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Longer Arm

December 1, 2021

The bright star Deneb stands high in the west-northwest at nightfall. It represents both the tail of Cygnus, the swan, and the top of the Northern Cross. And if you have a dark sky, you can see that it’s immersed in the glow of the Milky Way — the light of millions of stars that outline the disk of our home galaxy.

Many of those stars are in the same feature of the galaxy as the solar system — the Local Arm. It’s one of several spiral arms that make the Milky Way look like a pinwheel spinning through space.

For a long time, it wasn’t considered an “arm” at all. Instead, it was classified as a “spur” — a short spike sticking off one of the major spiral arms. But over the last couple of years, several studies have found that it’s a lot longer than thought.

One study plotted the locations of hundreds of thousands of stars, plus hundreds of star clusters. The positions came from Gaia, a space telescope that’s mapping more than a billion stars. Another study plotted the positions of “masers” — bright beams of microwaves energized by hot stars.

Both studies found that the Local Arm extends for at least 25,000 light-years. That’s not nearly long enough to loop all the way around the galaxy. And it may not “loop” at all — it may extend straight across the galaxy. Yet it’s clear that the Local Arm is bigger than expected — an impressive neighborhood for the Sun, Deneb, and most of the stars we see in the night sky.

Script by Damond Benningfield

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