物理学 1912

小麦哲伦云中 25 颗变星的周期

亨丽埃塔·斯旺·勒维特

一颗星脉动的快慢，透露出它真正有多亮——于是一只秒表，就能丈量宇宙。

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In depth · the introduction

有些星像心脏一样搏动，以稳定的节律一胀一暗。一位失聪的女性，在玻璃底片上一颗颗数着它们，注意到：搏得最慢的，正是最亮的星——并把这一点，变成了一把丈量宇宙的尺。

核心想法

有一类星，叫造父变星，会以几天到几周的规则节律变亮、变暗。亨丽埃塔·勒维特在它们身上发现了一条干净的规则：一颗造父变星完成一次脉动用的时间越长，它真正放出的光就越多。那次脉动，就像印在星上的一张标签，写着它究竟有多亮。

这就解开了天文学最古老的难题。一颗暗淡的星，可能是近处一盏昏灯，也可能是远方一盏明灯，你通常分不清是哪一种。可一旦星的脉动告诉了你它真实的亮度，再把它和「它看上去有多暗」一比，就知道它必定在多远。勒维特，造出了第一把丈量宇宙的尺。

它是如何诞生的

勒维特在哈佛天文台工作，是「计算员」中的一员——一支受雇查看天空照相底片的女性团队，活儿极费心力，薪水却很低。她被分去做变星，而且做得极好，最终编录了一千多颗。为了让比较公平，她专挑小麦哲伦云里的星——那是南天的一团光斑，里面的星离我们大致等远——于是亮度上的任何差别，都必定是真实的。

1908 年，她近乎顺带地记下：越亮的，周期越长。到 1912 年，凭 25 颗仔细测过的造父变星，那点端倪已成了一条定律：图上的一条直线。这份三页的通告，以她主管的名义发出，只有一句话把这项工作归给「勒维特小姐」。

它为何重要

在勒维特之前，对于离我们最近的近邻之外的任何东西，没人能可靠地说出它有多远。她的定律，成了一架距离测量阶梯最底下的一级，而这架阶梯一路伸向整个可观测宇宙。不到十年，埃德温·哈勃就用造父变星证明了那些旋涡「星云」其实是远在银河系之外的一个个完整星系，进而证明它们正彼此飞离——宇宙在膨胀。这一切，没有一把丈量宇宙距离的尺都到不了，而勒维特，给出了第一把。

一个可以想象的画面

想象一群人举着一模一样的灯笼，每盏的亮度都由一条简单的规则定死——闪得慢，说明这盏灯笼亮；闪得快，说明它暗。你站在夜里的旷野上，没法直接量出哪盏灯笼离你多远。可你盯着一盏看它怎么闪，规则就告诉了你它真正烧得多亮；再拿它和「从你站的地方看上去有多暗」一比，你就能一步步量出那段距离。造父变星就是那些灯笼，它们的脉动，就是那闪烁。

它的位置

几个世纪以来，天文学里的距离只能伸到视差——近处恒星那极微小的摆动——所能承载的尽头，至多几百光年。勒维特定律打破了这层天花板，把埃德温·哈勃（1929，也在本馆）所需要的工具递到了他手里，让他得以丈量星系、发现宇宙膨胀，进而铺就了勒梅特与大爆炸宇宙学。直到今天，造父变星仍在干着这份活，正处在那场尚未了结的「哈勃张力」——关于宇宙究竟膨胀得多快——的正中心。

The original document

Original source text

H. S. Leavitt (signed by E. C. Pickering) · Harvard College Observatory Circular 173 (1912): 1–3

Whose work this is

The following statement regarding the periods of 25 variable stars in the Small Magellanic Cloud has been prepared by Miss Leavitt.

The Circular carries the director Edward C. Pickering's signature, but its opening sentence is unusually frank about authorship: the science is Leavitt's. She had measured these stars on photographic glass plates of the southern sky exposed at Harvard's station in Arequipa, Peru.

The straight line

A straight line can be readily drawn among each of the two series of points corresponding to maxima and minima, thus showing that there is a simple relation between the brightness of the variables and their periods.

Leavitt plotted each star's brightness — once at its brightest, once at its faintest — against its period. Against the period itself the points curve; against the logarithm of the period they fall on two clean parallel straight lines. That is the period–luminosity relation, now called Leavitt's law.

Why the trick works

Since the variables are probably at nearly the same distance from the earth, their periods are apparently associated with their actual emission of light, as determined by their mass, density, and surface brightness.

This single sentence is the whole idea. A star looks fainter both when it is dimmer and when it is farther away, and normally you cannot tell which. But all 25 stars sit together in the Small Magellanic Cloud, at one common distance — so the distance cancels, and the differences in brightness must be real differences in how much light the stars pour out. Period, something easy to measure, became a proxy for true luminosity.

Foreshadowed four years earlier

In her 1908 catalogue "1777 Variables in the Magellanic Clouds" (Annals of Harvard College Observatory 60: 87), Leavitt had already noticed the pattern in a single line:

It is worthy of notice that the brighter variables have the longer periods.

[ … ]

[Editorial] The Circular closes by noting that the relation should let astronomers determine distances once the line is calibrated against a few Cepheids of known parallax — a calibration Leavitt herself was never assigned to do. The full three-page scan, with her table and the two-line graph, is at the source below.

Harvard College Observatory · March 3, 1912