In my previous column, I suggested that you step out after dark to count the stars. If you did this, I hope you learned just how few stars we can see with the unaided human eye.
You might like to try it again this week with the light of the waxing moon. I guarantee you’ll come up with a significantly smaller number.
That’s because the moonlight scatters around the atmosphere and decreases the contrast of the stars against the sky.
The sun does this in the daytime as well, so no stars are visible then (though they’re all there to see if you’ve got a hefty telescope!)
If we expect to see stars during the early evening hours this week, we’ll have to be satisfied with only the brightest of them, since that’s all that can be seen over the moon’s “light pollution.”
Fortunately, the seven stars of the Big Dipper are bright enough. To find the Dipper, look fairly high in the northwestern sky shortly after dark and you’ll see the four stars that form a rectangular bowl and the three stars of its curved handle just above.
Like all other stars, these are distant thermonuclear fusion reactors. In their cores, they convert hydrogen to helium and release a tremendous amount of energy in the process.
Some of that energy arrives at the Earth in the form of the starlight we see.
Another interesting fact about stars is that only about 15% of them are single bodies. The rest are part of what we call “binary” or “multiple-star” systems, in which two or more stars orbit a common center of gravity.
Even a glance around the night sky will reveal many stars that seem to have a “companion” nearby, but more often than not, these stars are not physically connected by gravitation.
Many stars simply appear to lie roughly along the same line of sight as other, more distant stars.
Despite the bright moonlight this week, we can easily find one good example of such a stellar pair during the early evening hours.
First, locate the Big Dipper and look toward the middle star of its bent handle. If you look carefully, you might be able to see two stars there; if you have trouble, aim binoculars in their direction.
The main star at the bend of the Dipper’s handle is known as Mizar, and its fainter companion is Alcor; they’re also known informally as the “horse and rider.”
The 13th-century Persian writer Zakariyya’ al-Qazwini wrote that “people tested their eyesight by this star,” and I’ve heard it said that those who saw them both were inducted into the Persian army!
Though they appear close together in the sky, Alcor and Mizar are not connected by gravitation. 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 part of a binary system, and even a small telescope will show you its faint companion star right next to it. Its name is Zeta Ursa Majoris (aka UMaj).
