The Milky Way is a vast collection of stars, gas, and dust. Understanding where our Solar System fits into this complicated structure requires grasping the immense scale and shape of the entire system.
Understanding the Barred Spiral Structure
The Milky Way is classified as a barred spiral galaxy, distinguished by a central, elongated bar-shaped structure composed primarily of older stars. This bar extends outward from the dense, spherical central bulge, which contains the supermassive black hole, Sagittarius A. Surrounding this core is the galactic disk, a flattened region where most gas, dust, and younger stars reside, and where the spiral arms originate.
Astronomers believe the Milky Way possesses two main spiral arms: the Scutum-Centaurus Arm and the Perseus Arm, which connect to the ends of the central bar. These major arms contain the highest densities of stars and trace the largest spirals throughout the disk. Other features, such as the Norma and Sagittarius Arms, are often reclassified as minor arms residing between the larger structures. These spiral features are dynamic regions of higher density that move through the disk, triggering star formation as they sweep up material.
The Local Arm: Our Immediate Location
The Solar System is not situated within a major spiral arm. Instead, we reside in a smaller feature known as the Orion Arm, also called the Orion Spur or Local Arm. This feature is best described as a minor arm or a partial bridge lying between the two main arms. The Orion Arm is located between the major Perseus Arm, which is further out from the galactic center, and the Sagittarius Arm, which is closer to the center.
The Orion Arm is estimated to be approximately 3,500 light-years wide and 20,000 light-years long. It was once considered merely a temporary side branch, or “spur,” but recent evidence suggests it may be a more significant, independent segment. Our Solar System is positioned close to the inner rim of this local feature, embedded within a low-density region known as the Local Bubble. Mapping this region is challenging because dust and gas within the galactic plane obscure the view of the surrounding spiral structure.
Navigating the Galactic Neighborhood
Our Sun and its planets orbit the center of the Milky Way galaxy at a distance of approximately 27,000 light-years. This position places us roughly halfway out from the galactic core to the disk’s edge. The Solar System is moving at an estimated speed of about 515,000 miles per hour (828,000 kilometers per hour) as it traces its path around the center.
At this velocity, the Solar System takes an immense amount of time to complete a single orbit, a period known as a “Galactic Year.” One complete revolution takes approximately 225 to 230 million Earth years. Since the Earth is about 4.54 billion years old, it has completed roughly 20 full orbits. We also oscillate slightly above and below the galactic plane as we orbit.