A Double Star Puzzle

What well-known double star has a companion that appears to orbit the primary once every 24 hours?

The answer is a well-known star that looks like a single star to our naked eye but is actually a binary system with a smaller companion star. The companion star is visible in a telescope and appears to orbit the primary once every day. If you view this system through a 24 hour period, perhaps in Winter from above the Arctic Circle where it remains dark all day, then the companion would trace one complete orbit around the primary and end up back where it started. If you are impatient then scroll down to the video below for the answer.


Albireo – one star blue and the other golden – as captured by Tom Wildoner in July, 2015. Visit Tom’s blog, Leisurely Scientist.

A double star is a pair of stars that appear close to each other as viewed from Earth, especially with the aid of a telescope. In binary systems, the stars are physically bound together by gravity so they are in orbit around each other. Binary stars are important to astronomers as knowledge of their motions allows direct calculation of stellar mass.

The star Alberio, the head of swan in the constellation Cygnus, is considered to be one of the most beautiful double stars. It is a binary system with the brighter gold star and the dimmer blue star orbiting around their common center of mass.

The video below shows the answer to the double star puzzle produced by simulating the binary system using the SkySafari app.

Polaris, the north star, is a binary system with a companion SAO 305, also known as Alpha Ursae Minoris B or Polaris B. If you pay attention to the date/time at the bottom of video then you can see that SAO 305 does, in fact, appear to orbit Polar once every 24 hours.

But the puzzle is a deception, of course, because all the stars that we can view from the Northern Hemisphere appear to orbit Polaris every 24 hours as seen in the following video with a wider field.

These “orbits” appear because of the rotation of the Earth rather than the motion of the stars.

But the universe keeps things interesting – In 1929, another very close companion was found near Polaris making it a system of three stars that are gravitationally bound together.

Credit: This puzzle came from Glenn Chaple‘s Oberving Basics column in the May issue of Astronomy Magazine, published Tuesday, April 2, 2019.