We continue on with the discussion of Ms. Henrietta Leavitt, whose careful studies of the brightnesses of stars led to a means to measure the distances between the galaxies. Following her certificate for studies at Radcliffe College, Ms. Leavitt secured a research position at Harvard College Observatory measuring the brightnesses of stars.
In this discipline known as stellar photometry, images of stars are acquired and one by one compared to ‘standards.’ Ms. Leavitt noticed that while the vast majority of stars that she could study in the Milky Way shone steadily, a small subset of stars instead varied in brightness with a regular cycle of dimming and then brightening up again that lasted weeks to months.
In an epic comment of understatement, she wrote that “It is worthy of notice that the brighter variables have the longer periods.” Then she studied stars in the nearby galaxy called the Large Magellanic Cloud, and found additional variable stars with similar properties as the ones found in the Milky Way.
She understood that these were the same types of stars in both galaxies, yet the ones in the Large Magellanic Clould were fainter. If you could get a stopwatch and count the number of beats a star makes in the distant galaxy then you could use it to measure the intrinsic brightness. Compare that to the brightness it appears to have to get the distance.
This is similar to saying that because we can read the wattage on a flashlight we can put the flashlight at the other side of a football field obtain an estimate of its distance. If we did not know the wattage of the flashlight ahead of time, then if we saw a faint light coming from a some distance away we would not know if the light hailed from a tiny keychain flashlight right in front of our noses or from a spotlight placed a large distance away. Ms. Leavitt gave us the means to read that ‘wattage’ of the star!