Man's physical universe

xanabras

84 THE UNIVERSE A VAST SYSTEM OF PARTS

class of these variable stars known as the Cepheid Variables, named

after one of the stars in the group, Delta Cephei, discovered that the

period of variation is related to the intrinsic brightness. Once the

intrinsic brightness of a star is known, its distance can be calculated, of

course, by methods already outlined.

W^k^.

Fig. 18. The Spiral Nebula in Triangulum,

Messier 33. (Photograph from the Yerkes

Observatory, reprinted by permission of the

Chicago University Press.)

It was found that many of the exterior galaxies contain Cepheid

Variables which enable us to form our present conclusions concerning

the distance of these

galaxies from ours. Note

that it would no longer

add meaning to refer such

distances to the earth or

the sun.

Extragalactic Nebulae

Move at Various

Speeds.

There is a definite rotational

motion in spiral nebulae.

Photographs taken

at ten-year intervals show

that the spiral nebula, Andromeda,

makes one rotation

about every 17,000,000

years.

Some astronomers have

suggested that our galaxy,

too, is spiral in shape; but

little evidence is available.

We are too much a part of

it to see it in the proper

perspective. There is evidence,

however, that our galaxy rotates once in about every 100,000,-

000,000 years.

The exterior galaxies are the

"speed demons" of the universe; at

least the shift in the Fraunhofer lines indicates that Andromeda is

approaching our galaxy at a speed of nearly 200 miles per second.

of these galaxies seem to be moving in the same general direction, perhaps

as a single organization of supergalaxies, which Shapley would

call a metagalaxy.

The fact that all of the galaxies seem to be receding from one another

has led to the theory of an expanding universe.

All

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