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. Fig. 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. 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!
Luminosity 光度
1924年，我們現代的宇宙圖象才被奠定。那是因為美國天文學家埃得溫·哈勃證明了，我們的星系不是唯一的星系。事實上，還存在許多其他的星系，在它們之間是巨大的空虛的太空。為了證明這些，他必須確定這些星系的距離。這些星系是如此之遙遠，不像鄰近的恆星那樣，它們確實顯得是固定不動的。所以哈勃被迫用間接的手段去測量這些距離。眾所周知，恆星的表觀亮度決定於兩個因素：多少光被輻射出來（它的絕對星等）以及它離我們多遠。對於近處的恆星，我們可以測量其表觀亮度和距離，這樣我們可以算出它的絕對亮度。
相反，如果我們知道其他星系中恆星的絕對亮度，我們可用測量它們的表觀亮度的方法來算出它們的距離。哈勃注意到，當某些類型的恆星近到足夠能被我們測量時，它們有相同的絕對光度；所以他提出，如果我們在其他星系找出這樣的恆星，我們可以假定它們有同樣的絕對光度——這樣就可計算出那個星系的距離。如果我們能對同一星系中的許多恆星這樣做，並且計算結果總是給出相同的距離，則我們對自己的估計就會有相當的信賴度。
埃得溫·哈勃用上述方法算出了九個不同星系的距離。現在我們知道，我們的星系只是用現代望遠鏡可以看到的幾千億個星系中的一個，每個星系本身都包含有幾千億顆恆星。圖3.1所示的便是一個螺旋星系的圖，從生活在其他星系中的人來看我們的星系，想必也是類似這個樣子。我們生活在一個寬約為10萬光年並慢慢旋轉著的星系中；在它的螺旋臂上的恆星繞著它的中心公轉一圈大約花幾億年。我們的太陽只不過是一個平常的、平均大小的、黃色的恆星，它靠近在一個螺旋臂的內邊緣。我們離開亞里士多德和托勒密的觀念肯定是相當遙遠了，那時我們認為地球是宇宙的中心！
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