The American Association of Variable Star Observers created the Citizen Sky program to enlist amateurs around the world and study the 2009-2011 eclipse of the binary star system epsilon Aurigae. By the end of the eclipse amateurs had submitted over eight thousand observations, making epsilon Aurigae the most thoroughly-observed variable star in history.
Stars twinkle in the night sky thanks to turbulence in the atmosphere. That’s one of the reasons space agencies put telescopes in space. But even then the light from distant stars isn’t constant. Starlight brightens and fades over time. It may take millions of years as a star ages or it may happen in minutes. Some of the variation is due to the nature of the star. Nuclear fusion in a star’s core may make the star throb. As it grows and shrinks, the light from the star dims and brightens. In other cases two stars may orbit each other so closely and from such a distance that their light merges into a single point in our night sky. The light from one of these binary stars is blocked as it passes behind the other, reducing the combined light arriving at Earth. Planets circling other stars block light as their orbit carries them between the star and the Earth.
Observing variable stars is like observing the tides. If you sit on a dock and measure the sea’s level over the course of the day, you can chart the rise and fall of high and low tides. You see how the patterns change as the Moon orbits Earth by observing the tides over the course of months and years. Variable star astronomers do the same thing by measuring a star’s brightness over time. They display the data as a light curve that shows the rising and falling brightness levels over time. That pattern gives astronomers insights into the nature of a variable star’s properties.
You can see several variable stars with your own eyes - astronomers began studying variable stars centuries before anyone invented the telescope. One of these visible variables is the binary star system epsilon Aurigae. Every twenty-seven years the stars go into eclipse and its light dims for about six hundred days. The most recent eclipse coincided with the 2009 International Year of Astronomy. The AAVSO turned that coincidence as an opportunity to create an outreach program, Citizen Sky, that would engage amateurs of all skill levels in a single, massively-coordinated observing campaign.
But this wasn’t just an outreach project - the amateur observations would be part of a serious scientific research project to study a very complex object. The epsilon Aurigae system consists of a primary star that pulses due to its own internal fusion reactions and a smaller secondary star that be a binary system of its own. Whether one or two stars, the secondary system is circled by a disk of gas and dust that has its own effect during the eclipse. Citizen Sky’s intensive observations would add to 160 years of epsilon Aurigae eclipse observations in AAVSO’s variable star database.
A combination of live workshops, chat sessions, and online tutorials showed amateurs that they didn’t need expensive equipment to observe epsilon Aurigae. You just compare the star to nearby stars with constant, well-documented brightness levels. In the case of epsilon Aurigae, you can use this relative magnitude method with your bare eyes. The Citizen Sky project developed tutorials for using binoculars and consumer digital cameras. By the end of the eclipse, hundreds of people around the world contributed over seven thousand visual observations and one thousand digital observations.
AAVSO formally merged Citizen Sky into its organization to train new variable astronomers. Its “10-Star Observing Tutorial” walks you through conducting relative magnitude observations of visible variables. The “5-Star Analysis with VSTAR Tutorial” shows you how to use the AAVSO’s VSTAR software to process observations into scientifically useful data that you can contribute to the association’s database. The “DSLR Photometry Tutorial” shows you how to use a consumer camera and lens to make digital observations. Using a camera and lens you already have is an inexpensive alternative to using the combination of telescopes, mounts, and astronomical CCD cameras that advanced amateurs use to observe variable stars. The Citizen Sky tutorials let new amateurs build their observational skills before moving on to the AAVSO’s manuals for visual observations and CCD observations.
The AAVSO published an edition of The Journal of the American Association of Variable Star Observers dedicated to the epsilon Aurigae campaign. You can read individual articles on the AAVSO website or purchase a printed version.* Members can download the journal free. Astrophysicist John Percy describes the scientific importance of epsilon Aurigae and sets the observing campaign in historical context. An article by Aaron Price and the Citizen Sky team gives a behind-the-scenes report on the project’s origins and future. Over a dozen other research papers from amateur and professional astronomers around the world describe what they’ve learned - and what new questions they have raised.
* This is not an affiliate link. All proceeds from the sale of the journal support the AAVSO.