The light that we currently see from Castor left the star and began it’s journey towards Earth 51 years ago.

Castor C is an eclipsing binary system, exhibiting variations in brightness as a result of the two components periodically eclipsing each other as they orbit.

Once you know the luminosity and temperature (or color) of a star, you can plot the star as a point on the H-R diagram. The primary star of the Castor A binary pair, Castor Aa, is a main sequence blue-white star of spectral class A1 V, similar to Sirius. The bright, blue-white star has a visual luminosity between 17 and 34 times that of our Sun. Once you know the luminosity and temperature (or color) of a star, you can plot the star as a point on the H-R diagram. 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. The brightest star in the constellation is normally given the Alpha designation although there are exceptions such as Pollux which is Beta ... Castor Luminosity. Star type. A Simple H-R Diagram. Star with planet.In northern spring evenings, the "twin" stars Castor and Pollux of the constellation Gemini descend the northwestern sky looking like a pair of eyes staring down at the Earth. The inverse square law applies to gravity as well as apparent brightness of a star. Pollux, which is slightly closer to us, is a single star, while Castor is a star system consisting of six stars. Each component of the Alpha Geminorum system is in itself a spectroscopic binary star, which makes Castor a four-star system. The “b” indicates that the star is less luminous than average for its class. 51 years ago it was 1968 (as of 2019). Castor is a complex sextuple system of hot, bluish-white A-type stars and dim red dwarfs, while Pollux is a single, cooler yellow-orange giant. The two components are separated by 6” and have a revolution period of about 467 years. Though Castor is designated as the alpha star of its constellation, it is, in fact, Pollux– Beta Geminorum – who is the brightest star. However, this might also suggest that one or maybe both

They are twins only in mythology, these warriors, Pollux fathered by Zeus and divine, Castor mortal, both placed in the sky to allow them to be together for all time. Castor and Pollux are the two "heavenly twin" stars giving the constellation Gemini (Latin, 'the twins') its name.The stars, however, are quite different in detail.

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A Simple H-R Diagram. Castor is a complex sextuple system of hot, bluish-white A-type stars and dim red dwarfs, while Pollux is a single, cooler yellow-orange giant. Castor and Pollux are the two "heavenly twin" stars giving the constellation Gemini (Latin, 'the twins') its name. Castor’s Distance in Perspective: Castor is about 51 lightyears away. Pollux, the brightest star in the constellation Gemini, blazes in a golden light next to its bluish-white heavenly twin, Castor, in the evening skies of the Northern Hemisphere's spring.