happens is that in some region of the
sky where no star, or only a very faint star, had been registered on our
charts, we almost suddenly perceive a bright star. In a few days it may
rise to the highest brilliancy. By the spectroscope we learn that this
distant blaze means a prodigious outpour of white-hot hydrogen at
hundreds of miles a second. But the star sinks again after a few months,
and we then find a nebula round it on every side. It is natural to
suppose that a dead or dying sun has somehow been reconverted in whole
or in part into a nebula. A few astronomers think that it may have
partially collided with another star, or approached too closely to
another, with the result we described on an earlier page. The general
opinion now is that a faint or dead star had rushed into one of those
regions of space in which there are immense stretches of nebulous
matter, and been (at least in part) vaporised by the friction.

But the difficulties are considerable, and some astronomers prefer to
think that the blazing star may merely have lit up a dark nebula which
already existed. It is one of those problems on which speculation is
most tempting but positive knowledge is still very incomplete. We may be
content, even proud, that already we can take a conflagration that has
occurred more than a thousand trillion miles away and analyse it
positively into an outflame of glowing hydrogen gas at so many miles a
second.

THE SHAPE OF OUR UNIVERSE

Sec. 4

Our Universe a Spiral Nebula

What is the shape of our universe, and what are its dimensions? This is
a tremendous question to ask. It is like asking an intelligent insect,
living on a single leaf in the midst of a great Brazilian forest, to say
what is the shape and size of the forest. Yet man's ingenuity has proved
equal to giving an answer even to this question, and by a method exactly
similar to that which would be adopted by the insect. Suppose, for
instance, that the forest was shaped as an elongated oval, and the
insect lived on a tree near the centre of the oval. If the trees were
approximately equally spaced from one another they would appear much
denser along the length of the oval than across its width. This is the
simple consideration that has guided astronomers in determining the
shape of our stellar universe. There is one direction in the heavens
along which the stars appear denser than in the directions at right
angles to it. That direction is the direction