MARK GARLICK / SCIENCE PHOTO LIBRARY MARK GARLICK / SCIENCE PHOTO LIBRARY
Polar binary star. Artwork of a binary star system known as a polar. Polars are short period variable stars which emit X-rays. They consist of a normal star (left) in mutual orbit with a white dwarf (right). Their periods of variation are typically around 2 hours, and correlate with the orbital period of the white dwarf. Their light is strongly polarised. The gravitational field of the white dwarf pulls material off the larger star, and the dwarf's strong magnetic field channels it towards its poles, as seen at lower right. This material emits X-rays as it nears the white dwarf. Polars are also known as AM Her stars, after the first such star to be discovered, AM Herculis.
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