Castor (α Gem / α Geminorum / Alpha Geminorum) is the second brightest star in the constellation Gemini and one of the brightest stars in the nighttime sky. Although it has the Bayer designation "alpha", it is actually fainter than Beta Geminorum (Pollux). In astronomy, an epoch is a moment in time for which celestial coordinates or orbital elements are specified. ... Pierce this website is not very reliable!!!!!!!!!!!!!!!!! Some well-known constellations contain striking and familiar patterns of bright stars. ... Gemini (Latin for twins, symbol , Unicode ♊) is one of the constellations of the zodiac. ... Right ascension (RA; symbol α: Greek letter alpha) is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. ... In astronomy, declination (dec) is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. ... // Headline text HEY!! HOW ARE YOU ALL?? Its nice of you to come read this page. ... This article does not cite its references or sources. ... Radial velocity is the velocity of an object in the direction of the line of sight. ... kilometre per second is an SI derived unit of both speed (scalar) and velocity (vector), signified by the symbol km/s or km s-1. ... The proper motion of a star is the motion of the position of the star in the sky (the change in direction in which we see it, as opposed to the radial velocity) after eliminating the improper motions of the stars, which affect their measured coordinates but are not real... A milliarcsecond (m, mas) , or a thoundsanth of an arcsecond. ... A year is the time between two recurrences of an event related to the orbit of the Earth around the Sun. ... A milliarcsecond (m, mas) , or a thoundsanth of an arcsecond. ... A year is the time between two recurrences of an event related to the orbit of the Earth around the Sun. ... Parallax (Greek: παραλλαγή (parallagé) = alteration) is the change of angular position of two stationary points relative to each other as seen by an observer, due to the motion of an observer. ... A milliarcsecond (m, mas) , or a thoundsanth of an arcsecond. ... The distance between two points is the length of a straight line segment between them. ... A light year, abbreviated ly, is the distance light travels in one year: roughly 9. ... The parsec (symbol pc) is a unit of length used in astronomy. ... In astronomy, absolute magnitude is the apparent magnitude, m, an object would have if it were at a standard luminosity distance away from us. ... In astronomy, many stars are referred to simply by catalogue numbers. ... In astronomy, many stars are referred to simply by catalogue numbers. ... In astronomy, many stars are referred to simply by catalogue numbers. ... In astronomy, many stars are referred to simply by catalogue numbers. ... The Henry Draper Catalogue is an astronomy catalogue with astrometric and spectroscopic data about more than 225,000 stars. ... In astronomy, many stars are referred to simply by catalogue numbers. ... In astronomy, many stars are referred to simply by catalogue numbers. ... The Smithsonian Astrophysical Observatory (SAO) is a research institute of the Smithsonian Institution headquartered in Cambridge, Massachusetts, where it is joined with the Harvard College Observatory (HCO) to form the Harvard-Smithsonian Center for Astrophysics (CfA). ... In astronomy, many stars are referred to simply by catalogue numbers. ... Alpha (uppercase Α, lowercase α) is the first letter of the Greek alphabet. ... Pierce this website is not very reliable!!!!!!!!!!!!!!!!! Some well-known constellations contain striking and familiar patterns of bright stars. ... Gemini (Latin for twins, symbol , Unicode ♊) is one of the constellations of the zodiac. ... Bright stars can be bright because they produce more light, because they are closer to us, or both. ... Many of the brighter stars are given names which are known as Bayer designations. ... Pollux (β Gem / β Geminorum / Beta Geminorum) is one of the brightest star in the constellation Gemini and one of the brightest stars in the nighttime sky. ...

Castor and Pollux are the two "heavenly twin" stars that give the constellation Gemini (meaning "twins" in Latin) its name. It also has the arabic name Al-Ras al-Taum al-Muqadim, which literally means "The head of the foremost twin".

Astronomically, Castor was discovered to be a visual binary in 1719, with the magnitude of its components being 2.8 and 2.0. The separation of the components is about 6" and the period of revolution is around 350 years. Each of the components of Castor is itself a spectroscopic binary, making Castor a quadruple star system. Castor has a faint companion separated from it by about 72" but having the same parallax and proper motion; this companion is also a spectroscopic binary with a period slightly less than 1 day. Castor can thus be considered to be a sextuple star system, with six individual stars gravitationally bound together. Component C has the variable star designation YY Geminorum. A visual binary star is a binary star for which the angular separation between the two components is great enough to permit them to be observed as a double star in a telescope. ... // Events January 23 - The Principality of Liechtenstein is created within the Holy Roman Empire April 25 - Daniel Defoe publishes Robinson Crusoe June 10 - Battle of Glen Shiel Prussia conducts Europes first systematic census Miners in Falun, Sweden find an apparently petrified body of Fet-Mats Israelsson in an unused... // Headline text HEY!! HOW ARE YOU ALL?? Its nice of you to come read this page. ... A spectroscopic binary star is a binary star which cannot be resolved as a visual binary, even with telescopes of the highest existing resolving power. ... Parallax (Greek: παραλλαγή (parallagé) = alteration) is the change of angular position of two stationary points relative to each other as seen by an observer, due to the motion of an observer. ... The proper motion of a star is the motion of the position of the star in the sky (the change in direction in which we see it, as opposed to the radial velocity) after eliminating the improper motions of the stars, which affect their measured coordinates but are not real... Most stars are of almost constant luminosity. ...

Data for the six stars that make up Castor: