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 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.
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.
Deborah Byrd created the EarthSky radio series in 1991 and EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.