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A fourth magnitude star in Canis Major, lying a little over seven degrees southwestward of brilliant Sirius. Xi1Canis Majoris forms a naked-eye optical double with Xi2 Canis Majoris, which lies less than a degree to the northeast. In reality, the two stars are unrelated to one another, and Xi1 is about a thousand light years more distant from Earth than its apparent 'neighbour' in the sky.

Xi1 Canis Majoris is a blue star on the borderline between a subgiant and giant classification. Deep within the star is a layer of iron, and opposing physical forces acting on this layer cause it to periodically expand and contract. This makes Xi1 Canis Majoris a pulsating variable of the Beta Cephei type, with its luminosity oscillating over a period of very slightly more than five hours.

This is a star with several notable features, including an unsually high magnetic field and intense X-ray emissions. It also has a remarkably slow rotation speed. For comparison, the Sun rotates on its axis over a period averaging twenty-eight days, while Xi1 Canis Majoris takes no less than thirty years to complete a single rotation. This rotation rate is in fact gradually slowing, and the star may eventually cease to rotate altogether.


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