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.) More-over, 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.
Pinpoint 針尖
visible light 可見光
恆星離開我們是如此之遠，以致使我們只能看到極小的光點，而看不到它們的大小和形狀。這樣怎麼能區分不同的恆星種類呢？對於絕大多數的恆星，只有一個特徵可供觀測——光的顏色。牛頓發現，如果太陽光通過一個稱為稜鏡的三角形狀的玻璃塊，就會被分解成像彩虹一樣的分顏色（它的光譜）。將一個望遠鏡聚焦在一個單獨的恆星或星系上，人們就可類似地觀察到從這恆星或星系來的光譜線。不同的恆星具有不同的光譜，但是不同顏色的相對亮度總是剛好和一個紅熱的物體發出的光譜完全一致。（實際上，從一個不透明的灼熱的物體發出的光，有一個只依賴於它的溫度的特徵光譜——熱譜。這意味著可以從恆星的光譜得知它的溫度。）並且，我們發現，某些非常特定的顏色在恆星光譜裡找不到，這些失去的譜線可以因不同的恆星而異。既然我們知道，每一化學元素都有非常獨特的吸收光譜線族，將它們和恆星光譜中失去的譜線相比較，我們就可以準確地確定恆星大氣中存在什麼元素。
在20年代天文學家開始觀察其他星系中的恆星光譜時，他們發現了最異的現象：它們和我們的銀河系一樣具有吸收的特徵線族，只是所有這些線族都向光譜的紅端移動了同樣相對的量。為了理解這個含意，我們必須先理解多普勒效應。我們已經知道，可見光即是電磁場的起伏或波動。
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