Pi Herculis (π Her, π Herculis) is a third-magnitude

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the e ...

Hercules. As one of the four stars in the Keystone asterism (see yellow quadrangle), it is one of the constellation's more easily recognized. It has an apparent visual magnitude of +3.2, which is visible to the naked eye and makes it one of its brighter members. The

Hipparcos ''Hipparcos'' was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial obj ...

satellite mission estimated its distance at roughly 115 parsecs from Earth, or about 377

light year A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...

s away. The overall reduction in the star's visual magnitude due to extinction from the intervening matter is 0.11.

Properties

Pi Herculis is a bright giant

with a

stellar classification In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a Prism (optics), prism or diffraction grati ...

of K3 II. P.C. Keenan and R. E. Pitts (1980) graded it as a spectral type K3 IIab and it is sometimes listed with this alternate classification. The star is enormous compared to the Sun, having a mass that is 4.5 times solar and a radius approximately 60 times depending on which wavelength the star's angular diameter is measured at. Due to limb darkening, all giant and supergiant stars present unique challenges when measuring their

photosphere The photosphere is a star's outer shell from which light is radiated. The term itself is derived from Ancient Greek roots, φῶς, φωτός/''phos, photos'' meaning "light" and σφαῖρα/''sphaira'' meaning "sphere", in reference to it ...

. This orange-hued giant shines with 1,330 times the luminosity of the Sun. It is a low-amplitude photometric variable star showing a typical change of roughly 0.0054 in magnitude over a 24-hour period.

Possible planetary system

Low-amplitude radial velocity variations with a period of 613 days in the bright giant have suggested the possible presence of a substellar companion. If this is really due to a low-mass object, such a companion would be as small as 0.027

Solar mass The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass ...

es (27 times the mass of Jupiter, probably a brown dwarf) and 3 astronomical units away from the bright primary. A substellar companion is only one of several hypotheses to explain the star's behaviour. Most likely the cause of the variation is weak pulsation of the star's atmosphere. With a luminosity more than 1,000 times that of the Sun, an orbit where a planet could be habitable would be located 37 AU away from Pi Herculis—in Solar System terms, halfway between

Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...

and Pluto's orbits. On the other hand, a putative companion would orbit in a scorching region and would be as hot as a planet would at 0.08 AU around a Sun-like star. In any case it's likely that it would soon be swallowed up by the expanding giant.

References

External links

* http://stars.astro.illinois.edu/sow/piher.html * http://www.dibonsmith.com/her_p.htm

Properties

Possible planetary system

References

External links

See also