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Castor B (Companion Star) Facts

Castor B Facts

Castor B's Alternative Names

(Alpha Geminorum B) is the Bayer Classification for the star. The Bayer Classification was created by Johann Bayer in the early nineteenth century. The brightest star in the constellation is normally given the Alpha designation although there are exceptions such as Pollux which is Beta Geminorum.

The Id of the star in the Yale Bright Star Catalogue is HR2890. The Id of the star in the Henry Draper catalogue is HD60178.

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

Location of Castor B

The location of the subgiant 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 B, the location is 07h 34m 36.10 and +31 ° 53` 18.57 .

Radial Velocity and Proper Motion of Castor B

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 -148.20 milliarcseconds/year towards the north and -206.30 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 -1.20 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 B

Castor B Colour and Temperature

Castor B has a spectral type of kA0hA2:mA1IVs B.

Castor B Apparent and Absolute Magnitudes

Castor B has an apparent magnitude of 2.97 which is how bright we see the star from Earth. Apparent Magnitude is also known as Visual Magnitude. 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.

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Additional Castor B Facts and Figures

Visual Facts


Primary / Proper / Traditional NameCastor B
Alternative NamesAlpha Geminorum B, HD 60178, HR 2890
Spectral TypekA0hA2:mA1IVs B
Constellation's Main StarNo
Multiple Star SystemYes
Star TypeSubgiant Star
GalaxyMilky Way
ConstellationGemini
Visual / Apparent Magnitude2.97
Naked Eye VisibleYes - Magnitudes
Right Ascension (R.A.)07h 34m 36.10
Declination (Dec.)+31 ° 53` 18.57
Proper Motion Dec.-148.20 milliarcseconds/year
Proper Motion RA.-206.30 milliarcseconds/year
Radial Velocity-1.20 km/s
Associated / Clustered StarsCastor
Castor C

Companions (Multi-Star and Exoplanets) Facts


Exoplanet CountNone/Unaware

Sources and Links


Sourcehttp://simbad.u-strasbg.fr/simbad/sim-id?Ident=Castor%20B

Location of Castor B in Gemini


Castor B Location in Gemini

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

Related Stars


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