Man's physical universe

xanabras

76 THK rMX'ERSE A VAST SYSTEM OF PARTS

for they are composed of stars too far apart to influence each other's

motion appreciably. There are some groups of stars, however, other

than the globular star clusters, such as the Pleiades, which move

through space as a unit.

Stars Move through Space at Tremendous Velocities.

The speed of

stars varies considerably, some of them hurrying along with a velocity

FiG. 11. The Pleiades and accompanying

nebula. (Photograph from the Yerkes Observatory,

reprinted by permission of the

Chicago University Press.)

much light and heat as a

small, hot one. The tem-

Thus the surface temperature of some

perature of stars varies greatly.

of 200 miles per second,

others at very much less.

The average rate is about

18 miles per second. Our

own sun is moving toward

the constellation

Hercules

at the leisurely speed of

12 miles per second. So

great is the distance of the

stars from the earth, however,

that any two of the

more distant stars moving

apart at the rate of

600,000,000 miles per year,

characteristic of the speed

of many stars, would not

show any appreciable

change of position within

a thousand years.

Stars Differ in Temperature,

The true brightness

of a star depends on its size

and temperature; a large,

cool star may radiate as

stars is only 2500° C, which is 1000° less than the temperature obtained

in our best electric furnaces. Other stars have a surface temperature

as high as 16,000° C, while the interior temperature of many

stars is thought to be as high as 20,000,000° C.

Stars Differ in Size and Density. From the knowledge of the temperatures

of stars and other data, it seems to be certain that some stars

are merely volumes of very hot gases, so rarefied that they would pass

as a good vacuum on earth, while other stars are denser than anything

known on earth. Stars do not appear to differ as much as a hundred-

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