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21 Capricorni

21 Capricorni Facts

  • 21 Capricorni is a giant star that can be located in the constellation of Capricornus. The description is based on the spectral class.
  • 21 Capricorni is not part of the constellation outline but is within the borders of the constellation.
  • Based on the spectral type (K3III) of the star, the star's colour is orange to red .
  • The star can be seen with the naked eye, that is, you don't need a telescope/binoculars to see it.
  • Using the most recent figures given by the 2007 Hipparcos data, the star is 523.54 light years away from us. Distance

21 Capricorni's Alternative Names

HIP103703 is the reference name for the star in the Hipparcos Star Catalogue. The Id of the star in the Henry Draper catalogue is HD199947.

Flamsteed designations are named after the creator, Sir John Flamsteed. Sir John named the stars in the constellation with a number and its latin name, this star's Flamsteed designation is 21 Capricorni. The Flamsteed name can be shortened to 21 Cap.

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-18 5831.

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

Location of 21 Capricorni

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 21 Capricorni, the location is 21h 00m 51.78 and -17° 31` 51.1 .

Radial Velocity and Proper Motion of 21 Capricorni

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 -17.15 ± 0.29 milliarcseconds/year towards the north and -31.35 ± 0.51 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 1.10000 km/s with an error of about 2.90 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 21 Capricorni

21 Capricorni Temperature and Colour

Based on the star's spectral type of K3III , 21 Capricorni's colour and type is orange to red giant star. The star has a B-V Colour Index of 1.26 which means the star's temperature is about 4,388 Kelvin. The temperature was calculated using information from Morgans @ Uni.edu.

21 Capricorni 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 153.34 that I have given is based on the value in the Simbad Hipparcos Extended Catalogue at the University of Strasbourg from 2012.

21 Capricorni Radius

21 Capricorni estimated radius has been calculated as being 16.53 times bigger than the Sun. The Sun's radius is 695,800km, therefore the star's radius is an estimated 11,504,164.50.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 15.862432071361063754513212562. 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.

21 Capricorni Apparent and Absolute Magnitudes

21 Capricorni has an apparent magnitude of 6.07 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 -0.05 If you used the 2007 Parallax value, you would get an absolute magnitude of 0.04. 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 21 Capricorni

Using the original Hipparcos data that was released in 1997, the parallax to the star was given as 5.98000 which gave the calculated distance to 21 Capricorni as 545.42 light years away from Earth or 167.22 parsecs. If you want that in miles, it is about 3,206,319,851,041,804.57, based on 1 Ly = 5,878,625,373,183.61 miles.

In 2007, Hipparcos data was revised with a new parallax of 6.23000 which put 21 Capricorni at a distance of 523.54 light years or 160.51 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 33,107,333.36 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 7,290.00 Parsecs or 23,777.31 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 21 Capricorni

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)
Walking487,773,657,486.67
Car1202,925,788,582.89
Airbus A380736477,030,747.21
Speed of Sound (Mach 1)767.269457,590,010.74
Concorde (Mach 2)1,534.54228,794,707.17
New Horizons Probe33,00010,639,231.21
Speed of Light670,616,629.00523.54

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 21 Capricorni Facts and Figures

Visual Facts


Primary / Proper / Traditional Name21 Capricorni
Alternative NamesHD 199947, HIP 103703, 21 Cap, BD-18 5831
Spectral TypeK3III
Constellation's Main StarNo
Multiple Star SystemNo / Unknown
Star TypeGiant Star
ColourOrange to Red
GalaxyMilky Way
ConstellationCapricornus
Absolute Magnitude -0.05 / 0.04
Visual / Apparent Magnitude6.07
Naked Eye VisibleYes - Magnitudes
Right Ascension (R.A.)21h 00m 51.78
Declination (Dec.)-17° 31` 51.1
Galactic Latitude-36.26601860 degrees
Galactic Longitude30.41684670 degrees
1997 Distance from Earth5.98000 Parallax (milliarcseconds)
 545.42 Light Years
 167.22 Parsecs
2007 Distance from Earth6.23000 Parallax (milliarcseconds)
 523.54 Light Years
 160.51 Parsecs
 33,107,333.36 Astronomical Units
Galacto-Centric Distance23,777.31 Light Years / 7,290.00 Parsecs
Proper Motion Dec.-17.15000 ± 0.29000 milliarcseconds/year
Proper Motion RA.-31.35000 ± 0.51000 milliarcseconds/year
B-V Index1.26
Radial Velocity1.10000 ± 2.90 km/s
Eccentricity0.15470
Semi-Major Axis7528.0000000
Stellar Luminosity (Lsun)153.3400000

Companions (Multi-Star and Exoplanets) Facts


Exoplanet CountNone/Unaware

Estimated Calculated Facts


Radius (x the Sun)15.86
Effective Temperature4,388 Kelvin

Sources and Links


SIMBAD SourceLink

Related Stars


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