How often does the Sun pass through the spiral arm it was born in?

Our Milky Way looks like a giant pinwheel in space, but the arms that make up this spiral are not solid objects that rotate around the hub of the Milky Way. Instead, think of the 'wave' you might be a part of at your local baseball stadium. Every few million years as one of these arms advances in the Milky Way it involves a different collection of clouds, gas and stars in the disk of the Milky Way. There are about two main arms that cross the solar circle, and since the Sun takes about 240 million years to orbit the center, this means that every 110 million years or so, the Sun enters a new arm system. But a lot has changed in the Milky Way in the 26 Galactic Years that have come and gone since the Sun was first born. We may never find ourselves in exactly the same neighborhood of our Sun’s youth.

Everyone has moved away.

This answer was updated in 2011. See my books: The Astronomy Cafe (1998) and Back to the Astronomy Cafe (2003) for more FAQs in printed form. Author: Dr. Sten Odenwald, Copyright 2011

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