In 1912 Henrietta Swan Leavitt noted that 25 stars, called Cepheid stars (their location in the HR diagram is given in Fig. 8.10), in the Magellanic cloud (see Fig, 8.8) are variable, that is, they brighten and dim periodically.
Many stars are variable, but the Cepheids are special because their period (the time
they for them to brighten, dim and brighten again, see
Fig. 8.9) is
Leavitt was able to measure the period by just looking at the stars and timing the ups and downs in brightness,
But in order to obtain the brightness at the distance of one light year she needed to fist measure the maximum brightness on Earth and then, using the HR method, determine the distance from Earth to these stars (as it turns out, the Magellanic cloud is about 105 light years away from us). What she obtained is that the brighter the Cepheid the longer its period, and that the relation between brightens and period was very simple: a straight line (Fig. 8.11). This means that the period and brightness are proportional to each other
These stars are quite distinct, reasonably abundant and very bright. One can identify them not only in our galaxy, but in many other galaxies as well.
If one requires the distance to a given galaxy one first locates the Cepheid variables in this galaxy. From these observations one determines the period of each of these stars. Leavitt's data states that a given period has a unique brightness associated to it. So form the period and Leavitt's plot we get the brightness at the distance of one light year. We can also measure the brightness on Earth. The brightness at the distance of one light year will be larger than the observed brightness due to the fact that this quantity drops like the square of the distance (Sect. 8.2.1). From these numbers one can extract the distance to the stars. This method works up to 13 million l.y. when Earth-bound telescopes are used; for larger distances these stars become too dim to be observed.
Much more recently the Hubble telescope has used this same type of indicators to much farther distances (the Hubble is outside the Earth's atmosphere and can detect much fainter stars). Looking at a galaxy in the Virgo cluster (the galaxy is ``called'' M100), Wendy L. Freedman found (1994-5) that the Cepheid variables in this galaxy could be used to determine its distance; the result is 56 million l.y..