Page 22 - A Brief History of Time - Stephen Hawking

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

THE EXPANDING UNIVERSE

If one looks at the sky on a clear, moonless night, the brightest objects one sees are likely to be the planets
Venus, Mars, Jupiter, and Saturn. There will also be a very large number of stars, which are just like our own
sun but much farther from us. Some of these fixed stars do, in fact, appear to change very slightly their
positions relative to each other as earth orbits around the sun: they are not really fixed at all! This is because
they are comparatively near to us. As the earth goes round the sun, we see them from different positions
against the background of more distant stars. This is fortunate, because it enables us to measure directly the
distance of these stars from us: the nearer they are, the more they appear to move. The nearest star, called
Proxima Centauri, is found to be about four light-years away (the light from it takes about four years to reach
earth), or about twenty-three million million miles. Most of the other stars that are visible to the naked eye lie
within a few hundred light-years of us. Our sun, for comparison, is a mere light-minutes away! The visible stars
appear spread all over the night sky, but are particularly concentrated in one band, which we call the Milky
Way. As long ago as 1750, some astronomers were suggesting that the appearance of the Milky Way could be
explained if most of the visible stars lie in a single disklike configuration, one example of what we now call a
spiral galaxy. Only a few decades later, the astronomer Sir William Herschel confirmed this idea by
painstakingly cataloging the positions and distances of vast numbers of stars. Even so, the idea gained
complete acceptance only early this century.

Our modern picture of the universe dates back to only 1924, when the American astronomer Edwin Hubble
demonstrated that ours was not the only galaxy. There were in fact many others, with vast tracts of empty
space between them. In order to prove this, he needed to determine the distances to these other galaxies,
which are so far away that, unlike nearby stars, they really do appear fixed. Hubble was forced, therefore, to
use indirect methods to measure the distances. Now, the apparent brightness of a star depends on two factors:
how much light it radiates (its luminosity), and how far it is from us. For nearby stars, we can measure their
apparent brightness and their distance, and so we can work out their luminosity. Conversely, if we knew the
luminosity of stars in other galaxies, we could work out their distance by measuring their apparent brightness.
Hubble noted that certain types of stars always have the same luminosity when they are near enough for us to
measure; therefore, he argued, if we found such stars in another galaxy, we could assume that they had the
same luminosity – and so calculate the distance to that galaxy. If we could do this for a number of stars in the
same galaxy, and our calculations always gave the same distance, we could be fairly confident of our estimate.

In this way, Edwin Hubble worked out the distances to nine different galaxies. We now know that our galaxy is
only one of some hundred thousand million that can be seen using modern telescopes, each galaxy itself
containing some hundred thousand million stars. Figure 3:1 shows a picture of one spiral galaxy that is similar
to what we think ours must look like to someone living in another galaxy.

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