Myths and Marvels of Astronomy
only fell short of Venus in splendour. It preserved its lustre for about
three weeks, after which time it gradually grew fainter and fainter
until some time between October 1605 and February 1606, when it
disappeared. The exact day is unknown, as during that interval the
constellation of the Serpent-Bearer is above the horizon in the day-time
only. But in February 1606, when it again became possible to look for
the new star in the night-time, it had vanished. It probably continued
to glow with sufficient lustre to have remained visible, but for the
veil of light under which the sun concealed it, for about sixteen months
altogether. In fact, it seems very closely to have resembled Tycho's
star, not only in appearance and in the degree of its greatest
brightness, but in the duration of its visibility.
In the year 1670 a new star appeared in the constellation Cygnus,
attaining the third magnitude. It remained visible, but not with this
lustre, for nearly two years. After it had faded almost out of view, it
flickered up again for awhile, but soon after it died out, so as to be
entirely invisible. Whether a powerful telescope would still have shown
it is uncertain, but it seems extremely probable. It may be, indeed,
that this new star in the Swan is the same which has made its appearance
within the last few weeks; but on this point the evidence is uncertain.
On April 20, 1848, Mr. Hind (Superintendent of the Nautical Almanac,
and discoverer of ten new members of the solar system) noticed a new
star of the fifth magnitude in the Serpent-Bearer, but in quite another
part of that large constellation than had been occupied by Kepler's
star. A few weeks later, it rose to the fourth magnitude. But afterwards
its light diminished until it became invisible to ordinary eyesight. It
did not vanish utterly, however. It is still visible with telescopic
power, shining as a star of the eleventh magnitude, that is five
magnitudes below the faintest star discernible with the unaided eye.
This is the first new star which has been kept in view since its
apparent creation. But we are now approaching the time when it was found
that as so-called new stars continue in existence long after they have
disappeared from view, so also they are not in reality new, but were in
existence long before they became visible to the naked eye.
On May 12, 1866, shortly before midnight, Mr. Birmingham, of Tuam,
noticed a star of the second magnitude in the Northern Crown, where
hitherto no star visible to the naked eye had been known. Dr. Schmidt,
of Athens, who had been observing that region of the heavens the same
night, was certain that up to 11 P.M., Athens local time, there was no
star above the fourth magnitude in the place occupied by the new star.
So that, if this negative evidence can be implicitly relied on, the new
star must have sprung at least from the fourth, and probably from a much
lower magnitude, to the second, in less than three hours--eleven o'clock
at Athens corresponding to about nine o'clock by Irish railway time. A
Mr. Barker, of London, Canada, put forward a claim to having seen the
new star as early as May 4--a claim not in the least worth
investigating, so far as the credit of first seeing the new star is
concerned, but exceedingly important in its bearing on the nature of the
outburst affecting the star in Corona. It is unpleasant to have to throw
discredit on any definite assertion of facts; unfortunately, however,
Mr. Barker, when his claim was challenged, laid before Mr. Stone, of the
Greenwich Observatory, such very definite records of observations made
on May 4, 8, 9, and 10, that we have no choice but either to admit these
observations, or to infer that he experienced the delusive effects of a
very singular trick of memory. He mentions in his letter to Mr. Stone
that he had sent full particulars of his observations on those early
dates to Professor Watson, of Ann Arbor University, on May 17; but
(again unfortunately) instead of leaving that letter to tell its own
story in Professor Watson's hands, he asked Professor Watson to return
it to him: so that when Mr. Stone very naturally asked Professor Watson
to furnish a copy of this important letter, Professor Watson had to
reply, 'About a month ago, Mr. Barker applied to me for this letter, and
I returned it to him, as requested, without preserving a copy. I can,
however,' he proceeded, 'state positively that he did not mention any
actual observation earlier than May 14. He said he thought he had
noticed a strange star in the Crown about two weeks before the date of
his first observation--May 14--but not particularly, and that he did not
recognise it until the 14th. He did not give any date, and did not even
seem positive as to identity.... When I returned the letter of May 17, I
made an endorsement across the first page, in regard to its genuineness,
and attached my signature. I regret that I did not preserve a copy of
the letter in question; but if the original is produced, it will appear
that my recollection of its contents is correct.' I think no one can
blame Mr. Stone, if, on the receipt of this letter, he stated that he
had not the 'slightest hesitation' in regarding Mr. Barker's earlier
observations as 'not entitled to the slightest credit.'[33]
It may be fairly taken for granted that the new star leapt very quickly,
if not quite suddenly, to its full splendour. Birmingham, as we have
seen, was the first to notice it, on May 12. On the evening of May 13,
Schmidt of Athens discovered it independently, and a few hours later it
was noticed by a French engineer named Courbebaisse. Afterwards,
Baxendell of Manchester, and others independently saw the star. Schmidt,
examining Argelander's charts of 324,000 stars (charts which I have had
the pleasure of mapping in a single sheet), found that the star was not
a new one, but had been set down by Argelander as between the ninth and
tenth magnitudes. Referring to Argelander's list, we find that the star
had been twice observed--viz., on May 18, 1855, and on March 31, 1856.
