Star in the constellation Hercules
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! style="text-align: center; background-color: #FFFFC0;" colspan="2" | Observation dataEpoch J2000      Equinox J2000
! style="text-align:left" | Constellation
! style="text-align:left" | Right ascension
! style="text-align:left" | Declination
! style="text-align:left" | Apparent magnitude (V)
! style="background-color: #FFFFC0; text-align: center;" colspan="2"| Characteristics
! style="text-align:left" | Spectral type
! style="text-align:left" | U−B 
! style="text-align:left" | B−V 
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 ;/th ; ;/tr ;
 ;indicator name="01-sky-coordinates" ; ;templatestyles src="Template:Sky/styles.css" / ;Coordinates: &amp;show_grid=1&amp;show_constellation_lines=1&amp;show_constellation_boundaries=1&amp;show_const_names=1&amp;show_galaxies=1&amp;img_source=IMG_all 16h 42m 53.80s, +38° 55′ 20.116″ ;/indicator ;
Eta Herculis (η Her, η Herculis) is a third-magnitude star in the constellation Hercules. It is located at 110 light-years from Earth.
Properties.
Eta Herculis is a G-type giant star. With a stellar classification G7.5IIIb, it is considerably larger and more evolved than the Sun, having a mass that is two times solar and a radius 8.9 times. Though it only shines with an apparent magnitude of 3.48, it is part of the "Keystone" asterism, visible overhead in the mid-summer night sky to northern observers, allowing it to be easily recognized. Eta Herculis is 46 times more luminous than the Sun. The Hipparcos satellite mission estimated its distance at roughly 34.4 parsecs from Earth, or 112 light years away.
If one follows the line connecting Eta Herculis with Zeta Herculis one comes across one of the earliest and most stunning globular clusters in the nighttime sky, M13, discovered in 1714 by Edmond Halley.
Eta Herculis is a double star once thought to be part of a binary star system.
References.
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