Tau Boötis

Stellar components

The system is a binary. The primary component is a yellow-white dwarf (spectral type F6V) and secondary is a dim red dwarf (spectral type M2). The system is relatively nearby, distance being about 51 light-years. The primary star should be easily visible to the unaided eye under dark skies.

The primary star, Tau Boötis A is a yellow-white dwarf. It is 35 percent more massive and 42 percent larger than the Sun and thus is somewhat brighter and hotter. It is about 1.3 billion years old, making it younger than the Sun as well. Since it is more massive than the Sun, its lifespan is shorter—less than 6 billion years. Tau Boötis is the first star apart from the Sun to be observed changing the polarity of its magnetic field. It is also listed as a suspected variable star. The magnetic activity cycle for this star shows a period of 122 days—much shorter than the solar cycle.

Tau Boötis B (with a capital B, as opposed to the planet) is a dim, 11 mag red dwarf with only about half the mass and radius of the Sun. It orbits the primary star at an average distance of about 220 AU (14 arcseconds) but comes as close as about 28 AU to the primary, giving its orbit a very high eccentricity of about 0.87. One orbit around the primary would take approximately 2400 years to complete.