Castor and Pollux - Alpha and Beta Geminorum

 

The constellation Gemini with the bright blue Castor and light orange hued Pollux (both top left)
A stack of light subs captured with a Star Adventurer EQ mounted Canon 600d DSLR with a fixed Canon 50mm lens. Image Credit: Pip Stakkert

 

" The stars Castor and Pollux are quite different and appear so to the naked eye.. 

Castor looks to be a single star but is in fact six stars bound gravitationally. Two of the larger and brighter component stars are of A spectral class and dominate the combined Castor spectrum. The  127mm refractor at the Jodrell Plank Observatory is easily able to separate these two stars.

Pollux is an evolved red giant star, that having used up it's supply of hydrogen, has commenced fusing helium at its core. The change requires the temperature to rise at the core and the whole star to increase in size. Pollux is of spectral class K.

The Jodrell Plank Observatory 127mm Refractor and 3d printed transmission grating spectrometer were used to capture spectral profiles of the two stars. Even a cursory inspection, shows how different the spectra are and the marked difference in surface temperature of Castor and Pollux. Although Pollux is much cooler than Castor, it appears brighter, mainly because its diameter and consequent surface area are both much larger than the equivalents for Castor." - Kurt Thrust current Director of the Jodrell Plank Observatory

Comparative low resolution spectral profiles with Planck Curves
 for Castor and Pollux.
Credit: Kurt Thrust and BASS Spectro software

A cropped section from the above widefield image of the Constellation
 Credit : Kurt Thrust