Tales of the North Star

The North Star intrigues us and guides us. It is a beacon in the night sky for peoples and cultures around the world. Even though the North Star, also called Polaris, is but one star in a sky of millions and billions of other stars Polaris is special. Polaris is a guide star for cosmology, helping astronomers measure the expansion of the Universe.

Polaris is one of the brightest stars in the night sky because it is both a supergiant star and, relatively speaking, close to us. Polaris also does something else, it pulsates. That is, the amount of light we see from the star brightens and dims over a period of about 4 days and this variation repeats over and over. This variation is almost perfectly periodic and we classify Polaris to be one of a type of stars called classical Cepheids.

Classical Cepheids are found throughout our Galaxy and beyond in numerous other galaxies. We find Cepheids as far away as the Virgo (54,000,000 lightyears) and Coma clusters (100 million lightyears). We know that these stars are Cepheids because we see their periodic changes in brightness that is their characteristic ID. All Cepheids pulsate and their periods range from just a few days to more than one hundred days.

The one thing that makes this pulsation really interesting is that the period of variation is relative to how much light a Cepheid emits. That is Cepheids with long periods of tens of days emit more light than Cepheids with short periods of days. This relation was first discovered by Henrietta Swan Leavitt (https://www.amazon.ca/Miss-Leavitts-Stars-George-Johnson/dp/0393051285/ref=sr_1_4?ie=UTF8&qid=1547669838&sr=8-4&keywords=Henrietta+Swan+Leavitt) more than a century ago and we call this relation the Leavitt Law. The Leavitt Law has changed the way we understand astronomy and cosmology. This is because if we can measure the period of pulsation for a Cepheid then we can know how much light the Cepheid is emitting. We can measure how much light we see and if we take these two pieces of information then we can determine just how far away a Cepheid is from us. This makes Cepheids powerful standard candles and Polaris is the closest one to us.

Because it is the closest Cepheid, Polaris helps anchor the Leavitt Law. Unfortunately, we cannot just know the Leavitt Law but have to calibrate it using Cepheids that we can measure their distances using other methods. As such, the Leavitt Law is anchored by the nearby Cepheids and for most of the past century we could use only a handful of Cepheids to anchor the Law. Polaris is one of these anchor stars. Today, however, thanks to the success of the Gaia mission, the Leavitt Law can be anchored by thousands of Cepheids in our Galaxy.

With this success, you might think that Polaris is less important now for the Leavitt Law. But, Polaris continues to be a special Cepheid because we have recorded observations of its variations for about two hundred years. This allows us to watch the pulsation change and evolve. This is a new direction for understanding the Leavitt Law and Cepheids and how our calibrators change over time. Polaris is telling us a story about the Universe and the many connections between us and measuring the expansion of the Universe.