Birmingham wrote at once to Mr. Huggins, who, in conjunction with the
late Dr. Miller, had been for some time engaged in observing stars and
other celestial objects with the spectroscope. These two observers at
once directed their telescope armed with spectroscopic adjuncts--the
telespectroscope is the pleasing name of the compound instrument--to the
new-comer. The result was rather startling. It may be well, however,
before describing it, to indicate in a few words the meaning of various
kinds of spectroscopic evidence.
The light of the sun, sifted out by the spectroscope, shows all the
colours but not all the tints of the rainbow. It is spread out into a
large rainbow-tinted streak, but at various places (a few thousand)
along the streak there are missing tints; so that in fact the streak is
crossed by a multitude of dark lines. We know that these lines are due
to the absorptive action of vapours existing in the atmosphere of the
sun, and from the position of the lines we can tell what the vapours
are. Thus, hydrogen by its absorptive action produces four of the bright
lines. The vapour of iron is there, the vapour of sodium, magnesium, and
so on. Again, we know that these same vapours, which, by their
absorptive action, cut off rays of certain tints, emit light of just
those tints. In fact, if the glowing mass of the sun could be suddenly
extinguished, leaving his atmosphere in its present intensely heated
condition, the light of the faint sun which would thus be left us would
give (under spectroscopic scrutiny) those very rays which now seem
wanting. There would be a spectrum of multitudinous bright lines,
instead of a rainbow-tinted spectrum crossed by multitudinous dark
lines. It is, indeed, only by contrast that the dark lines appear dark,
just as it is only by contrast that the solar spots seem dark. Not only
the penumbra but the umbra of a sun-spot, not only the umbra but the
nucleus, not only the nucleus but the deeper black which seems to lie at
the core of the nucleus, shine really with a lustre far exceeding that
of the electric light, though by contrast with the rest of the sun's
surface the penumbra looks dark, the umbra darker still, the nucleus
deep black, and the core of the nucleus jet black. So the dark lines
across the solar spectrum mark where certain rays are relatively faint,
though in reality intensely lustrous. Conceive another change than that
just imagined. Conceive the sun's globe to remain as at present, but the
atmosphere to be excited to many times its present degree of light and
splendour: then would all these dark lines become bright, and the
rainbow-tinted background would be dull or even quite dark by contrast.
This is not a mere fancy. At times, local disturbances take place in the
sun which produce just such a change in certain constituents of the
sun's atmosphere, causing the hydrogen, for example, to glow with so
intense a heat that, instead of its lines appearing dark, they stand out
as bright lines. Occasionally, too, the magnesium in the solar
atmosphere (over certain limited regions only, be it remembered) has
been known to behave in this manner. It was so during the intensely hot
summer of 1872, insomuch that the Italian observer Tacchini, who noticed
the phenomenon, attributed to such local overheating of the sun's
magnesium vapour the remarkable heat from which we then for a time
suffered.
Now, the stars are suns, and the spectrum of a star is simply a
miniature of the solar spectrum. Of course, there are characteristic
differences. One star has more hydrogen, at least more hydrogen at work
absorbing its rays, and thus has the hydrogen lines more strongly
marked than they are in the solar spectrum. Another star shows the lines
of various metals more conspicuously, indicating that the glowing
vapours of such elements, iron, copper, mercury, tin, and so forth,
either hang more densely in the star's atmosphere than in our sun's, or,
being cooler, absorb their special tints more effectively. But speaking
generally, a stellar spectrum is like the solar spectrum. There is the
rainbow-tinted streak, which implies that the source of light is glowing
solid, liquid, or highly compressed vaporous matter, and athwart the
streak there are the multitudinous dark lines which imply that around
the glowing heart of the star there are envelopes of relatively cool
vapours.
We can understand, then, the meaning of the evidence obtained from the
new star in the Northern Crown.
In the first place, the new star showed the rainbow-tinted streak
crossed by dark lines, which indicated its sun-like nature. _But,
standing out on that rainbow-tinted streak as on a dark background, were
four exceedingly bright lines--lines so bright, though fine, that
clearly most of the star's light came from the glowing vapours to which
these lines belonged._ Three of the lines belonged to hydrogen, the
fourth was not identified with any known line.
Let us distinguish between what can certainly be concluded from this
remarkable observation, and what can only be inferred with a greater or
less degree of probability.
It is absolutely certain that when Messrs. Huggins and Miller made their
observation (by which time the new star had faded from the second to the
third magnitude), enormous masses of hydrogen around the star were
glowing with a heat far more intense than that of the star itself within
the hydrogen envelope. It is certain that the increase in the star's
light, rendering the star visible which before had been far beyond the
range of ordinary eyesight, was due to the abnormal heat of the
hydrogen surrounding that remote sun.
But it is not so clear whether the intense glow of the hydrogen was