Tianji (Zeta Herculis)
Tianji (Zeta Herculis) is a star in the constellation Hercules, magnitude 2.8.
Two Sun-like stars in the Keystone of Hercules, followed for nearly six orbits since 1826.
Two stars much like our Sun circle each other in Zeta Herculis. The companion is about the Sun’s size and mass. The brighter one is heavier, about 1.45 times the Sun’s mass, and has begun to run out of core hydrogen, so it has grown to nearly three times the Sun’s width and shines more than seven times as brightly. Its surface, at around 5,760 K, is almost exactly as hot as the Sun’s.
They take 34.45 years to complete one orbit, on average about 15 times the Earth–Sun distance apart. F. G. W. Struve reported the pair in 1826, so astronomers have now followed it around nearly six full orbits.
In May 2026 the IAU gave the brighter star a name, Tianji, after the Chinese star group Tiān Jì, “Celestial Discipline”, in which it is the second star.
Tianji (Zeta Herculis) is 35 light-years away: the light we see from it set off in 1991, around the time the Hubble Space Telescope was launched.
Did you know?
- A family on the move. Zeta Herculis gives its name to a moving group: stars scattered across the sky, including Zeta Reticuli and Phi² Pavonis, that travel through space together.
- One name, three stars. Tianji, with different meanings, is also the Chinese name for Phecda in the Big Dipper and for Suhail in Vela.
A yellowish-white star of magnitude 2.8 at the south-western corner of the Keystone of Hercules. Its companion, about 1.5 arcseconds away on average and noticeably fainter, is a demanding test for larger telescopes on steady nights.
When and where to see it
It is best placed in the evening sky around July, when it crosses the meridian at about 10 pm local time (around midnight in June). It can be seen from everywhere north of 58°S and never sets north of 58°N.
At its highest it stands 72° up from 50°N, 87° up from 35°N, 58° up from the equator, 23° up from 35°S.
Facts
- Type
- Star
- Constellation
- Hercules
- Designations
- Zeta Herculis, Rutilicus, ζ Her, 40 Her, HR 6212, HD 150680, HIP 81693
- Magnitude
- 2.8
- Position
- RA 16h 41.3m, Dec +31° 36′ (J2000)
- Spectral type
- G0IV
- Colour
- yellow (B−V 0.63)
- Distance
- 35 light-years
- Discovered
- Known since antiquity; F. G. W. Struve reported its companion in 1826
Open AstroPilot to see when Tianji (Zeta Herculis) is highest tonight from your location, how easy it is with your telescope or binoculars, and an eyepiece view.