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Castor (Alpha Geminorum, 66 Geminorum A) Star Facts

Castor Facts

  • Castor is a main sequence star that can be located in the constellation of Gemini. The description is based on the spectral class.
  • Castor is a main star of the constellation outline.
  • Based on the spectral type (A2Vm) of the star, the star's colour is blue - white .
  • Castor is the 24th brightest star in the night sky and the 2nd brightest star in Gemini 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.
  • Castor has a radius that is 2.40 times bigger than the Suns. Radius
  • Using the most recent figures given by the 2007 Hipparcos data, the star is 50.87 light years away from us. Distance

Information on Castor

The information for Castor on this site is based on the Hipparcos Catalogue which doesn`t differentiate the three stars that make up the star system. If you wanted to break the stars down, you would need to search on the Bonner Durchmusterung number. The numbers are A - BD+32°1581A, B - BD+32°1581B and C - BD +32° 1582

Although the star is referred to as the Alpha star in the constellation, it is not as bright as its twin, Pollux which is a yellow supergiant.

Castor's Alternative Names

Alpha Geminorum (Alf Gem) 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 HR2891. HIP36850 is the reference name for the star in the Hipparcos Star Catalogue. The Id of the star in the Henry Draper catalogue is HD60179. The Gliese ID of the star is GL 278A. The star was part of the original catalogue devised by German Astronomer Wilheim Gliese of stars located within 20 parsecs of Earth. Star Names

Castor has alternative name(s) :- Castor A.

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 66 Geminorum A. The Flamsteed name can be shortened to 66 Gem A.

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+32 1581.

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

Location of Castor

The location of the main sequence 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 Castor, the location is 07h 34m 36.00 and +31° 53` 19.1 .

Radial Velocity and Proper Motion of Castor

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 -145.19 ± 2.63 milliarcseconds/year towards the north and -191.45 ± 3.75 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 5.40 km/s with an error of about 0.50 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 Castor

Castor Temperature and Colour

Based on the star's spectral type of A2Vm , Castor's colour and type is blue - white main sequence star. The star's effective temperature is 10,286 Kelvin which is hotter than our own Sun's effective Temperature which is 5,777 Kelvin.

Castor 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 53.40 that I have given is based on the value in the Simbad Hipparcos Extended Catalogue at the University of Strasbourg from 2012.

Castor Radius

Castor Radius has been calculated as being 2.40 times bigger than the Sun. The Sun's radius is 695,800km, therefore the star's radius is an estimated 1,669,920.00.km. If you need the diameter of the star, you just need to multiple the radius by 2.

Castor Mass

The Castor's solar mass is 2.76 times that of our star, the Sun. The Sun's Mass is 1,989,100,000,000,000,000,000 billion kg. which to calculate using this website is too large. To give idea of size, the Sun is 99.86% the mass of the solar system.

Castor Apparent and Absolute Magnitudes

Castor has an apparent magnitude of 1.58 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 0.59 If you used the 2007 Parallax value, you would get an absolute magnitude of 0.61. 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 Castor

Using the original Hipparcos data that was released in 1997, the parallax to the star was given as 63.27 which gave the calculated distance to Castor as 51.55 light years away from Earth or 15.81 parsecs. If you want that in miles, it is about 303,043,137,987,615.10, based on 1 Ly = 5,878,625,373,183.61 miles.

In 2007, Hipparcos data was revised with a new parallax of 64.12 which put Castor at a distance of 50.87 light years or 15.60 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 3,217,708.56 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 7,414.00 Parsecs or 24,181.75 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 Castor

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)
Walking48,528,566,979.31
Car120284,285,565.98
Airbus A38073646,350,907.50
Speed of Sound (Mach 1)767.26944,461,939.58
Concorde (Mach 2)1,534.5422,230,940.81
New Horizons Probe33,0001,033,765.69
Speed of Light670,616,629.0050.87

Meteor Showers Radiating from near Castor

The Geminids Meteor Shower radiants from a point near this star. The meteor shower runs typically between December 6-19 with a peak date of Dec. 13/14. The speed of a meteor in the shower is 34 Km/s. The amount of meteors predicted to be seen per hour (Zenith Hourly Rate) is 34.

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 Castor Facts and Figures

Visual Facts


Primary / Proper / Traditional NameCastor
Alternative NamesAlpha Geminorum, Alf Gem, Castor A, HD 60179, HIP 36850, HR 2891, 66 Geminorum A, 66 Gem A, BD+32 1581, Gliese 278A
Spectral TypeA2Vm
Constellation's Main StarYes
Multiple Star SystemYes
Star Type Main Sequence Dwarf Star
ColourBlue
GalaxyMilky Way
ConstellationGemini
Absolute Magnitude 0.59 / 0.61
Visual / Apparent Magnitude1.58
Naked Eye VisibleYes - Magnitudes
Right Ascension (R.A.)07h 34m 36.00
Declination (Dec.)+31° 53` 19.1
Galactic Latitude22.48 degrees
Galactic Longitude187.44 degrees
1997 Distance from Earth63.27 Parallax (milliarcseconds)
 51.55 Light Years
 15.81 Parsecs
2007 Distance from Earth64.12 Parallax (milliarcseconds)
 50.87 Light Years
 15.60 Parsecs
 3,217,708.56 Astronomical Units
Galacto-Centric Distance24,181.75 Light Years / 7,414.00 Parsecs
Proper Motion Dec.-145.19 ± 2.63 milliarcseconds/year
Proper Motion RA.-191.45 ± 3.75 milliarcseconds/year
B-V Index0.03
Radial Velocity5.40 ± 0.50 km/s
Eccentricity0.08
Semi-Major Axis8011.00
Stellar Luminosity (Lsun)53.40
Brightest in Night Sky24th
Associated / Clustered StarsCastor B
Castor C

Companions (Multi-Star and Exoplanets) Facts


Exoplanet CountNone/Unaware

Estimated Calculated Facts


Radius (x the Sun)2.40
Effective Temperature9,140 Kelvin
Mass Compared to the Sun2.76

Sources and Links


SIMBAD SourceLink

Multi-Star System

The star has been identified as being a multi-star system, one in which there is at least one star in close orbit to another star or two or more stars orbiting a central point. The stars may be of equal mass, unequal mass where one star is stronger than the other or be in groups orbiting a central point which doesn't necessarily have to be a star. More information can be found on my dedicated multiple star systems page. The source of the info is Simbad. The file is dated 2000 so any differences between this and any other source will be down to the actual source from where the information came from.


Proper Motion mas/yr
H.D. IdB.D. IdStar CodeMagnitudeR.A.Dec.SpectrumColourYear
60179+32 1581.0A2.00000-171.00000-103.00000A0White
60178B2.90000A0White1826
+32 1582.0C8.80000-210.00000-112.00000M1Red1835
+32 1580.0D4.00000-23.000001902

Location of Castor in Gemini


Castor Location in Gemini

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

Gemini Main Stars


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