We use pulsating stars to measure how fast the Universe is expanding, to probe our understanding of the physics of stars, and the structure of nearby galaxies.
Stars change their brightness over time for a myriad of reasons and over many different timescales. Among these variable stars, we are especially interested in the so-called “Cepheids” that change in brightness by up to a factor of two of the course of a few days up to a couple of months. Several such stars are visible by eye, for example the famous North Star, Polaris. However, Polaris is an odd-ball among Cepheids, and its light variations are far too small to be seen by eye. A better candidate is the star δ Cephei, which can be observed year-round from the Northern hemisphere and takes about 5 1/2 days to vary from brightest to faintest and back to brightest.
The physics of stars are important for most aspects of astronomy, and also for understanding how life could develop in the Universe. For example, all the carbon, nitrogen, and oxygen that exists today was created a long time ago in stars that no longer exist. With modern telescopes, observational techniques, theoretical models, and simulations, we can now understand the physics of stars better than ever before, and we exploit all of these methods to figure out exactly how and why stars vary over time.
Cepheids are especially versatile astronomical tools because they adhere to the Leavitt Law, named in honor of Henrietta Swan Leavitt, who found that a Cepheid’s variability period corresponds to its true luminosity. Thanks to the Leavitt Law, we can use Cepheids for measuring distances by comparing the true luminosity of Cepheids with how bright they appear. Cepheids thus form the backbone of the distance ladder used for measuring the expansion of the cosmos whose acceleration is driven by the mysterious “dark energy” that makes up 75% of today’s Universe. We now seek to improve the design of the distance ladder based on new insights into the physics and properties of Cepheids afforded by very detailed observations and state-of-the-art theoretical models. In this way, we will contribute to a better understanding of the nature of dark energy and how the cosmos has evolved since the Big Bang.