Dear Moon, Please Take Us Porrima

Tonight’s moon pulls right up alongside Porrima, a beautiful telescopic double star. Stellarium

I like working the moon to death. Because it “gets around,” we’ve used it on previous occasions to help us find bright stars and planets. Tonight, the waxing gibbous sits just 1° above and left of one of the finest double stars in the sky — Porrima. Also known as Gamma (γ) Virginis, Porrima is an unassuming 3rd magnitude star in the Virgo located about a fist above Spica, the constellation’s brightest star.

The name Porrima comes to us all the way from ancient Rome, when Joe-citizen knew it as one of the goddesses of prophecy. Boy, they had a lot of deities back then. Maybe the Romans assigned them to stars just to make it easier to remember all those names! Anyway, Porrima is simply beautiful in a small telescope.

Porrima is a binary star with each sun revolving about the system’s common center of gravity with a period of 169 years. This causes the separation and orientation of the stars to change over time. Separation between the stars is given in arc seconds. One arc second = 1/60th of an arc minute. 30 arc minutes = the apparent diameter of the full moon. Damian Peach

It wasn’t always so. The star is one of the few where it’s easy to see the motion of the stars about their common center of gravity. Many doubles take many human lifetimes to reveal their orbital motion. Gamma Vir makes a complete spin-around every 169 years or about two lifetimes. They were closest in 2005 when only larger amateur telescope had the resolving power to split the two apart. Now, they’ve widened to the point that a 4-inch or larger scope will will get the job done.

Diagram showing the orbit of the secondary companion around the primary star in the Gamma Virginis double. The two were tight in 2005. Since then they’ve been opening up. This season, the stars are north-south of each other and much easier to cleave. www.dibonsmith.com

To find Porrima, go out tonight (May 24), and use a lower power eyepiece in your telescope. 50x or less is perfect. Center it on the moon and then slide just a degree to the south and west (down and to the right) to corral Porrima. At low power, it will look like a single star, but if you increase the magnification to about 100-150x, you’ll see two pure white, equally bright suns glimmering side by side.

Each star is 1.5 times as massive than the sun, and they’re relatively close at 38 light years away, about the same distance as Arcturus. Although both orbit around a common center, the orbit is tilted with respect to the sky, so it looks like the one is looping around the other. They’re separated from each other by 43 astronomical units or 43 times the distance between the Earth and the sun which happens to be about the distance between the sun and Pluto.

As you can see from the orbit diagram at right, the separation of the two stars has been increasing since the squeaky tight days of 2005. Later this century, they’ll be about 5″ apart — twice as wide as this year  —  and splittable in even the smallest telescope.

So go ahead, make the moon deliver up a beautiful entree for you tonight.

2 Responses

  1. Alan

    Bob,
    You really have a rare gift for explaining Astronomy and so much of it’s fascinating phenomena clearly and thoroughly with an economy of words.
    Thanks so much,
    Alan Delman, M.D

    1. astrobob

      Dr. Delman,
      Thank you for such a kind compliment. I always strive for economy and occasionally succeed. I’m glad you enjoy the blog.

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