The Queen of the Cefeidas

What we know worse is often what we have closer. It seems like a paradox, but it actually has all the logic. It is very difficult to see what your house consists of if you only see it from the inside. The trees prevent you from seeing the forest. That's why it cost God and help us to understand that the earth was one of those bright lights that move slowly in the twilight, the planets already observed by the astronomers of antiquity, and surely the first Homo erectus who left Africa to Know the world. The forest effect has been repeated on a galactic scale. We already knew some galaxies that, singing views, 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 in the matter. Here we are going to review the crucial discovery of the woman who has made all this possible. Henrietta Swan Leavitt (1868-1921), Queen of the Cefeidas.

The first Cefeida (Delta Cephei, in the constellation Cepheus) was descubiera in 1784 by the British astronomer John Goodricke. The brightness of this star, and of all of its kind to be described later, shows beats with a period that goes from a day and a half to about 50 days, according to the specific star (for each cefeida, the period is extremely regular). They are not very common: some 400 cefeidas have been described in the Milky Way, which has 200 billion of stars. And so there were things when Leavitt addressed the problem, and in the worst imaginable conditions: In his time not even the advanced Harvard University admitted it to study physics, and managed as he could to form in astronomy and placed in a group of Women known, somewhat contemptuously, as the "calculators", dedicated to cataloguing the night sky, especially the southern hemisphere, the great forgotten by classical European astronomy.

Leavitt knew that there were cefeidas more and less brilliant, and with slower or faster pulsation periods, without these two properties showing the least relationship. But in a particular sector of the southern sky, the Magellanic Clouds, there was a precise relationship between the brightness and the pulsating period. What did this mean? Think the reader a minute before you continue reading.

Leavitt deduced the correct answer in a trait of genius. The period of pulsation of a cefeida always depends on its brightness (i.e., its mass), but not its apparent brightness, the brightness that we see from the earth, but its intrinsic luster, which we would see if we were at his side. With the cefeidas of the Milky Way, this relationship is occluded by the very different distances to those that are of the Earth. But the Magellanic clouds are so far away that the differences in distance to the land of one cefeida or another cease to matter. For practical purposes, all the cefeidas of the Magellanic clouds are at the same distance from us, and therefore 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 cefeida and you will know how far away it is from us without more than comparing its apparent brightness with its intrinsic luster. This was the tape measure that Hubble used to discover that the cosmos is expanding, and that the astronomers have now used to reveal that our galaxy is shaped like a crooked veneer. I already told you, great.

* The science of the week is a space in which Javier Sampedro analyzes the scientific news. Subscribe to the Materia newsletter and you will receive it every Saturday in your mail, along with a selection of our best news of the week.

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