Page 23 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 3

Figure 3:1

We live in a galaxy that is about one hundred thousand light-years across and is slowly rotating; the stars in its
spiral arms orbit around its center about once every several hundred million years. Our sun is just an ordinary,
average-sized, yellow star, near the inner edge of one of the spiral arms. We have certainly come a long way
since Aristotle and Ptolemy, when thought that the earth was the center of the universe!

Stars are so far away that they appear to us to be just pinpoints of light. We cannot see their size or shape. So
how can we tell different types of stars apart? For the vast majority of stars, there is only one characteristic
feature that we can observe – the color of their light. Newton discovered that if light from the sun passes
through a triangular-shaped piece of glass, called a prism, it breaks up into its component colors (its spectrum)
as in a rainbow. By focusing a telescope on an individual star or galaxy, one can similarly observe the spectrum
of the light from that star or galaxy. Different stars have different spectra, but the relative brightness of the
different colors is always exactly what one would expect to find in the light emitted by an object that is glowing
red hot. (In fact, the light emitted by any opaque object that is glowing red hot has a characteristic spectrum
that depends only on its temperature – a thermal spectrum. This means that we can tell a star’s temperature
from the spectrum of its light.) Moreover, we find that certain very specific colors are missing from stars’
spectra, and these missing colors may vary from star to star. Since we know that each chemical element
absorbs a characteristic set of very specific colors, by matching these to those that are missing from a star’s
spectrum, we can determine exactly which elements are present in the star’s atmosphere.

In the 1920s, when astronomers began to look at the spectra of stars in other galaxies, they found something
most peculiar: there were the same characteristic sets of missing colors as for stars in our own galaxy, but they
were all shifted by the same relative amount toward the red end of the spectrum. To understand the
implications of this, we must first understand the Doppler effect. As we have seen, visible light consists of
fluctuations, or waves, in the electromagnetic field. The wavelength (or distance from one wave crest to the
next) of light is extremely small, ranging from four to seven ten-millionths of a meter. The different wavelengths
of light are what the human eye sees as different colors, with the longest wavelengths appearing at the red end
of the spectrum and the shortest wavelengths at the blue end. Now imagine a source of light at a constant
distance from us, such as a star, emitting waves of light at a constant wavelength. Obviously the wavelength of

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