A Briefer History of Time by Stephen Hawking Read Free Book Online Page B

positions against the background of more distant stars. The effect is the same one you see when you are driving down an open road and the relative positions of nearby trees seem to change against the background of whatever is on the horizon. The nearer the trees, the more they seem to move. This change in relative position is called parallax. (See illustration on page 52.) In the case of stars, it is fortunate, because it enables us to measure directly the distance of these stars from us.
    As we mentioned in Chapter 1, the nearest star, Proxima Centauri, is about four light-years, or twenty-three million million miles, away. 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 eight 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 cataloguing the positions and distances of vast numbers of stars. Even so, this idea gained complete acceptance only early in the twentieth century We now know that the Milky Way-our galaxy—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 we thought that the earth was the center of the universe!
    Our modern picture of the universe dates back only to 1924, when the American astronomer Edwin Hubble demonstrated that the Milky Way was not the only galaxy. He found, in fact, many others, with vast tracts of empty space between them. In order to prove this, Hubble needed to determine the distances from the earth to the other galaxies. But these galaxies were so far away that, unlike nearby stars, their positions really do appear fixed. Since Hubble couldn’t use the parallax on these galaxies, he was forced to use indirect methods to measure their distances. One obvious measure of a star’s distance is its brightness. But the apparent brightness of a star depends not only on its distance but also on how much light it radiates (its luminosity). A dim star, if near enough, will outshine the brightest star in any distant galaxy. So in order to use apparent brightness as a measure of its distance, we must know a star’s luminosity.
    The luminosity of nearby stars can be calculated from their apparent brightness because their parallax enables us to know their distance.

    Parallax
    Whether you are moving down a road or through space, the relative position of nearer and farther objects changes as you go A measure of that change can be used to determine the relative distance of the objects
    Hubble noted that these nearby stars could be classified into certain types by the kind of light they give off. The same type of stars always had the same luminosity. He then argued that if we found these types of stars in a distant galaxy, we could assume that they had the same luminosity as the similar stars nearby. With that information, we could 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, Hubble worked out the distances to nine different galaxies.
    Today we know that stars visible to the naked eye make up only a minute fraction of all the stars. We can see about five thousand stars, only about .0001 percent of all the stars in just our own galaxy, the Milky Way.