We now know that Cepheids throb as they do because they are elderly stars that have moved past their “main sequence phase,” in the parlance of astronomers, and become red giants. The chemistry of red giants is a little weighty for our purposes here (it requires an appreciation for the properties of singly ionized helium atoms, among quite a lot else), but put simply it means that they burn their remaining fuel in a way that produces a very rhythmic, very reliable brightening and dimming. Leavitt's genius was to realize that by comparing the relative magnitudes of Cepheids at different points in the sky you could work out where they were in relation to each other. They could be used as “standard candles”—a term she coined and still in universal use. The method provided only relative distances, not absolute distances, but even so it was the first time that anyone had come up with a usable way to measure the large-scale universe.
我们现在知道，造父变星之所以搏动，是因为——用天文学家的行话来说——它们已经走过“主序阶段”，变成了红巨星。红巨星的化学过程有点儿难懂，已经超出了本书的宗旨（它要求了解很多东西，其中之一就是单离子化的氦原子的性质）。但是，简而言之，在燃烧剩余的燃料的过程中，它们产生了一种很有节奏、不停地一亮一暗的现象。莱维特的天才在于，她发现，通过比较造父变星在天空中不同角度的大小，就可以计算出它们之间的相对位置。它们可以被作为标准烛光——这个名称也是她创造的，现在依然广泛使用。用这种方法得到的只是相对距离，不是绝对距离。但是，即使这样，这也是第一次有人想出了一个计算浩瀚宇宙的实用方法。

(Just to put these insights into perspective, it is perhaps worth noting that at the time Leavitt and Cannon were inferring fundamental properties of the cosmos from dim smudges on photographic plates, the Harvard astronomer William H. Pickering, who could of course peer into a first-class telescope as often as he wanted, was developing his seminal theory that dark patches on the Moon were caused by swarms of seasonally migrating insects.)
（为了合理评价这些深邃的见解，也许值得注意的是，当莱维特和坎农在根据照片上远方星星的模糊影子推定宇宙的基本特性的时候，哈佛大学的天文学家威廉·H.皮克林——他当然能从一流的天文望远镜里想观察多少次就观察多少次——却在建立自己的理论，认为月球上的黑影是由大群大群的、随着季节迁徙的昆虫形成的。）