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| Space on May 20, 2014

Which spiral arm of the Milky Way contains our sun?

Many know our Milky Way is a spiral galaxy. But where within this vast spiral structure do our sun and Earth reside?

We live in an island of stars called the Milky Way, and many know that our Milky Way is a spiral galaxy. In fact, it’s a barred spiral galaxy, which means that our galaxy probably has just two major spiral arms, plus a central bar that astronomers are only now beginning to understand. But where within this vast spiral structure do our sun and its planets reside? Our galaxy is about 100,000 light-years wide. We’re about 25,000 light-years from the center of the galaxy. It turns out we’re not located in one of the Milky Way’s two primary spiral arms. Instead, we’re located in a minor arm of the galaxy. Our local spiral arm is sometimes Orion Arm, or sometimes the Orion Spur. It’s between the Sagittarius and Perseus Arms of the Milky Way. The image below shows it.

Our sun is located the Orion Arm, or Orion Spur, of the Milky Way galaxy. It’s a minor spiral arm, located between two other arms. Image updated in 2010 by R. Hurt on Wikimedia Commons.

Our local Orion Arm of the Milky Way is some 3,500 light years across. It’s approximately 10,000 light years in length. Our sun, the Earth, and all the other planets in our solar system reside within this Orion Arm. We’re located close to the inner rim of this spiral arm, about halfway along its length.

The Orion Arm, or Orion Spur, has other names as well. It’s sometimes simply called the Local Arm, or the Orion-Cygnus Arm, or the Local Spur. The Orion Arm is named for the constellation Orion the Hunter, which is one of the most prominent constellations of Northern Hemisphere winter (Southern Hemisphere summer). Some of the brightest stars and most famous celestial objects of this constellation (Betelgeuse, Rigel, the stars of Orion’s Belt, the Orion Nebula) are neighbors of sorts to our sun, located within the Orion Arm. That’s why we see so many bright objects within the constellation Orion – because when we look at it, we’re looking into our own local spiral arm.

Go here to see illustrations of Orion’s stars within the Orion Arm

Artist’s concept of what astronomers now believe is the overall structure of the spiral arms in our Milky Way galaxy. Image via NASA and Wikimedia Commons.

Botton line: The sun resides within a minor spiral arm of the galaxy, called the Orion Arm. This post explains more and has artist’s illustrations.

This is not the Milky Way. We can’t, of course, get outside the Milky Way to take its picture. This is another barred spiral galaxy, known as NGC 1300. See how it has just two major spiral arms? The Milky Way’s structure is apparently more complex, but, like this galaxy, it is thought to have just two major spiral arms. Photo via Hubble Space Telescope.