The Day We Found the Universe - Marcia Bartusiak [65]
Globular Cluster M80
(The Hubble Heritage Team [AURA/STScI/NASA])
As his collection of photographs mushroomed, though, Shapley began to identify Cepheids which he knew would serve as his measuring tape out to the globular clusters. He was quite aware of the paper that Henrietta Leavitt had published just a couple of years earlier and intended to apply it. “Her discovery … is destined to be one of the most significant results of stellar astronomy,” Shapley later wrote to her boss, Pickering.
What was needed was a reliable distance to a Cepheid—any Cepheid, anywhere in the sky—that could serve as the calibration for determining the distances to all other Cepheids using Leavitt's period-luminosity law. That was the beauty of her discovery: Know the distance to just one Cepheid and you know the rest.
Distance measurements have long been a problem for astronomers. To our eye, the celestial sky resembles a dark bowl with pinpoints of light affixed to it—everything appears to be the same distance away. But in reality the stars we see reside at vastly different ranges. Bluish-white Sirius, the brightest star in the heavens, is located 8.6 light-years from Earth; Vega, the prominent summertime star in the constellation Lyra, lies 25 light-years away. How do astronomers arrive at these numbers? “Parallax” is one surveying technique. Parallax is the apparent change in a star's position on the sky when observed first at one end of Earth's orbit and then six months later at the other end (similar to the way an object close by will appear to shift when you view it first with one eye, then the other). By setting the radius of Earth's orbit as a baseline and knowing the angle of shift in the star's parallax, a bit of geometric triangulation determines the star's distance from the Sun directly. Astronomers devised the term parsec to describe the distance between Earth and a celestial object that displays a parallax of one arcsecond of angular measurement on the sky. (One parsec equals 3.26 light-years.) The parallax method is useful out to several hundred light-years. After that, the change in a star's position is too small to be discernible by ground-based telescopes, which is why Leavitt's law was so treasured. It would enable astronomers to extend their distance surveys much farther outward. It would have been nice if a Cepheid resided fairly close to our Sun; then astronomers could have measured the star's parallax and gotten their calibration fairly easily. Unfortunately, there was no Cepheid within reach of a direct parallax measurement from Earth in Shapley's day. Nature was not so accommodating to astronomers. (The closest Cepheid to us is Polaris, the North Star, located about 430 light-years away. Polaris is actually a three-star system, one of which is a large yellow Cepheid that completes its dim/bright cycle every four days.)
The first person to try to confront the Cepheid distance problem was Ejnar Hertzsprung, who had initially recognized that Leavitt's twenty-five variables in the Small Magellanic Cloud were specifically Cepheid stars. He began to look at the Cepheids best studied within the Milky Way, thirteen in all. He couldn't measure their parallax (they were too far away), but he could consult a chronological sequence of astronomical atlases to see how far the stars had moved across the sky, at right angles to our line of sight, in their travels through the Milky Way. It was a matter of determining how their celestial coordinates had changed over the years. Astronomers refer to this advance as a star's “proper motion.” From another type of catalog he looked up how fast they were moving either toward or away from Earth based on the stars' blueshifts or redshifts (a rough gauge of their overall velocity). In an imaginative leap, he then estimated the Cepheid's distance by comparing the star's measured velocity with how fast it appears to be