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Arkab Posterior (Beta-2 Sagittarii) Star Facts

Arkab Posterior Facts

  • Arkab Posterior is a giant star that can be located in the constellation of Sagittarius. The description is based on the spectral class.
  • Arkab Posterior is not part of the constellation outline but is within the borders of the constellation.
  • Based on the spectral type (F2III) of the star, the star's colour is yellow to white .
  • The star can be seen with the naked eye, that is, you don't need a telescope/binoculars to see it.
  • The star has an estimated age of 1.00 Billion of Years but could be as young as 0.90 to 1.10 according to Hipparcos.
  • Using the most recent figures given by the 2007 Hipparcos data, the star is 134.17 light years away from us. Distance

Arkab Posterior's Alternative Names

Beta-2 Sagittarii (Bet02 Sgr) is the Bayer Classification for the star. The Bayer Classification was created by Johann Bayer in 1603. The brightest star in the constellation is normally given the Alpha designation, there are exceptions such as Pollux which is Beta Geminorum.

The Id of the star in the Yale Bright Star Catalogue is HR7343. HIP95294 is the reference name for the star in the Hipparcos Star Catalogue. The Id of the star in the Henry Draper catalogue is HD181623.

The Gould star designation is one that was designed by American astronomer, Benjamin Apthorp Gould. Gould stars are predominantly in the Southern and Equatorial constellations but do appear in northern constellations such as Bootes and Orion. The star has the designation 172 G. Sagittarii. There are no stars with a Gould designation in Ursa Major for example.

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

Location of Arkab Posterior

The location of the giant 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 object is compared to the celestial equator and is expressed in degrees. For Arkab Posterior, the location is 19h 23m 13.06 and -44° 47` 58.7 .

Radial Velocity and Proper Motion of Arkab Posterior

Proper Motion

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 -54.09 ± 0.10 milliarcseconds/year towards the north and 93.45 ± 0.19 milliarcseconds/year east if we saw them in the horizon.

Radial Velocity

The Radial Velocity, that is the speed at which the star is moving away/towards the Sun is 19.00000 km/s with an error of about 4.10 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.

Physical Properties of Arkab Posterior

Arkab Posterior Temperature and Colour

Based on the star's spectral type of F2III , Arkab Posterior's colour and type is yellow to white giant star. The star has a B-V Colour Index of 0.35 which means the star's temperature is about 6,967 Kelvin. The temperature was calculated using information from Morgans @ Uni.edu.

Arkab Posterior 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 27.57 that I have given is based on the value in the Simbad Hipparcos Extended Catalogue at the University of Strasbourg from 2012.

Arkab Posterior Radius

Arkab Posterior estimated radius has been calculated as being 3.81 times bigger than the Sun. The Sun's radius is 695,800km, therefore the star's radius is an estimated 2,650,239.46.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 3.6880772036823473138821876834. The figure is derived at by using the formula from SDSS rather than peer reviewed papers. It has been known to produce widely incorrect figures.

Arkab Posterior Iron Abundance

Arkab Posterior Iron Abundance is -0.17 with an error value of 0.15 Fe/H with the Sun has a value of 1 to put it into context. The value comes from the Hipparcos Extended Catalog.

Arkab Posterior Estimated Age

The stars age according to Hipparcos data files put the star at an age of about 1.00 Billion years old but could be between 0.90 and 1.10 Billion years old. In comparison, the Sun's age is about 4.6 Billion Years Old.

Arkab Posterior Apparent and Absolute Magnitudes

Arkab Posterior has an apparent magnitude of 4.27 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 1.13 If you used the 2007 Parallax value, you would get an absolute magnitude of 1.20. 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 Arkab Posterior

Using the original Hipparcos data that was released in 1997, the parallax to the star was given as 23.52000 which gave the calculated distance to Arkab Posterior as 138.67 light years away from Earth or 42.52 parsecs. If you want that in miles, it is about 815,188,980,499,371.20, based on 1 Ly = 5,878,625,373,183.61 miles.

In 2007, Hipparcos data was revised with a new parallax of 24.31000 which put Arkab Posterior at a distance of 134.17 light years or 41.14 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 8,485,675.00 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 7,363.00 Parsecs or 24,015.41 Light Years. The Galacto-Centric Distance is the distance from the star to the Centre of the Galaxy which is Sagittarius A*.

Travel Time to Arkab Posterior

The time it will take to travel to this star is dependent on how fast you are going. U.G. has done some calculations as to how long it will take going at differing speeds. A note about the calculations, when I'm talking about years, I'm talking non-leap years only (365 days).

The New Horizons space probe is the fastest probe that we've sent into space at the time of writing. Its primary mission was to visit Pluto which at the time of launch (2006), Pluto was still a planet.

DescriptionSpeed (m.p.h.)Time (years)
Walking422,494,158,278.23
Car120749,805,275.94
Airbus A380736122,250,860.21
Speed of Sound (Mach 1)767.269117,268,693.40
Concorde (Mach 2)1,534.5458,634,270.28
New Horizons Probe33,0002,726,564.64
Speed of Light670,616,629.00134.17

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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Additional Arkab Posterior Facts and Figures

Visual Facts


Primary / Proper / Traditional NameArkab Posterior
Alternative NamesBeta-2 Sagittarii, Bet02 Sgr, HD 181623, HIP 95294, HR 7343, 172 G. Sagittarii
Spectral TypeF2III
Constellation's Main StarNo
Multiple Star SystemNo / Unknown
Star TypeGiant Star
ColourYellow - White
GalaxyMilky Way
ConstellationSagittarius
Age1.00 Billion Years Old
Age Range0.90 - 1.10 Billion Years Old
Absolute Magnitude 1.13 / 1.20
Visual / Apparent Magnitude4.27
Naked Eye VisibleYes - Magnitudes
Right Ascension (R.A.)19h 23m 13.06
Declination (Dec.)-44° 47` 58.7
Galactic Latitude-24.11619348 degrees
Galactic Longitude353.28292469 degrees
1997 Distance from Earth23.52000 Parallax (milliarcseconds)
 138.67 Light Years
 42.52 Parsecs
2007 Distance from Earth24.31000 Parallax (milliarcseconds)
 134.17 Light Years
 41.14 Parsecs
 8,485,675.00 Astronomical Units
Galacto-Centric Distance24,015.41 Light Years / 7,363.00 Parsecs
Proper Motion Dec.-54.09000 ± 0.10000 milliarcseconds/year
Proper Motion RA.93.45000 ± 0.19000 milliarcseconds/year
B-V Index0.35
Radial Velocity19.00000 ± 4.10 km/s
Iron Abundance-0.1700 ± 0.15 Fe/H
Eccentricity0.13710
Semi-Major Axis8041.0000000
Stellar Luminosity (Lsun)27.5700000

Companions (Multi-Star and Exoplanets) Facts


Exoplanet CountNone/Unaware

Estimated Calculated Facts


Radius (x the Sun)3.69
Effective Temperature6,967 Kelvin

Sources and Links


SIMBAD SourceLink

Location of Arkab Posterior in Sagittarius


Arkab Posterior Location in Sagittarius

The map was generated using Night Vision, an awesome free application by Brian Simpson.

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