reflected from the nebula was following it on its way to us all
these years at that same interval of a few months.
[Sidenote: An objection.]
Now, let me refer before leaving this point to the chief objection which
has been urged against this theory. It has been maintained that the
illumination would necessarily appear to travel outwards from the centre
with an approach to uniformity, whereas the observed rate of travel is not
uniform, and has been even towards the centre instead of away from it;
which would seem as though portions of the nebula more distant from the
centre were lighted up sooner than those closer to it. By a simple
illustration from our solar system, we shall see that these curious
anomalies may easily be explained. Let us consider for simplicity two
planets only, say the Earth and Saturn. We know that Saturn travels round
the sun in an orbit which is ten times larger than the orbit of the earth.
Suppose now that the sun were suddenly to be extinguished; light takes
about eight minutes to travel from the sun to the earth, and consequently
we should not get news of the extinction for some eight minutes; the sun
would appear to us to still go on shining for eight minutes after he had
really been extinguished. Saturn being about ten times as far away from
the sun, the news would take eighty minutes to reach Saturn; and from the
earth we should see Saturn shining more[3] than eighty minutes after the
sun had been extinguished, although we ourselves should have lost the
sun's light after eight minutes. I think we already begin to see
possibilities of curious anomalies; but they can be made clearer than
this. Instead of imagining an observer on the earth, let us suppose him
removed to a great distance away in the plane of the two orbits; and let
us suppose that the sun is now lighted up again as suddenly as the new
star blazed up in February 1901. Then such an observer would first see
this blaze in the centre; eight minutes afterwards the illumination would
reach the earth, a little speck of light near the sun would be
illuminated, just as we saw a portion of the dark nebula round Nova Persei
illuminated; eighty minutes later another speck, namely, Saturn, would
begin to shine. But now, would Saturn necessarily appear to the distant
observer to be farther away from the sun than the earth was? Looking at
the diagram, we can see that if Saturn were at S{1} then it would present
this natural appearance of being farther away from the sun than the earth;
but it might be at S{2} or S{3}, in which case it would seem to be nearer
the sun, and the illumination would seem to travel inwards towards the
central body instead of outwards. Without considering other cases in
detail, it will be tolerably clear that almost any anomalous appearance
might be explained by choosing a suitable arrangement of the nebulous
matter which we suppose lighted up by the explosion of Nova Persei.
Another objection urged against the theory I have sketched is that the
light reflected from such a nebula would be so feeble that it would not
affect our photographic plates. This depends upon various assumptions
which we have no time to notice here; but I think we may say that there is
certainly room for the acceptance of the theory.
[Illustration: FIG. 6.]
[Sidenote: Did the nebula cause the outburst?]
Now, if this dark nebula was previously existing in this way all round the
star which blazed up, the question naturally arises whether the nebula had
anything to do with the conflagration. Was there previously a star, either
so cold or so distant as not to be shining with appreciable light, which,
travelling through space, encountered this vast nebula, and by the
friction of the encounter was suddenly rendered so luminous as to outshine
a star of the first magnitude? The case of meteoric stones striking our
own atmosphere seems to suggest such a possibility. These little stones
are previously quite cold and invisible, and are travelling in some way
through space, many of them probably circling round our sun. If they
happen in their journey to encounter our earth, even the extremely tenuous
atmosphere, so thin that it will scarcely bend the rays of light
appreciably, even this is sufficient by its friction to raise the stones
to a white heat, so that they blaze up into the falling stars with which
we are familiar. This analogy is suggested, but we must be cautious in
accepting it; for we know so very little of the nature of nebulae such as
that of which we have been speaking. But in any case, a totally new series
of phenomena have been laid open to our study by those wonderful
photographs taken at the Yerkes Observatory and the Lick Observatory in
the few years which the present century has as yet run.
[Sidenote: Importance of new stars]
One thing is quite certain: we must lose no opportunity of studying such
stars as may appear, and no diligence spent in discovering them at the
earliest possible moment is thrown away. We have only known up to the
present, as already stated, less than a score of them, and of these many
have told us but little; partly because they were only discovered too late
(after they had become faint), and partly because the earlier ones could
not be observed with the spectroscope, which had not then been invented.
It seems clear that in the future we must not allow accident to play so
large a part in the discovery of these objects; more must be done in the
way of deliberate search. Although we know beforehand that this will
involve a vast amount of apparently useless labour, that months and years
must be spent in comparing photographic plates, or portions of the sky
itself, with one another without detecting anything remarkable, it will
not be the first time that years have been cheerfully spent in such
searches without result. We need only recall Hencke's fifteen years of
fruitless search, before finding a minor planet, to realise this fact.
[Sidenote: Superposition of plates.]
[Sidenote: The stereo-comparator.]
One thing of importance may be done; we may improve our methods of making
the search, so as to economise labour, and several successful attempts
have already been made in this direction. The simplest plan is to
superpose two photographs taken at different dates, so that the stars on
one lie very close to those on the other; then if an image is seen to be
unpaired we _may_ have found a new star, though of course the object may
be merely a planet or a variable. The superposition of the plates may be
either actual or virtual. A beautiful instrument has been devised on the
principle of the stereoscope for examining two plates placed side by side,
one with each eye. We know that in this way two photographs of the same
object from different points of view will appear to coalesce, and at the
same time to give an appearance of solidity to the object or landscape,
portions of which will seem to stand out in front of the background.
Applying this principle to two photographs of stars, what happens is this:
if the stars have all remained in the same positions exactly, the two
pictures will seem to us to coalesce, and the images all to lie on a flat
background; but if in the interval between the exposures of the two plates
one of the stars has appreciably moved or disappeared, it will seem, when
looked at with this instrument, to stand out in front of this background,
and is accordingly detected with comparatively little trouble. This new
instrument, to which the name Stereo-comparator has been given, promises
to be of immense value in dredging the sky for strange bodies in the
future. I am glad to say that a generous friend has kindly presented the
University Observatory at Oxford with one of these beautiful instruments,
which have been constructed by Messrs. Zeiss of Jena after the skilful
designs of Dr. Pulfrich. Whether we shall be able to repeat by deliberate
search the success which mere accident threw in our way remains to be
seen.