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Orion Arm

February 18, 2015

Orion twinkles in the south on February evenings. The constellation includes some of the brightest stars in the night sky, and some of the most magnificent stars in the entire Milky Way galaxy. They’re much bigger and heavier than the Sun, and tens of thousands of times brighter.

These stars belong to one of the galaxy’s spiral arms, which is named the Orion Arm for the bright constellation. In fact, our own solar system is in this same arm.

A spiral arm isn’t a permanent band of stars. Instead, it’s a wave that travels through the galaxy, with stars passing into and out of the arm as the wave moves.

Material in the wave is denser than the regions around it. That squeezes giant clouds of gas and dust, causing them to collapse and give birth to new stars. In fact, that’s happening right now in the Orion Nebula, a complex of thousands of young stars plus enough gas and dust to make many more. Many newborn stars are massive and bright, like those of Orion, so a spiral arm shines brighter than the rest of the galaxy’s disk.

The Orion Arm isn’t one of the Milky Way’s major arms, which wrap almost all the way around the galaxy’s center. Instead, it branches off one of the major arms, so it’s described as a minor arm or a spur. It’s not nearly as long or as wide as the major arms, and it contains far fewer stars. But many of the stars it does contain are young, bright, and vigorous, so they shine brightly — like the stars of Orion.


Script by Damond Benningfield, Copyright 2014

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