larger motion, the mean of the two being 430 kilometres per second.

Very different was it with the accompanying darkline spectrum. The two hydrogen lines agreed in giving a motion toward the earth of about 780 kilometres per second. The difference of these two results is enormous, more than 600 English miles per second.

The problem of reconciling these rapid motions with any easily conceivable constitution of the body or bodies was no easy one, and the proposed solutions can hardly be considered as better than speculations. The view most generally received was that two bodies had suddenly approached very closely together, perhaps come into collision, and then separated. While this view is by no means. impossible, it is far from being established. The great change in the character of the spectrum, while not conclusive against it, certainly seems to throw difficulties in the way of its reception. The history of the star leaves us in great doubt on the question whether, even if the displacement of the lines was due to a rapid motion, the latter was the integral motion of a body. It might have been only that of an incandescent gas escaping from under pressure, in a direction from our system, in fact, an eruption of hydrogen and calcium vapours. If these vapours, after cooling, fell back again in such a way as to cut off the light of the brighter region beyond, they would absorb the dark lines and give the spectrum of a dark body moving toward us.

The most recent investigations showing to what changes the form, position, and brightness of spectral lines are subject through changes in the physical condition of the bodies which emit the light lead to great caution in attributing the displacement of broadened lines in any spectrum to motion.

The fact that these objects blaze up only once in their history shows that the phenomenon is due to some cataclysm of a rather extraordinary kind. The first and most interesting question raised by this fact is whether one star is more likely to be subject to such a cataclysm than another. If new stars were known to vary, or to have any special kind of spectrum before their sudden outburst, we should know that the latter was a catastrophe to which only a particular kind of star is subject. If we could find no peculiarity in the spectrum of the star we should conclude that the catastrophe was due to some external cause. But unfortunately we have thus far no record of any new star before its appearance except, in a very few cases, its position in the heavens. It is true that the star may be studied after it has settled down again, but if the catastrophe was due to an external cause, we have no reason to suppose that it had relapsed to its former condition. Quite likely the cataclysm might have made a permanent change in its constitution.

Perhaps the most natural theory at first sight is that the outburst is due to a collision. It seems probable that stars like our sun, which are in a state of considerable condensation, have somewhat the

character of masses of gas confined under enormous

hollow globes of highly We do not mean by this

pressure, as if they were heated and compressed gas. that the shell is solid; what is possible is that it is composed of divided matter probably denser than the gases below, and compressing the latter by its weight rather than by its tension. If, by the fall of a foreign body, an opening is suddenly made in the shell, the interior gases will burst forth. What magnitude the outburst might assume it is impossible to say, and cautious thinkers will decline to accept this or any other solution until we have had more experience on the subject.

A general fact that seems supported by the most recent observations is that after their outbursts of light these bodies settle down to a nebular condition. This was the case with Nova Auriga, and the recent Nova Aquila of 1900. Campbell found the spectrum of the latter to consist of extremely faint continuous light in the green, and three bright bands in the positions of the three nebular lines.

On the night of February 21-22, 1901, Dr. Anderson of Edinborough noticed a previously unknown. star of magnitude 2.7, in the constellation

The New Star of 1901 in Perseus.

Perseus. In the course of the next two days it increased so rapidly as to become. about the third brightest star in the sky, being a little brighter than Capella. Then it began slowly to fade away. Early in March it was again of the third magnitude, and before the middle of April had dropped to the fifth.

It seems to have blazed out with extraordinary rapidity. It happened most fortunately that the region had been photographed at the Harvard Observatory several times during the month of February, the last photograph having been taken on the 19th. The plate showed stars as faint as the eleventh magnitude. It must therefore have risen from some magnitude below the eleventh to the first within about three days. This difference corresponds to an increase of the light ten thousandfold.

Its spectrum shows the mixture of dark and bright bands characteristic of new stars. But, in the beginning, Campbell found that the sodium lines were faint and dark. He was thus enabled to determine the radial velocity of the star, which was six kilometres per second away from the sun.

Nova Persei, as the star will hereafter be called, is the brightest new star that has been recorded since. the time of Kepler. But it is not impossible that, before the heavens were so carefully watched by observers, such an object might have reached an equal degree of brightness without exciting notice. The complete history of this star cannot yet be written, and there is no reason to suppose that it will differ very widely from that of Nova Auriga. Indeed on June 25, 1901 Professor Pickering reported that its spectrum had been gradually changing into that of a gaseous nebula.

IT

CHAPTER IX

THE PARALLAXES OF THE STARS

These mathematic men have thoughts that march
From sphere to sphere and measure out the blue
Of infinite space like roods of garden ground.

BLACKIE.

needs only the most elementary conceptions of space, direction, and motion to see that, as the earth makes its vast swing from one extremity of its orbit to the other, the stars, being fixed, must have an apparent swing in the opposite direction. The seeming absence of such a swing was in all ages before our own one of the great stumbling-blocks of astronomy. It was the base on which Ptolemy erected his proof that the earth was immovable in the centre of the celestial sphere. It was felt by Copernicus to be a great difficulty in the reception of his system. It led Tycho Brahe to suggest a grotesque combination of the Ptolemaic and Copernican systems, in which the earth was the centre of motion, round which the sun revolved, carrying the planets with it.

With every improvement in their instruments, astronomers sought to detect the annual swing of the

stars.

Each time that increased accuracy in observa