Eyes to the Sky
May 17 – 30, 2021
Look west to Gemini’s Castor and Pollux and to Mars, Mercury and Venus
Double star Castor photographed moving in space and time. Image courtesy of New Mexico-based astrophotographer Kent DeGroff.
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As twilight deepens tonight, beginning around 9:30pm, locate the crescent moon in the west mid-way between zenith and the horizon. Below the moon, a juxtaposed pair of bright stars stands out. Known as the Gemini twins, yellowish Pollux is on the left and blue-white Castor on the right. A ways to the left of Pollux find luminous Procyon the Little Dog. About the same distance to the right of Castor find Capella the Little Goat, brightest of the foursome.
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.
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