[ Pobierz całość w formacie PDF ]
.Roger Penrose and Ishowed that Einstein s general theory of relativity implied that the universe must have a beginning and,possibly, an end.PREVIOUS NEXTfile:///C|/WINDOWS/Desktop/blahh/Stephen Hawking - A brief history of time/a.html (12 of 12) [2/20/2001 3:14:16 AM]A Brief History of Time - Stephen Hawking.Chapter 3CHAPTER 3THE EXPANDING UNIVERSEIf one looks at the sky on a clear, moonless night, the brightest objects one sees are likely to be the planetsVenus, Mars, Jupiter, and Saturn.There will also be a very large number of stars, which are just like our ownsun but much farther from us.Some of these fixed stars do, in fact, appear to change very slightly theirpositions relative to each other as earth orbits around the sun: they are not really fixed at all! This is becausethey are comparatively near to us.As the earth goes round the sun, we see them from different positionsagainst the background of more distant stars.This is fortunate, because it enables us to measure directly thedistance of these stars from us: the nearer they are, the more they appear to move.The nearest star, calledProxima Centauri, is found to be about four light-years away (the light from it takes about four years to reachearth), or about twenty-three million million miles.Most of the other stars that are visible to the naked eye liewithin a few hundred light-years of us.Our sun, for comparison, is a mere light-minutes away! The visible starsappear spread all over the night sky, but are particularly concentrated in one band, which we call the MilkyWay.As long ago as 1750, some astronomers were suggesting that the appearance of the Milky Way could beexplained if most of the visible stars lie in a single disklike configuration, one example of what we now call aspiral galaxy.Only a few decades later, the astronomer Sir William Herschel confirmed this idea bypainstakingly cataloging the positions and distances of vast numbers of stars.Even so, the idea gainedcomplete acceptance only early this century.Our modern picture of the universe dates back to only 1924, when the American astronomer Edwin Hubbledemonstrated that ours was not the only galaxy.There were in fact many others, with vast tracts of emptyspace 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, touse 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 theirapparent brightness and their distance, and so we can work out their luminosity.Conversely, if we knew theluminosity 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 tomeasure; therefore, he argued, if we found such stars in another galaxy, we could assume that they had thesame luminosity  and so calculate the distance to that galaxy.If we could do this for a number of stars in thesame 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 isonly one of some hundred thousand million that can be seen using modern telescopes, each galaxy itselfcontaining some hundred thousand million stars.Figure 3:1 shows a picture of one spiral galaxy that is similarto what we think ours must look like to someone living in another galaxy.file:///C|/WINDOWS/Desktop/blahh/Stephen Hawking - A brief history of time/b.html (1 of 9) [2/20/2001 3:14:24 AM]A Brief History of Time - Stephen Hawking.Chapter 3Figure 3:1We live in a galaxy that is about one hundred thousand light-years across and is slowly rotating; the stars in itsspiral 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 waysince 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.Sohow can we tell different types of stars apart? For the vast majority of stars, there is only one characteristicfeature that we can observe  the color of their light.Newton discovered that if light from the sun passesthrough 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 spectrumof the light from that star or galaxy [ Pobierz całość w formacie PDF ]