Orion Arm

• 1. Introduction

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  The Orion Arm is the spiral arm of the Milky Way that contains Earth, the Solar System, and some of the most recognizable objects in the night sky. It spans roughly 3,500 light-years in width and extends about 20,000 light-years in length. For most of modern astronomy's history, scientists assumed this arm was something modest and secondary. Evidence that emerged in 2013, however, forced a reconsideration of that assumption. What exactly is the Orion Arm, and where does it sit within the larger architecture of our galaxy? How did astronomers measure its shape from the inside, when we can never step outside to look down at it? And why did a survey of radio signals from distant clouds of gas change the picture so completely?

• 2. Named for the Hunter

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  The arm takes its name from the constellation Orion, one of the most prominent constellations visible from the Northern Hemisphere in winter and from the Southern Hemisphere in summer. Several of the brightest stars in the entire sky belong to objects found within this arm, among them Betelgeuse and Rigel, along with the three stars of Orion's Belt and the Orion Nebula itself. The arm is also known as the Orion-Cygnus Arm, or sometimes the Local Arm. Earlier designations called it the Local Spur or the Orion Spur, names that reflect the older, diminished view of its status. It should not be confused with the Cygnus Arm, which is the outer terminus of the separate Norma Arm.

• 3. Sandwiched Between Giants

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  The Orion Arm sits between two much larger structures. On one side lies the Carina-Sagittarius Arm, whose local portion reaches toward the Galactic Center. On the other side is the Perseus Arm, which forms the outermost major arm in our part of the Milky Way. The Solar System sits close to the Orion Arm's inner rim, roughly halfway along its length. At that position, the Sun and its planets reside inside a relative cavity in the surrounding interstellar medium, a region called the Local Bubble. The Solar System is approximately 8,000 parsecs from the Galactic Center. For a long time, astronomers placed the arm structurally between those two larger neighbors as a simple spur, a minor connecting bridge rather than a feature with any independent character.

• 4. The BeSSeL Survey's Revelation

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  A project called the BeSSeL Survey, short for Bar and Spiral Structure Legacy Survey, changed the standing picture of the Orion Arm. The survey analyzed the parallax and proper motion of more than 30 masers, radio-emitting regions tuned to methanol at 6.7 GHz and water at 22 GHz, all located in high-mass star-forming regions within a few kiloparsecs of the Sun. The measurements achieved accuracy as fine as 3% in some cases, and no worse than plus or minus 10%. By pinpointing the precise locations of these masers, the survey found that the Local Arm wraps around less than a quarter of the Milky Way. Its pitch angle, the angle at which it curls away from a perfect circle, falls between 10.1 degrees plus or minus 2.7 degrees and 11.6 degrees plus or minus 1.8 degrees. Both that pitch angle and the arm's star formation rate turned out to be comparable to those of the galaxy's major spiral arms. The BeSSeL findings led researchers to describe the Local Arm as reasonably the fifth feature of the Milky Way, a designation that would have seemed implausible under the old spur model. The survey also raised the possibility that the Orion Arm is either a branch of the Perseus Arm or an entirely independent arm segment, rather than simply a bridge between Perseus and Carina-Sagittarius.

• 5. Stars Versus Gas

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  To probe the arm's structure through a different lens, researchers mapped the stellar density of stars roughly one billion years old, using data from the Gaia DR2 mission. The one-billion-year age group was chosen deliberately: stars of that vintage are more evolved than the gas clouds in high-mass star-forming regions, so they trace a different population from the masers in the BeSSeL Survey. The mapping covered a galactic longitude range from 90 to 270 degrees. Researchers found a marginally significant arm-like overdensity of stars close to the arm traced by gas and star-forming clouds, with the clearest signal appearing in the longitude range of 90 to 190 degrees. The pitch angle of the stellar arm turned out to be slightly larger than the pitch angle defined by the gas. There is also a measurable offset between where the gas-defined arm sits and where the stellar arm sits. These differences are consistent with a known physical expectation: star formation lags behind gas compression in a spiral density wave that lasts longer than the typical star formation timescale of between 10 million and 100 million years.

• 6. A Catalog of Objects Within

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  The Orion Arm is home to a large collection of Messier objects, the catalog of nebulae and star clusters compiled in the eighteenth century as a guide for astronomers who needed to distinguish fuzzy permanent objects from passing comets. Among the clusters lying within the arm are the Butterfly Cluster (M6), the Ptolemy Cluster (M7), the Beehive Cluster (M44), and the Pleiades (M45). Nebulae within the arm include the Orion Nebula itself (M42), the Dumbbell Nebula (M27), the Ring Nebula (M57), the Owl Nebula (M97), the Little Dumbbell Nebula (M76), and the De Mairan's Nebula (M43). The presence of so many of these objects within a single arm reflects both the arm's extent and its active star-forming character, a trait that the BeSSeL measurements showed is on par with the galaxy's most prominent spiral structures.