Orion_Arm - Wikiparency

# Orion Arm  **Microsim — three.js (Wikitube framework):** *Out to the nearest star (Solar System explorer)* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/solar/Solar_System.html?view=neighborhood&embed=1" data-title="Out to the nearest star (Solar System explorer)"></div> *The Solar System explorer locked on this article's state (`?view=neighborhood`); every object and population of the [[PORTAL_Solar_System|Solar System portal]] has its own state in the same scene.*  *Try: follow the ruler from the heliopause tick at 121 AU through the Oort cloud tick at about 2,000 AU to Proxima Centauri at 4.25 light-years, the farthest mark the explorer draws; set show to clouds and boundaries to see the Oort cloud points and the boundary shells alone; press l to hide the labels and drag the view edge-on, keeping in mind that the Orion Arm is some 800 times wider than this whole ruler.* The **Orion Arm**, also called the **Orion–Cygnus Arm**, the **Local Arm** or, in older work, the **Orion Spur**, is the spiral arm segment of the [[Milky_Way|Milky Way]] in which the [[Sun]] lies. It is about 3,500 light-years wide and roughly 20,000 light-years long, between the Sagittarius–Carina Arm on the inward side and the Perseus Arm on the outward side.[^xu2016][^reid2019] Once treated as a minor spur, it is now measured to have a pitch angle and a rate of star formation comparable to those of the Galaxy's major arms.[^xu2016] Much of the familiar night sky belongs to the arm: the bright stars and nebulae of Orion, the Pleiades and many other open clusters lie within it, and its gas is traced by radio masers in young, massive star-forming regions whose distances have been measured by parallax.[^reid2019][^menten2007] The Solar System sits near its inner edge, about 8,150 parsecs from the Galactic Centre, inside the low-density [[Local_Bubble|Local Bubble]].[^reid2019][^zucker2022] The explorer at the top of this page does not reach the arm's scale. Its neighbourhood view is a ruler ending at Proxima Centauri, 4.25 light-years out; the arm is roughly 800 times wider than that (derived) and is not drawn. ## Naming and brightness The arm, a band of stars and [[Interstellar_medium|interstellar gas]] in the disc of the [[Milky_Way|Milky Way]], takes its name from the constellation Orion, prominent in the evening sky of the northern winter and the southern summer. Looking toward Orion means looking along the arm, and many of the constellation's bright objects are among its members. The Orion Nebula, the nearest large region of massive star formation, lies about 414 parsecs (1,350 light-years) away, a distance measured by radio parallax of stars inside it.[^menten2007] The red supergiant Betelgeuse, at about 168 parsecs, is another of the arm's bright members.[^joyce2020] The other end of the arm lies toward Cygnus. There, sources in the arm stretch so far along the line of sight, and so nearly along the direction of the Sun's orbit, that they pile up on the sky and in radial velocity into the confusing complex called Cygnus X; parallax distances were needed to sort them out.[^xu2016] The two names, Orion and Cygnus, thus mark the two directions in which an observer on [[Earth]] looks lengthwise along the Sun's own arm. Because young, massive stars and bright nebulae are concentrated in spiral arms, the arm is also why these parts of the sky are so rich in bright objects. ## Location The Orion Arm lies between two of the [[Milky_Way|Milky Way's]] major arms: the Sagittarius–Carina Arm, whose nearby part lies toward the Galactic Centre, and the Perseus Arm, the main arm farther out.[^reid2019] The [[Sun]] is close to the inner rim of the Orion Arm and roughly halfway along its length.[^xu2016] Its distance from the Galactic Centre is about 8.15 kiloparsecs (26,600 light-years), and the circular speed of the Galaxy's rotation at the Sun's distance is about 236 km/s.[^reid2019] At that speed, one circuit of the Galaxy takes 2π × 8.15 kpc ÷ 236 km/s ≈ 212 million years (derived), the length of a [[Galactic_year|galactic year]]. Spiral arms are patterns, not fixed collections of stars. Stars and gas [[Orbit|orbit]] the centre under the Galaxy's [[Gravity|gravity]] and pass through the arms, and the Sun, moving at nearly the circular speed, drifts slowly relative to the arm it is in now. Its immediate surroundings, on the scale of a few hundred light-years, are the [[Local_Bubble|Local Bubble]] and, within that, the warm [[Local_Interstellar_Cloud|Local Interstellar Cloud]] through which it currently passes.[^zucker2022][^frisch2011] Whether the arm is a spur or a branch has been argued for decades. In 2013 Xu and colleagues found that its young star-forming regions do not look like a short spur and suggested that it is a branch of the Perseus Arm or an independent arm segment.[^xu2013][^nrao2013] ## Composition The arm is mapped by the objects that best mark spiral structure: regions where massive stars are forming. Such regions host methanol masers at 6.7 GHz and [[Water|water]] masers at 22 GHz, bright compact radio sources whose positions can be measured with very long baseline interferometry to within microarcseconds. Over a year, the [[Earth]]'s motion around the [[Sun]] makes each maser trace a small parallax ellipse, which gives its distance directly.[^reid2019] The Bar and Spiral Structure Legacy (BeSSeL) Survey with the Very Long Baseline Array and the Japanese VERA project together measured about 200 such parallaxes, many with uncertainties of 10 percent or less and some near 3 percent.[^reid2019][^hirota2020] For the Local Arm these measurements changed the picture. It emerges as an arm segment about 20,000 light-years long and about 3,000 light-years wide, wrapping less than a quarter of the way round the Galaxy, with pitch angles measured at about 10° to 12°, comparable to the major arms.[^xu2013][^xu2016][^reid2019] Its star formation rate is also similar to theirs, and Xu and colleagues identified a spur branching between it and the Sagittarius Arm.[^xu2016] Reid and Zheng describe the result as a four-armed spiral with the Local Arm as an extra, fifth feature.[^reid2020] ## Form Young gas and older stars need not trace the same arm. Miyachi and colleagues used the parallax distances of Gaia's second data release and the 2MASS infrared survey to select 4,654 stars about a billion years old, far older than the gas in star-forming regions, and mapped their density between galactic longitudes 90° and 270°.[^miyachi2019] They found a marginally significant arm-like overdensity of these stars near the gas-defined Local Arm, especially between longitudes 90° and 190°, with a slightly larger pitch angle than the gas and an offset from it.[^miyachi2019] The offset has a physical reading. In the density-wave picture, [[Hydrogen|hydrogen]] gas is compressed as it enters a long-lived spiral pattern and forms stars some time later, so the young stars and the older stellar arm should lie at slightly different places; a pattern that lasts longer than the star-formation timescale of 10 to 100 million years produces exactly such offsets.[^shen2020][^miyachi2019] The competing picture, in which arms are transient and continually re-formed, has more difficulty explaining an offset between stars and gas.[^miyachi2019] Together with the parallax results, this has moved the Local Arm from a minor spur toward being treated as a genuine, possibly long-lived, arm segment.[^shen2020] ## Messier objects Many of the objects catalogued by Charles Messier in the 18th century lie in the Orion Arm, since the nearest bright clusters and nebulae belong to the Sun's own arm. They include open clusters at a range of ages, emission and reflection nebulae where stars are forming now, and planetary nebulae, the shells of dying stars like the [[Sun]].[^seds] | Object | Name | Type | |---|---|---| | M42, M43 | Orion Nebula, De Mairan's Nebula | star-forming nebula | | M78 | — | reflection nebula in Orion | | M45 | Pleiades | young open cluster | | M44 | Beehive Cluster | open cluster | | M6, M7 | Butterfly Cluster, Ptolemy Cluster | open clusters in Scorpius | | M27, M57, M76, M97 | Dumbbell, Ring, Little Dumbbell and Owl nebulae | planetary nebulae | | M35, M41, M46, M47, M67 | — | open clusters | | M40 | Winnecke 4 | double star | Selected Messier objects in the Orion Arm, from the SEDS Messier catalogue.[^seds] Their distances show how local the arm's bright content is. The Pleiades, the most familiar of the young clusters, lie 136.2 parsecs (about 28 million [[Astronomical_unit|AU]], derived) away by radio parallax, a value that settled a long dispute with the Hipparcos satellite's shorter measurement.[^melis2014] The Orion Nebula, at about 414 parsecs, is some three times farther.[^menten2007] Both distances are tiny against the arm's length, a few percent of it (derived). ## Maps Every map of the arm is a reconstruction from inside it. The parallax map of Reid and colleagues fits logarithmic spirals to the maser positions and gives the arm's position, width and pitch angle with stated uncertainties.[^reid2019] Shen and Zheng combined those results with models of the Galactic bar into an updated scientifically constrained picture of the Milky Way seen from above.[^shen2020] Three-dimensional dust maps built from Gaia distances now show the clouds of the Local Arm, and of the [[Local_Bubble|Local Bubble]] within it, individually; Alves and colleagues found that many of the nearby star-forming clouds, once grouped into the Gould Belt, lie along a narrow, wave-like filament about 2.7 kiloparsecs long, the Radcliffe Wave.[^alves2020][^vergely2022] ### Interactive maps The same data sets feed three-dimensional, rotatable maps: the Gaia-based dust maps can be explored online, and the maser catalogues of BeSSeL and VERA are public, so that the arm's structure can be viewed from any angle rather than only from the Sun's position.[^hirota2020][^vergely2022] The explorer above works in the other direction, from the Sun outward to the nearest star, and stops long before the arm's scale. ## See also - [[Milky_Way]] - [[Local_Bubble]] · [[Local_Interstellar_Cloud]] - [[Galactic_year]] - [[Interstellar_medium]] - Gould Belt · Radcliffe Wave · Perseus Arm · List of Messier objects ## References [^xu2016]: Xu, Y.; Reid, M.; Dame, T.; Menten, K.; Sakai, N.; Li, J.; et al. (2016). "The local spiral structure of the Milky Way". *Science Advances* 2: e1600878. https://doi.org/10.1126/sciadv.1600878 [^reid2019]: Reid, M. J.; Menten, K. M.; Brunthaler, A.; et al. (2019). "Trigonometric parallaxes of high-mass star-forming regions: our view of the Milky Way". *The Astrophysical Journal* 885: 131. https://doi.org/10.3847/1538-4357/ab4a11 [^menten2007]: Menten, K. M.; Reid, M. J.; Forbrich, J.; Brunthaler, A. (2007). "The distance to the Orion Nebula". *Astronomy & Astrophysics* 474: 515–520. https://doi.org/10.1051/0004-6361:20078247 [^zucker2022]: Zucker, C.; Goodman, A. A.; Alves, J.; et al. (2022). "Star formation near the Sun is driven by expansion of the Local Bubble". *Nature* 601: 334–337. https://doi.org/10.1038/s41586-021-04286-5 [^joyce2020]: Joyce, M.; Leung, S.-C.; Molnár, L.; et al. (2020). "Standing on the shoulders of giants: new mass and distance estimates for Betelgeuse through combined evolutionary, asteroseismic, and hydrodynamic simulations with MESA". *The Astrophysical Journal* 902: 63. https://doi.org/10.3847/1538-4357/abb8db [^frisch2011]: Frisch, P. C.; Redfield, S.; Slavin, J. D. (2011). "The interstellar medium surrounding the Sun". *Annual Review of Astronomy and Astrophysics* 49: 237–279. https://doi.org/10.1146/annurev-astro-081710-102613 [^xu2013]: Xu, Y.; Li, J. J.; Reid, M. J.; et al. (2013). "On the nature of the Local spiral arm of the Milky Way". *The Astrophysical Journal* 769: 15. https://doi.org/10.1088/0004-637X/769/1/15 [^nrao2013]: Finley, D. (3 June 2013). "Earth's Milky Way neighborhood gets more respect". National Radio Astronomy Observatory press release (American Astronomical Society meeting, Indianapolis). [^hirota2020]: VERA Collaboration; Hirota, T.; Nagayama, T.; et al. (2020). "The first VERA astrometry catalog". *Publications of the Astronomical Society of Japan* 72: 50. https://doi.org/10.1093/pasj/psaa018 [^reid2020]: Reid, M. J.; Zheng, X.-W. (2020). "New view of the Milky Way". *Scientific American* 322 (4): 28–35. https://doi.org/10.1038/scientificamerican0420-28 [^miyachi2019]: Miyachi, Y.; Sakai, N.; Kawata, D.; et al. (2019). "Stellar overdensity in the Local Arm in Gaia DR2". *The Astrophysical Journal* 882: 48. https://doi.org/10.3847/1538-4357/ab2f86 [^shen2020]: Shen, J.; Zheng, X.-W. (2020). "The bar and spiral arms in the Milky Way: structure and kinematics". *Research in Astronomy and Astrophysics* 20: 159. https://doi.org/10.1088/1674-4527/20/10/159 [^seds]: Frommert, H.; Kronberg, C. "The Messier catalog". Students for the Exploration and Development of Space (SEDS). http://www.messier.seds.org/ [^melis2014]: Melis, C.; Reid, M. J.; Mioduszewski, A. J.; Stauffer, J. R.; Bower, G. C. (2014). "A VLBI resolution of the Pleiades distance controversy". *Science* 345: 1029–1032. https://doi.org/10.1126/science.1256101 [^alves2020]: Alves, J.; Zucker, C.; Goodman, A. A.; et al. (2020). "A Galactic-scale gas wave in the solar neighbourhood". *Nature* 578: 237–239. https://doi.org/10.1038/s41586-019-1874-z [^vergely2022]: Vergely, J. L.; Lallement, R.; Cox, N. L. J. (2022). "Three-dimensional extinction maps: inverting inter-calibrated extinction catalogues". *Astronomy & Astrophysics* 664: A174. https://doi.org/10.1051/0004-6361/202243319 ## External links - SEDS, Messier objects in the Milky Way: http://www.messier.seds.org/ - ESA, Gaia mission: https://www.esa.int/Science_Exploration/Space_Science/Gaia - NASA Science, "The Milky Way galaxy": https://science.nasa.gov/resource/the-milky-way-galaxy/ ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Orion_Arm) : [Wikitube](https://en.wikitube.io/wiki/Orion_Arm) · pinned revision [1357759388](https://en.wikipedia.org/w/index.php?oldid=1357759388) · 2026-09-18 ## Previous hub tags Hubs: `Life_Physics`. Portals: [[PORTAL_Solar_System]]. --- *Solar System portal child articles, wave 1 · 2026-09-18 · drafted · row SOL-029 · explorer state `?view=neighborhood`.*