Spring’s Newest ‘double-double’ Star

Saturn (left) and its 'companion' Porrima make a lovely naked eye "double star" during May and June. Details: 35mm lens at f/2.8, ISO 1600 and 20-second exposure. Photo: Bob King

We’ve got a new double star in the spring sky! OK, it’s only a made-up one and temporary at that, but Saturn and the star Gamma Virginis, also known as Porrima, make one of the most realistic close pairs of ‘stars’ I’ve ever seen. Their resemblance to a telescopic view of a true double star is remarkable.

Face due south at nightfall and look about halfway up to find Spica and Saturn. Less than 1/2 degree to the planet’s upper right is Porrima, named after a Roman goddess of prophecy. At magnitude 3.5, the star is considerably fainter than the planet, but its nearness makes it quite easy to see. The star stays put, but Saturn, being a planet, continues to move in Porrima’s direction until June 9. That’s when they’ll be closest at just 1/4 degree apart. Of course Saturn is very much in the foreground at a distance of 836 million miles (1.25 light hours) compared to Gamma’s 39 light years.

After that date, Saturn will appear to reverse direction and begin tracking back to the east. Their separation will increase and the thrill will be gone.

The double star Porrima photographed in 2003 when the two were closer together than they are now. Credit: Damian Peach

The beauty of this close pairing is that Porrima itself is a true binary or double star, making its proximity to Saturn a ‘double double’ delight. It’s also one of the few binaries whose companions amateur astronomers can watch move over the course of a few years.

The two stars are similar to the sun but hotter and nearly identical in size. They go around each other every 169 years in an elliptical orbit that cyclically brings them closer together and farther apart as seen from Earth. Closest approach was in 2005 when the duo was separated by the same distance Jupiter is from the sun or about 500 million miles. When farthest apart around the year 2080, they’ll be twice Pluto’s distance from each other. Check out this orbital diagram to get a clearer picture of what I mean.

Planets that are closer to the sun move faster than those that are in more distant orbits. The same is true with double stars. Porrima’s two stars are still moving relatively quickly since closest approach 6 years ago, making it possible for amateurs equipped with a modest telescope (4-inch and larger) to watch them separate and change position relative to one another in just a few years time.

Saturn is almost due south at nightfall in late May. It forms a triangle of bright stars with Spica and Arcturus. Created with Stellarium

As of this spring, the two are separated by a gap of just 1.7″ of arc. That’s pretty tiny. Remember that a second of arc is equal to 1/60 of a minute or 1/3600 of a degree. For reference, the full moon is 1/2 degree or 30 minutes of arc in diameter.

The slightly fainter companion lies to the northeast of the brighter ‘primary’ star. Use high power to split the pair and make a sketch. While the difference in position won’t be obvious in a year, six or seven years from now, the companion will be considerably farther away and due north of the primary star. Dust off that old sketch, compare the view and you’ll see that the stars have move of their own motion in just a few short years.