A team of astronomers writing in Astrophysical Journal Letters has shown that the two stars in the binary HM Cancri revolve around each other in a mere 5.4 minutes, making HM Cancri the binary star with by far the shortest known orbital period. It is also the smallest known binary; The system is no larger than 8 times the diameter of the Earth.

The team was able to prove the short binary period of the system by detecting the velocity variations in the spectral lines in the light of HM Cancri. These velocity variations are induced by the Doppler effect, caused by the orbital motion of the two stars revolving around each other. The Doppler effect causes the lines to periodically shift from blue to red and back.

The observations of HM Cancri were an ultimate challenge due to the extremely short period that needed to be resolved and the faintness of the binary system. At a distance of close to 16,000 light years from Earth, the binary shines at a brightness no more than one millionth of the faintest stars visible to the naked eye.

This image, made with the BinSim software, reflects the geometry of the system HM Cancri.

(Photo Credit: Rob Hynes)

"This is an intriguing system in a number of ways: it has an extremely short period; mass flows from one star and crashes down onto the equator of the other in a region comparable in size to the English Midlands where it liberates more than the Sun's entire power in X-rays. It could also be a strong emitter of gravitational waves which may one day be detected from this type of star system," Professor Tom Marsh from the University of Warwick said.

"The binary HM Cancri is a real challenge for our understanding of stellar and binary evolution," adds Dr Gijs Nelemans of the Radboud University."We know the system must have come from two normal stars that somehow spiralled together in two earlier episodes of mass transfer, but the physics of this process is very poorly known. The system is also a big opportunity for general relativity. It must be one of the most copious emitters of gravitational waves. These distortions of space-time we hope to detect directly with the future LISA satellite, and HM Cancri will be a cornerstone system for this mission."