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Rasalhague, Alpha Ophiuchi, 55 Ophiuchi, HD159561, HIP86032, HR6556

Rasalhague (Alpha Ophiuchi) Location in Ophiuchus

Rasalhague (Alpha Ophiuchi) is a blue giant star that can be located in the constellation of Ophiuchus. The description is based on the spectral class. Rasalhague is a main star of the constellation.

Rasalhague is the 56th brightest star in the night sky and is the brightest star in Ophiuchus based on the Hipparcos 2007 apparent magnitude. The star can be seen with the naked eye, that is, you don't need a telescope/binoculars to see it.

Rasalhague's Alternative Names

Alpha Ophiuchi (Alf Oph) is the Bayer Classification for the star. The Id of the star in the Yale Bright Star Catalogue is HR6556. HIP86032 is the reference name for the star in the Hipparcos Star Catalogue. The Id of the star in the Henry Draper catalogue is HD159561. The Gliese ID of the star is GL 681AB. The star was part of the original catalogue devised by German Astronomer Wilheim Gliese of stars located within 20 parsecs of Earth. Star Names In Arabic, it is known as Ra's al-Hawwa'.

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 55 Ophiuchi with it shortened to 55 Oph.

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+12 3252.

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

Location of Rasalhague

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 Rasalhague, the location is 17h 34m 56.00 and +12° 33` 38.1 .

Radial Velocity and Proper Motion of Rasalhague

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 -221.57 ± 0.71 miliarcseconds/year towards the north and 108.07 ± 1.06 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 the Sun is 11.70 km/s with an error of about 1.00 km/s . When the value is negative then the star and the Sun are getting closer to one another, likewise, a positive number means that two stars are moving away. Its nothing to fear as the stars are so far apart, they won't collide in our life-time, if ever.

Rasalhague 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 25.23 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 Rasalhague

Rasalhague has a spectral type of A5III. This means the star is a blue giant star. The star is 7,389.00 Parsecs from the Galactic Centre or terms of Light Years is 24,100.21 s. The star has a B-V Colour Index of 0.15 which means the star's temperature has been calculated using information from Morgans @ Uni.edu at being 8,084 Kelvin.

Rasalhague Radius has been calculated as being 2.62 times bigger than the Sun.The Sun's radius is 695,800km, therefore the star's radius is an estimated 1,820,214.03.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 2.73. 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 -0.16 with an error value of 9.99 Fe/H with the Sun has a value of 1 to put it into context.

Rasalhague Apparent and Absolute Magnitudes

Rasalhague has an apparent magnitude of 2.08 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 1.30 If you used the 2007 Parallax value, you would get an absolute magnitude of 1.21. 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 Rasalhague

Using the original Hipparcos data that was released in 1997, the parallax to the star was given as 69.84 which gave the calculated distance to Rasalhague as 46.70 light years away from Earth or 14.32 parsecs. It would take a spaceship travelling at the speed of light, 46.70 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 67.13 which put Rasalhague at a distance of 48.59 light years or 14.90 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.

Using the 2007 distance, the star is roughly 3,073,324.20 Astronomical Units from the Earth/Sun give or take a few. An Astronomical Unit is the distance between Earth and the Sun. The number of A.U. is the number of times that the star is from the Earth compared to the Sun.

The star's Galacto-Centric Distance is 7,389.00 Parsecs or 24,100.21 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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Rasalhague Facts

Visual Facts


Primary / Proper / Traditional NameRasalhague
Alternative NamesAlpha Ophiuchi, Alf Oph, Ra's al-Hawwa', HD 159561, HIP 86032, HR 6556, 55 Ophiuchi, 55 Oph, BD+12 3252, Gliese 681AB
Spectral TypeA5III
Constellation's Main StarYes
Multiple Star SystemYes
Star TypeGiant Star
Colour blue
GalaxyMilky Way
ConstellationOphiuchus
Absolute Magnitude 1.30 / 1.21
Visual / Apparent Magnitude2.08
Naked Eye VisibleYes - Magnitudes
Right Ascension (R.A.)17h 34m 56.00
Declination (Dec.)+12° 33` 38.1
Galactic Latitude22.57 degrees
Galactic Longitude35.89 degrees
1997 Distance from Earth69.84 Parallax (milliarcseconds)
 46.70 Light Years
 14.32 Parsecs
2007 Distance from Earth67.13 Parallax (milliarcseconds)
 48.59 Light Years
 14.90 Parsecs
 3,073,324.20 Astronomical Units
Galacto-Centric Distance24,100.21 Light Years / 7,389.00 Parsecs
Proper Motion Dec.-221.57 ± 0.71 milliarcseconds/year
Proper Motion RA.108.07 ± 1.06 milliarcseconds/year
B-V Index0.15
Radial Velocity11.70 ± 1.00 km/s
Iron Abundance-0.16 ± 9.99 Fe/H
Eccentricity0.22
Semi-Major Axis8711.00
Stellar Luminosity (Lsun)25.23
Brightest in Night Sky56th

Companions (Multi-Star and Exoplanets) Facts


Exoplanet CountNone/Unaware

Estimated Facts


Calculated Effective Temperature8,084 Kelvin

Sources and Links


SIMBAD SourceLink

Related Stars


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