What we know worst is often what we have closest to us. It seems a paradox, but in reality it has all the logic. It is very difficult to see what your house consists of if you only see it from within. The trees prevent you from seeing the forest. That's why God cost us and helps to understand that the Earth was one of those bright little lights that move slowly in the sunset, the planets that astronomers of antiquity have already observed, and surely the first Homo erectus They left Africa to know the world. The forest effect it has been repeated on a galactic scale. We already knew some galaxies that, seen in song, have the shape of an S. But we had no idea that our house, the Milky Way, was one of them. Read all the details. Here we are going to review the crucial discovery of the woman who has made all this possible. Henrietta Swan Leavitt (1868-1921), the queen of Cepheids.
The first Cepheid (Delta Cephei, in the constellation Cepheus) was discovered in 1784 by the British astronomer John Goodricke. The brightness of this star, and all of its kind that would be described later, shows pulsations with a period that goes from a day and a half to about 50 days, according to the specific star (for each Cepheid, the period is extremely regular) . They are not very common: about 400 Cepheids have been described in the Milky Way, which has 200,000 million stars. And that was the way things were when Leavitt tackled the problem, and in the worst imaginable conditions: at the time not even the advanced Harvard University admitted her to study physics, and she managed as best she could to be trained in astronomy and placed in a group of well-known women, somewhat contemptuously, as the "calculators", dedicated to cataloging the night sky, especially in the southern hemisphere, the great forgotten by classical European astronomy.
Leavitt knew that there were more and less bright Cepheids, and with slower or faster pulsation periods, without these two properties showing the least relationship. But in a specific sector of the southern sky, the Magellanic Clouds, there was a precise relationship between the brightness and the period of pulsation. What did this mean? Think the reader a minute before continuing reading.
Leavitt deduced the correct answer in a genius trait. The period of pulsation of a Cepheid depends forever of its brightness (that is, of its mass), but not of its apparent brightness, the brightness we see from Earth, but of its intrinsic brightness, the one we would see if we were by its side. With the Cepheids of the Milky Way, that relationship is occluded by very different distances from those on Earth. But the Magellanic Clouds are so far away that the differences in distance to the Earth of one Cepheid or another cease to matter. For practical purposes, all the Cepheids of the Magellanic Clouds are at the same distance from us, and that is why they reveal the hitherto hidden relationship of brightness with the period. Leavitt had found the measuring tape to measure the cosmos: look at the period of a Cepheid and you will know how far it is from us by comparing its apparent brightness with its intrinsic brightness. This was the tape measure used by Hubble to discover that the cosmos is expanding, and what astronomers have now used to reveal that our galaxy has the shape of a twisted sheet. I already told you: great.
* THE SCIENCE OF THE WEEK is a space in which Javier Sampedro analyzes scientific news. Subscribe to the Materia newsletter and you will receive it every Saturday in your email, along with a selection of our best news of the week.