Looking Up: Seeing Double

Feb 10, 2020

This image is available from the United States Library of Congress's Prints and Photographs division under the digital ID cph.3g10067. "Virgo", plate 21 in Urania's Mirror, a set of celestial cards accompanied by A familiar treatise on astronomy ... by Jehoshaphat Aspin. File:Sidney Hall. Restoration by Adam Cuerden.
Credit public domain / wikimedia commons

This week on Looking up Hal informs us about a star named Porrima. But is it one star or two?

High in the east in the Colorado night sky is the very interesting star Porrima. Or rather, I should say stars, because there are actually two stars orbiting each other, only 38 ly away. Or maybe I should just say “star” because whether you see one or two stars through a telescope depends on what year it is.

You see, there are lots of double stars out there that orbit each other. But the vast majority we know of orbit in long periods, often decades, centuries, or even longer. But our local neighbor Porrima is made up of two near-identical twin stars that complete an orbit every 169 years or so. Back in 2007, the stars got so close together that even in big telescopes they looked like a single point of light. But this year, 2020, they have now moved far enough apart that they can again be seen as two individual stars. These stars are one of a very few sets of double stars that a single human can actually observe moving over the course of a normal lifetime. Pretty cool, eh? 

Credit IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg) / wikimedia commons

Porrima is very close to the plane in which the planets and moons move around the Sun, so every now and then Porrima is occulted, or blocked, by the Moon or a planet passing in front of it. And if that sounds interesting, be sure to listen to next week’s episode about such an occultation happening in a solar system near you. Am I a tease?

If you’d like to take a closer look at Porrima, or any of the other wonderful and amazing things in the sky, please visit csastro.org for a link to information on our monthly meetings and our free public star parties.