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Monday, December 10 , 2018, 3:38 am | Fair 49º


Dennis Mammana: Finding the Horse and Rider in the Night Sky

Find the horse and rider after dark this week. Click to view larger
Find the horse and rider after dark this week. (Creators.com illustration)

We terrestrial stargazers have it easy. We orbit a single star, and we look out at the starry heavens and believe that all stars are like the sun.

In some ways that’s true. All are globes mostly composed of hydrogen that shine by a process of thermonuclear fusion occurring deep within their core. But, as astronomers have learned, that’s where the similarity often ends.

Today we know that stars can be larger or smaller, redder or bluer, or 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; they 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, especially with binoculars or a small telescope.

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 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 currently appears to be standing on its handle in the northeastern sky after dark. And it’s at the bend of the Dipper’s handle that an interesting object resides.

Look here to see whether 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 Zakariya al-Qazwini who wrote, “People tested their eyesight by this star.” The 14th-century Arabian writer El-Firuz Abadi referred to it as Al Sadak (meaning “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 each other. The pair is called an “optical double,” or 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.

Dennis Mammana is an astronomy writer, author, lecturer and photographer working from under the clear dark skies of the Anza-Borrego Desert in the San Diego County backcountry. Contact him at [email protected] and follow him on Twitter: @dennismammana. Click here to read previous columns. The opinions expressed are his own.

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