Astro Bob blog: How to double your enjoyment of the heavensOur solitary sun is a bit of an exception ... it could have been double. Learn more about double stars and see one for yourself tonight.
By: Bob King, Duluth News Tribune
Double your enjoyment of the heavens
I recently picked up a copy of the Cambridge Double Star Atlas that plots almost 2,400 double stars across the sky for binoculars and telescopes. It got me to thinking about our companionless sun. We live around a single star, and yet two-thirds of stars similar to the sun in size and temperature are double or multiple.
We don't lack for sunsets and sunrises, but the thought of seeing four or more of them a day is forever denied to Earthlings. What a variety of different lighting conditions our planet would have if we orbited around a binary star, one of which was an white giant and the other a red dwarf. The giant would sizzle with brilliant light but when it set, the dim dwarf would take over, illuminating our towns and forests with a softer radiance more akin to full moonlight. Somewhere, something wakes up to scenes like these.
The majority of stars in our galaxy -- more than three-quarters -- are red dwarfs with masses only a tenth to a half that of our sun. These little guys aren't often found in pairs. When they form deep within cold gas clouds, gravity causes the material to collapse into a single, dense blob that eventually evolves into a single star. Large stars are almost always double because they form in more turbulent conditions that tend to fragment the newly-forming dense star cores into twin or multiple cores. These evolve into double and multiple stars.
Four of the finest double stars for small telescopes. Rigel is in Orion, Albireo in the foot of the Northern Cross, Cor Caroli in Canes Venatici and Gamma in Aries. The thin, broken circles around each star are called diffraction rings, and all telescopes show them at higher magnifications. It's an optical effect caused by looking at point source like a star through a small tube. Illustration: Bob King
In true double stars, also called binaries, each star is gravitationally bound to the other and they orbit about their common center of gravity. There are also "false" doubles or what astronomers call optical double stars -- chance alignments of stars that minic the real thing. While there are only a handful of double stars that are splittable with the naked eye, hundreds are visible in the smallest telescopes. They are the gems of the heavens. Their delicate, pure colors and fiery appearance recall diamonds, rubies and sapphires. I looked at a tight pair Xi Cephei (zye SEPH-ee-eye) two nights ago. Something about those two stars so bright and so close together in a field awash with faint stars took my breath away. While galaxies generally require dark skies and a trained eye to appreciate, double stars tolerate average skies and are instantly accessible.
Some binaries are very close together and require high magnification to separate. These "challenge" doubles sharpen an amateur astronomer's vision, deepen the ability to concentrate and push our instruments to the limit. Most double stars revolve so slowly that they don't change position during a lifetime but there are some notable exceptions like the bright Castor in Gemini and Gamma in Virgo. In the case of Gamma, by using high magnification, you could see the companion's movement in the span of only a year!
The double star Mizar-Alcor is not only one of the brightest pairs in the sky but very easy to find. The map shows the sky as you face north around 9:30-10 p.m. Created with Stellarium
The sky's most famous double star will be shining tonight. The pair, Mizar and Alcor, are the pair of stars in the bend of the Big Dipper's Handle. Most people with reasonably good vision can split them without optical aid. Mizar, the brighter one, is double again through a small telescope. More sophisticated instruments have shown that each of these is also double forming an amazing quintuple system. Whether Alcor is a true companion or an optical one is still being debated, but there's no denying they form a pretty naked eye pair.
As you walk under the sun today, you may ponder why our star circles the galaxy alone when the chances were good it could have had a companion. Looks like the planets will have to do for company. Not such a bad way to go I think. After all, some of my favorite companions hail from Earth.