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 针尖
visible light 可见光
恒星离开我们是如此之远，以致使我们只能看到极小的光点，而看不到它们的大小和形状。这样怎么能区分不同的恒星种类呢？对于绝大多数的恒星，只有一个特征可供观测——光的颜色。牛顿发现，如果太阳光通过一个称为棱镜的三角形状的玻璃块，就会被分解成像彩虹一样的分颜色（它的光谱）。将一个望远镜聚焦在一个单独的恒星或星系上，人们就可类似地观察到从这恒星或星系来的光谱线。不同的恒星具有不同的光谱，但是不同颜色的相对亮度总是刚好和一个红热的物体发出的光谱完全一致。（实际上，从一个不透明的灼热的物体发出的光，有一个只依赖于它的温度的特征光谱——热谱。这意味着可以从恒星的光谱得知它的温度。）并且，我们发现，某些非常特定的颜色在恒星光谱里找不到，这些失去的谱线可以因不同的恒星而异。既然我们知道，每一化学元素都有非常独特的吸收光谱线族，将它们和恒星光谱中失去的谱线相比较，我们就可以准确地确定恒星大气中存在什么元素。
在20年代天文学家开始观察其他星系中的恒星光谱时，他们发现了最异的现象：它们和我们的银河系一样具有吸收的特征线族，只是所有这些线族都向光谱的红端移动了同样相对的量。为了理解这个含意，我们必须先理解多普勒效应。我们已经知道，可见光即是电磁场的起伏或波动。
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