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Rukbalgethi Genubi, Theta Herculis, 91 Herculis, HD163770, HIP87808, HR6695

Rukbalgethi Genubi (Theta Herculis) is a orange to red luminous giant star that can be located in the constellation of Hercules. The description is based on the spectral class. The star can be seen with the naked eye, that is, you don't need a telescope/binoculars to see it.

Rukbalgethi Genubi's Alternative Names

Theta Herculis (The Her) is the Bayer Classification for the star. The Id of the star in the Yale Bright Star Catalogue is HR6695. HIP87808 is the reference name for the star in the Hipparcos Star Catalogue. The Id of the star in the Henry Draper catalogue is HD163770.

Flamsteed designations are named after the creator, Sir John Flamsteed. Sir John numbered the stars in the constellation with a number and the latin name, this star's Flamsteed designation is 91 Herculis with it shortened to 91 Her.

BD number is the number that the star was filed under in the Durchmusterung or Bonner Durchmusterung, a star catalogue that was put together by the Bonn Observatory between 1859 to 1903. The star's BD Number is BD+37 2982.

More details on star alternative names can be found at Star Names .

Location of Rukbalgethi Genubi

The location of the star in the night sky is determined by the Right Ascension (R.A.) and Declination (Dec.), these are equivalent to the Longitude and Latitude on the Earth. The Right Ascension is how far expressed in time (hh:mm:ss) the star is along the celestial equator. If the R.A. is positive then its eastwards. The Declination is how far north or south the star is compared to the celestial equator and is expressed in degrees. For Rukbalgethi Genubi, the location is 17h 56m 15.18 and +37° 15` 01.9 .

Proper Motion of Rukbalgethi Genubi

All stars like planets orbit round a central spot, in the case of planets, its the central star such as the Sun. In the case of a star, its the galactic centre. The constellations that we see today will be different than they were 50,000 years ago or 50,000 years from now. Proper Motion details the movements of these stars and are measured in milliarcseconds. The star is moving 6.47 ± 0.11 miliarcseconds/year towards the north and 2.67 ± 0.13 miliarcseconds/year east if we saw them in the horizon. The Radial Velocity, that is the speed at which the star is moving away/towards us is -28.32000 km/s with an error of about 0.16 km/s .

Rukbalgethi Genubi Luminosity

Luminosity is the amount of energy that a star pumps out and its relative to the amount that our star, the Sun gives out. The figure of 2,795.67 that I have given is based on the value in the Simbad Hipparcos Extended Catalogue at the University of Strasbourg from 2012.

Physical Properties (Colour, Temperature) of Rukbalgethi Genubi

Rukbalgethi Genubi has a spectral type of K1IIvar. This means the star is a orange to red luminous giant star. The star is 7,310.00 Parsecs from the Galactic Centre or terms of Light Years is 23,842.54 s. The star has a B-V Colour Index of 1.35 which means the star's temperature has been calculated using information from Morgans @ Uni.edu at being 4,298 Kelvin.

Rukbalgethi Genubi Radius has been calculated as being 58.40 times bigger than the Sun.The Sun's radius is 695,800km, therefore the star's radius is an estimated 40,633,245.79.km. If you need the diameter of the star, you just need to multiple the radius by 2. However with the 2007 release of updated Hipparcos files, the radius is now calculated at being round 65.83. The figure is derived at by using the formula from SDSS and has been known to produce widely incorrect figures. The star's Iron Abundance is -0.12 with an error value of 9.99 Fe/H with the Sun has a value of 1 to put it into context.

Rukbalgethi Genubi Apparent and Absolute Magnitudes

Rukbalgethi Genubi has an apparent magnitude of 3.86 which is how bright we see the star from Earth. Apparent Magnitude is also known as Visual Magnitude. If you used the 1997 Parallax value, you would get an absolute magnitude of -2.70 If you used the 2007 Parallax value, you would get an absolute magnitude of -2.96. Magnitude, whether it be apparent/visual or absolute magnitude is measured by a number, the smaller the number, the brighter the Star is. Our own Sun is the brightest star and therefore has the lowest of all magnitudes, -26.74. A faint star will have a high number.

Distance to Rukbalgethi Genubi

Using the original Hipparcos data that was released in 1997, the parallax to the star was given as 4.87 which gave the calculated distance to Rukbalgethi Genubi as 669.74 light years away from Earth or 205.34 parsecs. It would take a spaceship travelling at the speed of light, 669.74 years to get there. We don't have the technology or spaceship that can carry people over that distance yet.

In 2007, Hipparcos data was revised with a new parallax of 4.33 which put Rukbalgethi Genubi at a distance of 753.26 light years or 230.95 parsecs. It should not be taken as though the star is moving closer or further away from us. It is purely that the distance was recalculated.

The star's Galacto-Centric Distance is 7,310.00 Parsecs or 23,842.54 Light Years. The Galacto-Centric Distance is the distance from the star to the Centre of the Galaxy which is Sagittarius A*.

Source of Information

The source of the information if it has a Hip I.D. is from Simbad, the Hipparcos data library based at the University at Strasbourg, France. Hipparcos was a E.S.A. satellite operation launched in 1989 for four years. The items in red are values that I've calculated so they could well be wrong. Information regarding Metallicity and/or Mass is from the E.U. Exoplanets. The information was obtained as of 12th Feb 2017.

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Rukbalgethi Genubi Facts

Visual Facts


Primary / Proper / Traditional NameRukbalgethi Genubi
Alternative NamesTheta Herculis, The Her, HD 163770, HIP 87808, HR 6695, 91 Herculis, 91 Her, BD+37 2982
Spectral TypeK1IIvar
Multiple Star SystemNo / Unknown
Star TypeLuminous Giant Star
Colour orange to red
GalaxyMilky Way
ConstellationHercules
Absolute Magnitude-2.70 / -2.96
Visual / Apparent Magnitude3.86
Naked Eye VisibleYes - Magnitudes
Right Ascension (R.A.)17h 56m 15.18
Declination (Dec.)+37° 15` 01.9
Galactic Latitude26.42 degrees
Galactic Longitude63.26 degrees
1997 Distance from Earth4.87 Parallax (milliarcseconds)
 669.74 Light Years
 205.34 Parsecs
2007 Distance from Earth4.33 Parallax (milliarcseconds)
 753.26 Light Years
 230.95 Parsecs
Galacto-Centric Distance23,842.54 Light Years / 7,310.00 Parsecs
Proper Motion Dec.6.47 ± 0.11 milliarcseconds/year
Proper Motion RA.2.67 ± 0.13 milliarcseconds/year
B-V Index1.35
Radial Velocity-28.32 ± 0.16 km/s
Iron Abundance-0.12 ± 9.99 Fe/H
Eccentricity0.07
Semi-Major Axis6945.00
Stellar Luminosity (Lsun)2,795.67

Companions (Multi-Star and Exoplanets) Facts


Exoplanet CountNone/Unaware

Estimated Facts


Calculated Effective Temperature4,298 Kelvin

Sources and Links


SIMBAD SourceLink

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