When we gaze at Pollux and Castor with the naked eye, we see two single stars. We learn more about Castor from the intriguing photograph of glistening streaks at the top of this page. As described by astrophotographer Kent DeGroff, “It is an image of a double star (Castor in Gemini) as it trails for a few seconds through the field of view of a telescope at high magnification… The trailing is caused by the Earth’s rotation with the telescope not tracking. The brighter of the two is referred to as Castor A and the fainter is Castor B, with magnitudes of 1.9 and 3.0 respectively. Actually, there are 6 stars with three being visible (only two in that image) and the others being spectroscopic binaries. The combined brightness of all six stars is +1.6. … If the telescope were moving with the stars, tracking, the double star would be seen as one star even at high magnification. When the telescope is in a set position, the star is seen moving through the eyepiece.”
Pollux, at magnitude +1.15, is slightly brighter than Castor—the smaller the number the brighter the celestial object. Recently, a giant planet was discovered orbiting Pollux.
Judy Isacoff
