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Historical view of the Hercules constellation showing the star π Her as one of the Keystone stars. |
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| Observation data Epoch J2000.0 Equinox J2000.0 |
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|---|---|
| Constellation | Hercules |
| Right ascension | 17h 15m 02.8343s[1] |
| Declination | +36° 48′ 32.983″[1] |
| Apparent magnitude (V) | +3.156[1] |
| Characteristics | |
| Spectral type | K3Iab[1] |
| U−B color index | +1.66 [2] |
| B−V color index | +1.45 [2] |
| Variable type | undetermined[3] |
| Astrometry | |
| Radial velocity (Rv) | -25.57[1] km/s |
| Proper motion (μ) | RA: -27.3[1] mas/yr Dec.: +2.70[1] mas/yr |
| Parallax (π) | 8.66 ± 0.12[4] mas |
| Distance | 377 ± 5 ly (115 ± 2 pc) |
| Absolute magnitude (MV) | -2.10 |
| Details | |
| Mass | ≥3.7 [3] M☉ |
| Radius | 72 [3] R☉ |
| Luminosity | 1400 L☉ |
| Temperature | 4170[5] K |
| Other designations | |
| Database references | |
| SIMBAD | data |
Coordinates: 17h 15m 02.80s, +36° 48′ 33.0″
Pi Herculis (π Her, π Herculis) is a fourth-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.
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Pi Herculis is an orange bright giant star with a stellar classification listed in the SIMBAD astronomical database as K3Iab. 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. Though its apparent magnitude is only 3.156, this orange giant shines with a luminousity that is 1,330L☉, yielding an absolute magnitude of -2.10, brighter in fact than the most of the hot B stars in the Pleiades open star cluster. The Hipparcos satellite mission estimated its distance at roughly 112 parsecs from Earth, or just under 370 light years away.
Low-amplitude radial velocity variations with a period of 613 days in the bright giant have suggested the possible presence of a substellar companion.[3] 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. 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.[3]
| Companion (in order from star) |
Mass | Semimajor axis (AU) |
Orbital period (days) |
Eccentricity |
|---|---|---|---|---|
| b (unconfirmed) | 27 MJ | 3 | 613 | 0.05 |
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