Man's physical universe

xanabras

MANY INSTRUMENTS SUPPLEMENT TELESCOPE 71

calcium, nickel, zinc, magnesium, oxygen, hydrogen, and many others

were common to both the earth and the sun.

The spectrometer also enables the astronomer to measure star distances.

The same Fraunhofer lines from different stars were observed

to vary in width. By checking these star distances by the method of

parallax,^ it was found that the width of the Fraunhofer lines revealed

the intrinsic or actual brightness of a star. Knowing the apparent

brightness of a star as measured by means of a telescope and the eye or

a photoelectric cell, and knowing its actual brightness as shown by the

Fraunhofer lines, it is easy to estimate star distances. One could compute

distances on the earth in the same way by the use of lights of

equal brightness, such as airplane beacons, and then measuring the

apparent brightness of all of the lights in sight. The knowledge of the

actual and apparent brightness of the stars enables one to estimate

their distance from the earth, for it is known that the intensity of light

varies inversely as the square of the distance from the observer.

The velocity of a source of light can also be observed by means of

the spectrometer. Here again the Fraunhofer lines or emission lines

come into use; they are observed to shift their position slightly. In

the study of sound it is found that the pitch of a musical note will be

raised or lowered according to whether the source of the sound is

approaching or receding from us. This effect, called the Doppler

effect, is found in light also.

It was noticed that the position of spectral

lines produced by light from a moving object is shifted from the position

they take when the light-source is standing still. If the source of

the light is approaching the observer, the lines are shifted toward the

violet end of the spectrum; and if it is receding, the lines are shifted

toward the red end. The amount of the shift depends upon the speed

at which the light-source is moving. By comparison of a spectrophotograph

of a particular star with that of a standard spectrophotograph,

not only the relative speed but the direction of the motion of the star

can be determined.

By way of summary, let us note that the spectrometer, in conjunction

with the telescope and a camera, enables the astronomer to

determine the composition of a star, its temperature, speed, and

direction of motion relative to the earth.

'

The method of parallax is used to determine star distances, using the principle employed

by surveyors in determining the distances across lakes, etc. If a given star is observed

at two intervals, six months apart, so that the observations are made on opposite

sides of the earth's orbit, and the angle of the telescope relative to the plane of the earth's

orbit is measured, then by trigonometry the distance from the earth to the star can be

determined. This method is accurate only for the nearer stars. The angles are so small for

more distant stars that results are unreliable for stars more than 300 light-years away.

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