Tau Boötis
Encyclopedia
Tau Boötis is a yellow-white dwarf
Yellow-white dwarf
An F-type main-sequence star is a main-sequence, hydrogen-fusing star of spectral type F and luminosity class V. These stars have from 1.0 to 1.4 times the mass of the Sun and surface temperatures between 6,000 and 7,600 K., Tables VII and VIII. This temperature range gives the F-type stars a...

 approximately 51 light-year
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...

s away in the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....

 of Boötes
Boötes
Boötes is a constellation in the northern sky, located between 0° and +60° declination, and 13 and 16 hours of right ascension on the celestial sphere. The name comes from the Greek Βοώτης, Boōtēs, meaning herdsman or plowman...

. The system is also a binary star
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...

 system, with the secondary star being a red dwarf
Red dwarf
According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type....

. As of 1999, an extrasolar planet
Extrasolar planet
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...

 has been confirmed to be orbiting the primary star.

Planetary system

In 1996 a planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

, designated as Tau Boötis b, was discovered orbiting the primary star. There are also some indications of another, more distant, planet orbiting the star. In an unusual case of role-reversal, it appears that Tau Boötis' rotation has been tidally locked
Tidal locking
Tidal locking occurs when the gravitational gradient makes one side of an astronomical body always face another; for example, the same side of the Earth's Moon always faces the Earth. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner...

 to Tau Boötis b. The planet was discovered by a team of astronomers led by Geoff Marcy and R. Paul Butler
R. Paul Butler
R. Paul Butler is an astronomer who searches for extrasolar planets.He received a BA and an MS from San Francisco State University, completing a Master's thesis with Geoffrey Marcy, and then completed his doctoral studies at the University of Maryland, College Park in 1993...

.

Stellar components

The system is a binary
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...

. The primary component is a yellow-white dwarf
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...

 (spectral type F7 V) and secondary is a dim red dwarf
Red dwarf
According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type....

 (spectral type M2 V). 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 20% more massive than our Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 and thus is somewhat brighter and hotter. It has a radius 1.9 times solar, and is probably about 1300 million years old. Since it is more massive than the Sun, its lifespan is shorter being less than 6000 million years. Tau Bootis 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
Variable star
A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...

. However, the star is not variable.

Tau Boötis B (with a capital B, as opposed to the planet) is a dim red dwarf
Red dwarf
According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type....

 orbiting the primary star at a distance of 240 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

. One orbit around the primary would take thousands of years to complete.
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