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Visual Binaries


Rigel - a triple star system in the constellation Orion.  

Photo courtesy of Prof Graeme White.

The photo featured here was taken recently by Prof Graeme White from his back yard in Wagga Wagga of the triple star system Rigel (Beta Orionis), the seventh brightest star in the night sky.

Interestingly enough, Rigel is often referred to as a visual binary.  The term ‘visual binary’ refers to two stars gravitationally bound together orbiting a centre-of-mass and can be individually seen (resolved) through a modest sized telescope.  Hence Rigel is often referred to as a visual binary made up of Rigel A and Rigel B.  

Rigel A is the brighter of the two and is called the primary star, Rigel B being referred to as its companion.  The intense brightness of Rigel A (120 000 times as luminous as the Sun) makes Rigel B difficult to see with smallish telescopes.  The distance separating Rigel A and B is approx. 2 200 times the distance from the Earth to the Sun (i.e. 2 200 au).  But what about the third star?  Rigel B is actually a spectroscopic binary system (two stars identified from the one blob using spectral analysis and Doppler methods) consisting of two stars each around double the mass of the Sun!