Pollux and Castor are two fairly bright stars not far apart on the face of the night sky, prominent on April evenings. These two are considered the “heads” in the constellation, Gemini the Twins.
They’re not exactly identical twins.
Despite the bright moonlight this week, they stand out, high in the southern sky between 8 and 9 p.m. (daylight savings time) They are visible most of the night, setting in the northwest a little after 3 a.m.
Orion, the majestic constellation with its famed trio of stars marking Orion’s Belt, can be used to point right at Pollux and Castor. Orion is a bit lower, in the south-southwest around 8-9 p.m. Trace an imagined line from Orion’s bright blue-white star Rigel at lower right of the “Belt,” through the bright red star Betelgeuse at upper left of the “Belt.” Continue this line to the upper left, to the Castor and Pollux duo.
(Betelgeuse, by the way, has triumphantly regained its brightness after a few months of unusual dimness this winter.)
Pollux is the brightest of the two “heads” of the Twins, and the lower one (assuming you aren’t in the Southern Hemisphere where the perspective is turned on its head). The star’s color is light yellow-orange. In 2006, a planet about 2.3 times the mass of Jupiter, was discovered circling the star. The International Astronomical Union has named this planet Thestias.
Castor is bluish-white and bright, but less so than Pollux. Castor’s claim to fame for amateur astronomers training their backyard telescopes at the sky is that there are actually six stars in the Castor system. We see it as one, with unaided eyes. There are three pairs of binary (double) stars, orbiting around a common center of mass.
Telescopes, however, only pick up three of these stars. Two are easily seen in a small telescope; you may also find a third star, which is a lot fainter. Each of them are double stars themselves, but they are so close it takes a spectroscope, splitting the starlight into their spectrum of colors, to resolve them as double stars. They don’t know anything about social distancing, a serious matter here on planet Earth lately.
The rest of the naked-eye stars making up the pattern of Gemini the Twins stretch back to the west (or right as seen on an April evening), towards Orion. There is a beautiful star cluster known as M35 located near the stars marking one of the “feet” of the Twins. M35 is easily visible in binoculars when the moon isn’t around, and is a fine sight in a small telescope.
It’s interesting to note that Pollux is 34 light-years from the Earth, and Castor is 51 light-years away. That means the two “heads” of Gemini the Twins are 17 light-years apart. It takes that many years for the starlight from one to reach the other. That would seem to make the Twins an incredibly immense couple of brothers.
Of course, we know that the constellations are imagined, two-dimensional patterns of our making. The stars across the sky are in three dimensions, at greatly varying distances. A space traveler heading out among the stars would observe that the constellation patterns as we know them become unrecognizable the farther we travel from the sun.
That doesn’t stop us from enjoying them!
Full moon is on Tuesday, April 7. The moon will be close to its monthly perigee, when it is closest to Earth. This will make the moon appear a little larger (by 14%) and brighter (by 30%) than when the full phase happens to occur close to the moon’s farthest point, apogee.
Be sure to see the trio of planets in the southeast in the hour before dawn. From left are Mars, (which is reddish), Saturn and Jupiter (which appears as the brightest). Venus is extremely bright in the evening, in the western sky.
Keep looking up at the sky!
Peter Becker is managing editor at The News Eagle in Hawley, Pennsylvania. Notes are welcome at firstname.lastname@example.org. Please mention in what newspaper or website you read this column.