Look up in the sky on a clear night from dark surroundings and you'll be able to see upwards of six thousand stars - all part of our galaxy, the Milky Way. Many, like our Sun, are orbiting on their own. Yet the rest, about half of them, are traveling in groups of two or more. Groups that not only give us some of the prettiest sights the sky has to offer but also a glimpse of how the laws of physics that we see on a daily basis are at work on the stellar level.
What causes these stars to travel in pairs? Stars, whether single or in multiple systems, are formed in clouds of gas called nebula. Over time, gravity causes huge quantities of gas to coalesce into a gas ball that, ultimately, creates enough heat and temperature to begin a nuclear fusion reaction at the center of this ball.
Double Stars Orbit
A frequent nickname for a nebula is stellar nursery. That is misleading, it implies a relatively tranquil, calm setting. But a nebula is subjected to very complex, even violent clashes of temperature, magnetism, gravity, and other forces that can be observed with our own Sun. In the course of all this turmoil most of the multiple star systems are created together as parts of the interstellar cloud collapse to form them. Whether or not these remain together or coalesce into a single star depends on the intricacies of the forces acting on or around them.
For some reason, double stars (or binaries) haven't drawn a great deal from amateur astronomers in general. There are splendid sights
to be seen amongst the array of galaxies, star clusters, and planets. But binaries provide some of the most striking glimpses of color
in the sky. They're easy to enjoy, even through the smallest telescopes or moonlit nights. And the very minute distances between many
of them provide a very easy way to test the resolution of a telescope.
Stars get their color from the temperature they have, the coolest stars appearing red and the hottest as blue. And these hues appear in crisp, pinpoint images for our eyes to enjoy. A good way to discern the dramatic differences that star hues can present is to view some double stars at a star party. If you ask most amateur astronomers, their idea of the prettiest doubles feature the widest color contrasts - say a hot bluish star matched with a cooler orange star. Having such a pair close together in the eyepiece illustrates the variations between stars wonderfully indeed.
One example you can see on your own is the system of Alcor & Mizar in the handle of the Big Dipper. It appears as the middle star in the handle. Consisting of three stars, the farthest pair can be split by just the naked eye. Aim binoculars at it, and you'll see the fine white colors that it is famous for.
Alcor and Mizar in the Big Dipper
Astronomy is one of those few remaining areas where amateurs can play an important role in contributing to science. Binary systems are abundant to the point that professional astronomers are hardly able to monitor the orbits of these pairs. Amateurs help to refine what we know of their orbits - how they interact with each other. How far apart they are, their angles of separation, orbital periods, and colors are all logged by active amateurs and submitted for scrutinizing. Doing the science is nice, but most of what you can get out of viewing binaries amounts to the same kind of pleasure a bird-watcher, collector, or a gardener must feel when their long effort and patience reveals a fresh display in their experience.
The following list has resources for more information about double stars: