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| Local Arm (Milky Way) | |
|---|---|
| Name | Local Arm |
| Type | Spiral arm segment |
| Galaxy | Milky Way |
| Other names | Orion–Cygnus Arm, Orion Spur |
| Length | ~10,000–20,000 ly |
| Distance from center | ~8,000–10,000 pc (Solar radius ~8,122 pc) |
| Notable objects | Solar System, Orion Nebula, Taurus Molecular Cloud, Perseus OB2 |
Local Arm (Milky Way) is a minor spiral arm segment in the Milky Way that contains the Solar System and prominent star-forming complexes such as the Orion Nebula. It lies between the major Perseus Arm and Sagittarius Arm and is often referred to as the Orion–Cygnus Arm or Orion Spur. Studies combining radio, infrared, and optical observations by teams associated with facilities like the Very Long Baseline Array, Spitzer Space Telescope, and Gaia have refined its extent, structure, and kinematics.
The Local Arm is a nearby arm segment hosting the Solar System, the Taurus Molecular Cloud, the Orion Molecular Cloud Complex, and stellar associations like Scorpius–Centaurus OB association. Surveys by projects such as the BeSSeL Survey, the VLBI Exploration of Radio Astrometry (VERA), and missions including Hipparcos and Gaia have mapped parallax distances and proper motions to establish the arm's position relative to the Galactic Center, the Galactic bar, and nearby spiral features such as the Perseus Arm and Sagittarius Arm. Historically, mapping efforts built on observations from institutions like the Royal Observatory Greenwich, the Copenhagen Observatory, and the Mount Wilson Observatory.
The Local Arm spans on the order of 10,000–20,000 light-years and has width and pitch angle estimates constrained by measurements from the Very Long Baseline Array, European VLBI Network, and the Atacama Large Millimeter/submillimeter Array. Its morphology appears as a spur or branch connecting to larger features, with proposed links to the Perseus Arm or the inner Sagittarius Arm noted in analyses by teams at institutions like the Max Planck Institute for Radio Astronomy and universities including Harvard University and the University of Tokyo. Models from groups using data from the Sloan Digital Sky Survey and the Two Micron All Sky Survey help estimate scale height, surface density, and mass distribution.
The Local Arm contains young OB associations such as Perseus OB2 and open clusters including Pleiades-associated groups, embedded clusters in the Orion Nebula, and older stellar populations traced by catalogues from Hipparcos and Gaia. Molecular gas reservoirs include the Taurus Molecular Cloud, the Orion Molecular Cloud Complex, and the Lupus cloud complex, studied with instruments like the James Clerk Maxwell Telescope and the IRAM 30m Telescope. Atomic hydrogen mapping by surveys with the Arecibo Observatory and the Green Bank Telescope traces HI structure, while carbon monoxide surveys from teams at the NRAO and the Five College Radio Astronomy Observatory characterize cold molecular mass.
Active star formation in the Local Arm is visible in regions such as the Orion Nebula, the Horsehead Nebula, and the Barnard 68 dark cloud, associated with H II regions identified in radio recombination line studies by groups using the Very Large Array and the Effelsberg Radio Telescope. Young stellar objects and protostellar outflows have been catalogued via observations from the Spitzer Space Telescope, the Hubble Space Telescope, and the Chandra X-ray Observatory. The influence of massive stars from associations like Scorpius–Centaurus OB association and feedback processes observed by teams at institutions including the European Southern Observatory drive local turbulence and trigger sequential star formation.
Proper motion and parallax measurements from Gaia and very long baseline interferometry projects such as BeSSeL Survey and VERA provide constraints on the Local Arm's rotation curve, streaming motions, and peculiar velocities relative to the Local Standard of Rest. Dynamical analyses by researchers at the Max Planck Institute for Astronomy and the Princeton University Observatory explore how perturbations from the Galactic bar and spiral density waves, as described in models by scientists associated with Cambridge University and Columbia University, influence arm morphology and stellar orbits. Studies referencing the Oort constants and resonance locations help interpret radial migration and stellar mixing.
The Local Arm is often characterized as a spur, bridge, or branch linking the Sagittarius Arm and the Perseus Arm, with competing interpretations from mapping efforts led by groups at the National Astronomical Observatory of Japan and the Harvard-Smithsonian Center for Astrophysics. Comparisons with spur-like features in external galaxies observed with the Hubble Space Telescope and the Spitzer Space Telescope inform whether the Local Arm is a long-lived structure or a transient feature induced by interactions involving the Galactic bar or satellite galaxies such as the Sagittarius Dwarf Spheroidal Galaxy and the Large Magellanic Cloud.
The arm's recognition grew from early radio mapping of HI by teams at the Morrison Observatory and the Dwingeloo Radio Telescope to modern parallax campaigns with the Very Long Baseline Array, VERA, and the European VLBI Network. Optical astrometry advanced with Hipparcos and revolutionized with Gaia, while infrared surveys like 2MASS and WISE revealed embedded populations. Molecular line surveys using the CO transition by groups at facilities including the IRAM 30m and the Nobeyama Radio Observatory provided mass estimates. Computational modeling by researchers at the Kavli Institute for Theoretical Physics and the Harvard & Smithsonian synthesizes observational constraints into coherent maps of local spiral structure.