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North Poles

StarDate: 
May 9, 2015

The Big Dipper stands high overhead early this evening, with the bowl upside down. As you take in the view, link the two stars at the outer edge of the bowl. If you follow that line down toward the horizon, the first moderately bright star you come to is Polaris. Earth’s north pole aims directly at the star, so it’s also known as the North Star or the Pole Star — it marks true north in the sky.

Another north pole stands much higher in the sky at that hour. It’s in Coma Berenices, a faint constellation that’s well above Arcturus, a bright yellow-orange star that’s high in the east.

That spot marks the north galactic pole — the projection of the north pole of the Milky Way galaxy.

The galaxy is shaped like a disk. It’s about a hundred thousand light-years wide, and about ten thousand light-years thick. If you draw a line through the center of the galaxy, at a right angle to the disk itself, and project it above the disk, it aims toward Coma Berenices.

So when we look in that direction, we’re looking through a thin section of the Milky Way, so there aren’t all that many brilliant stars in that part of the sky. Instead, we’re looking into intergalactic space. And in fact, a telescope reveals many other galaxies in that direction. More than a thousand of them form a giant cluster, known as the Coma Cluster — a beautiful collection of galaxies that stands due north.

We’ll have more about the Big Dipper tomorrow.

 

Script by Damond Benningfield, Copyright 2015

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