W Ursae Majoris star
W Ursae Majoris. Photo: ESO Online Digitized Sky Survey.
A W Ursae Majoris star is a type of eclipsing binary with a very short period of a few hours up to a day. In W UMa systems, the component stars, which are of spectral type F or G and lie on or near the main sequence, form a contact binary and have pulled each other into teardrop shapes. The primary and secondary minima are virtually the same (some 0.6 to 0.8 magnitude deep) and there is a continuous light variation throughout the orbit, with no clear start or end to an eclipse.
W Ursae Majoris itself lies 162 light-years away and consists of two stars not unlike the Sun, the primary with a mass of 0.99 solar mass, a radius of 1.14 solar radius, and a luminosity 1.45 times that of the Sun, and the secondary with a mass of 0.62 solar mass, a radius of 0.83 solar radius, and a solar-equivalent luminosity.