Sky Watch: Seeing double in the dipper

It's pretty easy for us terrestrial stargazers, orbiting a single star like the sun, to believe that the stars of our nighttime sky must be similar to it.

In some ways that's true. All are globes of mostly hydrogen gas that shine by a process of thermonuclear fusion occurring deep within their cores. But, as astronomers have learned over the past few centuries, that's where the similarity often ends.

Today we know that stars can be larger or smaller, redder or bluer, hotter or cooler, than the sun. Many stargazers are surprised to learn, however, that between half and three-quarters of all stars are actually part of systems in which two (or sometimes more) stars orbit a common center of gravity.

Even a quick glance around the night sky will reveal many stars that seem to have a companion nearby. More often than not, however, these stars are not physically related but simply appear close together in the sky. Astronomers know these as double stars.

We can find a classic example of these high in the northern sky after dark this week, especially with binoculars or a small telescope.

Let's begin by locating the seven stars that form the bowl and handle of the Big Dipper. Most people believe that the Big Dipper (or the Plough, as the British prefer) is a constellation, but it's really a group of stars that's only part of the large constellation Ursa Major, the Great Bear. It's what astronomers call an asterism, a group of stars that -- with enough imagination -- can appear as a familiar shape.

Whatever we call it, its seven nearly-equally-bright stars are not easy to miss. They form the shape of a bowl with a bent handle, which, right now, appears upside down. And it's at the bend of the Dipper's handle that an interesting object resides.

Look here to see if you can spot a double star. These two are known by their proper names Mizar and its fainter companion Alcor, also called the horse and rider.

If you cannot see both stars with just your eyes, aim binoculars in their direction and you should have no trouble at all.

It was the 13th century Persian writer Al Kazwini who wrote that "people tested their eyesight by this star." The 14th century Arabian writer Al Firuzabadi referred to it as Al Sadak (the test or the riddle).

Seems rather odd, though. Both stars are easily visible to most people today, even in moderately light-polluted regions. Has human eyesight improved that much over the past six or seven centuries? Or has the fainter star Alcor brightened significantly since medieval times? No one knows for sure.

What we do know is that Alcor and Mizar, though they appear close in the sky, are not physically related to one another. The pair is what's called an optical double, two stars that just happen to appear roughly along the same line of sight. Mizar, the brighter of the two, is actually part of a physical binary system (two stars that orbit a common center), and you can see its faint companion star with a small or medium-sized telescope.