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stars are all together. The same result is obtained from distance measurements
by the color-brightness method.
A study of many globular clusters gives another important bit of information.
Tt is found that there is a high concentration oŸ such clusters in a certain part of
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Fig. 5-6. A cluster of stars near the center of our galaxy. 'Their
distance from the earth is 30,000 light-years, or about 3 x 1022 meters.
the sky and that most of them are about the same distance from us. Coupling
this Information with other evidence, we conclude that this concentration of
clusters marks the center of our galaxy. We then know the distance to the center
of the galaxy——about 1029 meters.
lnowing the size of our own galaxy, we have a key to the measurement of
stiilH larger distances—the distances to other galaxies. Eigure 5-7 is a photograph
of a galaxy, which has much the same shape as our own. Probably it is the
same size, too. (Other evidence supports the idea that galaxies are all about the
same size.) IÝ it is the same size as ours, we can tell its distance. We measure
the angle it subtends in the sky; we know its diameter, and we compute its
distance—triangulation againl
Photographs of exceedingly distant galaxies have recently been obtained with
the giant Palomar telescope. One is shown in Pig. 5-8. It is now believed that
some of these galaxies are about halfway to the limit of the universe—10”8 meters
away——the largest distance we can contemplatel
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Fig. 5-7. A spiral galaxy like our own. Presuming that its diameter
Is similar to that of our own galaxy, we may compute Its distance from
its apparent size. lt is 30 million light-years (3 x 1023 meters) from the
earth.
5-7 Short distances
Now lets think about smaller distances. Subdividing the meter is easy.
'Without mụuch dificulty we can mark of one thousand equal spaces which add up
to one meter. With somewhat more difficulty, but in a similar way (using a good
microscope), we can mark off a thousand equal subdivisions of the millimeter
to make a scale of microns (millionths of a meter). It ¡is dificult to continue to
smaller scales, because we cannot “see” obJects smaller than the wavelength of
visible light (about 5 x 10~7 meter).
W© need not stop, however, at what we can see. With an electron microscope,
we can continue the process by making photographs on a still smaller scale,
say down to 10” meter (Eig. 5-9). By indirect measurements—by a kind of
triangulation on a microscopic scale—we can continue to measure to smaller and
smaller scales. First, from an observation oŸ the way light of short wavelength (x-
radiation) is reflected from a pattern oŸ marks of known separation, we determine
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Fig. 5-8. The most distant object, 3C295 in BOOTES (indicated by
the arrow), measured by the 200-inch telescope to date (1960).
the wavelength of the light vibrations. Then, from the pattern of the scattering
of the same light from a crystal, we can determine the relative location of the
atoms in the crystal, obtaining results which agree with the atomic spacings aÌso
determined by chemical means. We fñnd in this way that atoms have a diameter
of about 10~1 meter.
There is a large “gap” in physical sizes between the typical atomie dimension
of about 10~10 meter and the nuclear dimensions 10~!5 meter, 10—5 times smaller.
For nuclear sizes, a diferent way of measuring size becomes convenient. We
measure the øpparen‡ area, ơ, called the efective cross secfion. lf we wish the
radius, we can obtain it from ø = ør2, since nuclei are nearly spherical.
Measurement of a nuclear cross section can be made by passing a beam of
high-energy particles through a thin slab of material and observing the number
of particles which do not get through. 'These high-energy particles will plow right
through the thin cloud of electrons and will be stopped or deflected only If they
hit the concentrated weight of a nucleus. Suppose we have a piece of material
1 centimeter thick. There will be about 10Ẻ atomic layers. But the nuclei are
so small that there is little chance that any nucleus will lie behind another. We
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DISTANCES
LIGHT-YEARS METERS
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Edge of universe
106 To nearest neighbor galaxy
To center of our galaxy
To nearest star
Radius of orbit of Pluto
To the sun
To the moon
Height of a Sputnik
Height of a TV antenna tower
1 Height of a child
A grain of salt
A virus
Radius of an atom
10-15 Radius of a nucleus
???7?217???
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