Historical view of the Hercules constellation showing the star π Her as one of the Keystone stars.
Epoch J2000.0 Equinox J2000.0
|Right ascension||17h 15m 02.83436s|
|Declination||+36° 48′ 32.9843″|
|Apparent magnitude (V)||+3.15|
|Spectral type||K3 II|
|U−B color index||+1.66|
|B−V color index||+1.45|
|Radial velocity (Rv)||–25.57 ± 0.20 km/s|
|Proper motion (μ)||RA: –27.29 mas/yr
Dec.: +2.82 mas/yr
|Parallax (π)||8.66 ± 0.12 mas|
|Distance||377 ± 5 ly
(115 ± 2 pc)
|Absolute magnitude (MV)||+0.13
|Surface gravity (log g)||1.42 cgs|
|Metallicity [Fe/H]||–0.07 dex|
|Rotational velocity (v sin i)||6.12 km/s|
Pi Herculis (π Her, π Herculis) is a third-magnitude star in the constellation Hercules. As one of the four stars in the Keystone asterism (see yellow quadrangle) is one of the more easily recognized in the constellation. It has an apparent visual magnitude of +3.2, which is visible to the naked eye and makes it one of the brighter members of the constellation. The Hipparcos satellite mission estimated its distance at roughly 115 parsecs from Earth, or about 377 light years away. The overall reduction in the star's visual magnitude due to extinction from the intervening matter is 0.11.
Pi Herculis is a bright giant star with a stellar classification 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. 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.
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 masses (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 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.
(in order from star)
|b (unconfirmed)||27 MJ||3||613||0.05||—||—|
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