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AM CVn BinariesUnlike the Sun, most stars in our Galaxy orbit another star: these systems are called binary stars. Generally it takes many years for the two stars to orbit around each other. However, there is a group of binary systems (called AM CVn or ultra compact binaries) which have an orbital period less than one hour. The quickest is as fast as 5 minutes, with both stars being able to fit inside the volume of Saturn. These stars are called white dwarfs and have the same size as the Earth but weigh about the same mass as the Sun. Because the stars are so close together, gravity can pull material from one star which eventually lands on the other. As a result of this process, X-rays and optical light are emitted.
AM CVn binaries are interesting for many reasons. One is that they are predicted to emit lots of gravitational waves. A second is that their observed number is a good test of certain theoretical models. Although there is now over 50 AM CVn binaries known, the theoretical models still predict far more many systems. I have been involved with various surveys which have set out to detect new AM CVn systems through photometric synoptic surveys.
The European Space Agency plan to launch a space observatory called eLISA in around 15 years time. It is predicted that these ultra compacts will be the first objects it detects.