Man's physical universe

xanabras

78 THE UNIVERSE A VAST SYSTEM OF PARTS

so bright that they can be seen in the daytime.

One of these "novae,"

1^ ^>.

as such stars are called, appeared in the constellation of Aquila in 1918,

and another one was discovered in Cygnus a couple of years later.

This increase of brightness, which can be likened to the sudden change

from a small one-cell flashlight

to that of a huge

searchlight, occurs in a cosmic

instant, changing from

a very faint light to its

maximum. The light gradually

decreases to its original

intensity in about ten years.

Such a rapid change can be

accounted for only as an explosion.

Additional evidence

that explosions really take

place is ofifered by the expanding

shells of nebulous

matter that have been observed

around several novae.

About 150 small blue stars

have been observed which

are surrounded by shells of

luminous, nebulous material.

These planetary nebulae, as

_ ,,.„ „, they are called, are thought

Fig. 13. Dark nebulae in the Milky Way. ^ , ,j r^,

^o be old novae. The cause

(Photograph from the Yerkes Observatory,

reprinted by permission of the Chicago of these explosions has not

University Press.)

yet been found.

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