How the Size of Our Galaxy Was Measured

But if you knew the light was a 100-watt bulb, everything changes. A 100-watt bulb appearing this faint is at such-and-such a distance — that you can compute.

Triangulation will not do it. Stars are so far that even a baseline the width of Earth's orbit gives under one arcsecond. That reaches only the nearest few light years — nowhere near the scale of a galaxy.

Stars that tell you themselves

But some stars do announce their wattage.

Certain stars vary in brightness on a regular cycle, dimming and brightening over and over. And among them there was a rule: the brighter the star, the slower it blinks.

The person who found the rule was in a job with no access to a telescope: measuring and recording stellar brightness at the Harvard Observatory from plates that others had exposed.

A little further in

How do you know how far away a star is? Looking bright does not mean being near. Apparent brightness mixes two things: how bright the star truly is, and how far off it lies.

If you could know a star's true brightness, comparing it with the apparent brightness would give the distance — the way knowing a bulb is 100 watts lets you judge its distance from how dim it looks.

Stars that change brightness

Leavitt's job at the Harvard Observatory was examining photographic plates — not operating telescopes but measuring and recording stellar brightness from plates others had exposed. The women doing this work were called computers, paid by the hour, and not allotted observing time.

Her assignment was variable stars in the Small Magellanic Cloud, which brighten and dim in cycles. She found 1,777 of them and timed the periods of 25 precisely.

And she found a rule: the brighter the star, the longer its period. More precisely, brightness and period stood in a clean relation.

Why this was decisive

1. The stars in the Small Magellanic Cloud are all at roughly the same distance, being one clump.
2. At the same distance, differences in apparent brightness are differences in true brightness.
3. And that true brightness corresponds regularly to the period.
4. So for any such variable star, timing the period gives its true brightness.
5. Compare true with apparent brightness and the distance follows.

The method still needed calibrating. Leavitt's relation was relative — this period means so many times brighter than that one — not absolute. Fixing the scale required the distance to one nearby variable, measured by triangulation. Bessel's bottom rung comes in here.

The galaxy's size, and what lies beyond

With this ruler in hand, two things followed.

Shapley measured distances to globular clusters and mapped the size and shape of the Milky Way — finding that the Sun sits not at its centre but out towards the edge. After Copernicus displaced the Earth, the Sun was displaced too.

Hubble found variable stars in Andromeda. Timing them and computing the distance gave a figure far beyond the size of our own galaxy. Andromeda was not a nebula within the Milky Way but another galaxy entirely.

The universe grew by orders of magnitude in a stroke. And at the bottom of the calculation lay a table made by someone counting the blinking of stars on photographic plates. Leavitt died in 1921 without seeing the result.

The question that remainsApparent brightness alone told nothing; a signal the star gives about itself had to be found. What signals are we still failing to read?