Castor B (Alpha Geminorum B) is a subgiant star that can be located in the constellation of Gemini. The description is based on the spectral class. Castor B is not part of the constellation but is within the borders of the constellation.
The star can be seen with the naked eye, that is, you don't need a telescope/binoculars to see it.
(Alpha Geminorum B) is the Bayer Classification for the star. 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 star alternative names can be found at Star Names .
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 Castor B, the location is 07h 34m 36.10 and +31 ° 53` 18.57 .
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 miliarcseconds/year towards the north and -206.30 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 -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.
Castor B has a spectral type of kA0hA2:mA1IVs B.
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.
|Primary / Proper / Traditional Name||Castor B|
|Alternative Names||Alpha Geminorum B, HD 60178, HR 2890|
|Spectral Type||kA0hA2:mA1IVs B|
|Constellation's Main Star||No|
|Multiple Star System||Yes|
|Star Type||Subgiant Star|
|Visual / Apparent Magnitude||2.97|
|Naked Eye Visible||Yes - 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 Stars||Castor|