Observed structure of the Milky Way's spiral arms[1]

The Orion Arm is a minor spiral arm of the Milky Way Galaxy that is 3,500 light-years (1,100 parsecs) across and approximately 10,000 light-years (3,100 parsecs) in length,[2] containing the Solar System, including Earth. It is also referred to by its full name, the Orion–Cygnus Arm (not to be confused with the outer end of the Norma Arm the Cygnus Arm), as well as Local Arm, Orion Bridge, and formerly, the Local Spur and Orion Spur.

Milky Way centre from Sinai, Egypt

The arm is named for the Orion Constellation, 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 the constellation (e.g. Betelgeuse, Rigel, the three stars of Orion's Belt, the Orion Nebula) are within it as shown on the interactive map below.

The arm is between the Carina–Sagittarius Arm (the local portions of which are toward the Galactic Center) and the Perseus Arm (the local portion of which is the main outer-most arm and one of two major arms of the galaxy).

Long thought to be a minor structure, namely a "spur" between the two arms mentioned, evidence was presented in mid 2013 that the Orion Arm might be a branch of the Perseus Arm, or possibly an independent arm segment.[3]

Within the arm, the Solar System is close to its inner rim, in a relative cavity in the arm's interstellar medium known as the Local Bubble, about halfway along the arm's length, approximately 8,000 parsecs (26,000 light-years) from the Galactic Center.

Recently, the parallax and proper motion of more than 30 methanol (6.7-GHz) and water (22-GHz) masers in high-mass star-forming regions within a few kiloparsecs of the Sun were measured. Measurement accuracy was better than ±10% and even 3%, the best parallax measurement in the BeSSeL project (Bar and Spiral Structure Legacy Survey). The accuracy locations of interstellar masers in HMSFRs (high-mass star-forming regions) have been shown that the Local Arm appears to be an orphan segment of an arm between the Sagittarius and Perseus arms that wraps around less than a quarter of the Milky Way. The segment has the length of ~20,000 ly and the width of ~3,000 ly with the pitch angle from 10.1° ± 2.7° to 11.6° ± 1.8°. These results reveal that the Local Arm is larger than previously thought, and both its pitch angle and star formation rate are comparable to those of the Galaxy’s major spiral arms. The Local Arm is reasonably referred to as the fifth feature in the Milky Way. The “spur” interpretation may be incorrect.[4][5][6][7][8]

To understand the form of the Local Arm between the Sagittarius and Perseus arms, the stellar density of a specific population of stars with about 1 Gyr of age between 90° ≤ l ≤ 270° have been mapped using the Gaia DR2.[9] The 1 Gyr population have been employed because they are significantly more-evolved objects than the gas in HMSFRs tracing the Local Arm. Investigations have been carried out to compare both the stellar density and gas distribution along the Local Arm. Researchers have found a marginally significant arm-like stellar overdensity close to the Local Arm, identified with the HMSFRs especially in the region of 90° ≤ l ≤ 190°.[10] They have concluded that the Local Arm is the arm segment associated with only the gas and star-forming clouds, with a significant stellar overdensity. Additionally, they have found that the pitch angle of the stellar arm is slightly larger than the gas-defined arm, and also there is an offset between gas-defined and stellar arm. The offset and different pitch angles between the stellar and HMSFR-defined spiral arms are consistent with the expectation that star formation lags the gas compression in a spiral density wave lasting longer than the typical star formation timescale of 107 − 108 years.[11]

A depiction of the view from inside the Orion Arm into the Milky Way.

Messier objects

The shape of the Orion Arm[12]

The Orion Arm contains a number of Messier objects:

Interactive maps

Rosette NebulaCrab NebulaOrion NebulaTrifid NebulaLagoon NebulaOmega NebulaEagle NebulaNorth America NebulaRigelOrion's BeltPolarisSunBetelgeuseDenebPerseus ArmOrion ArmSagittarius Arm
Orion and neighboring arms (clickable map)
Rosette NebulaSeagull NebulaCone NebulaCalifornia NebulaHeart NebulaOrion NebulaSoul NebulaNorth America NebulaCocoon NebulaGamma Cygni NebulaVeil NebulaTrifid NebulaCrescent NebulaLagoon NebulaOmega NebulaEagle NebulaCat's Paw NebulaEta Carinae NebulaCrab NebulaMessier 37Messier 36Messier 38Messier 50Messier 46Messier 67Messier 34Messier 48Messier 41Messier 47Messier 44Messier 45Messier 39Messier 52Messier 93Messier 7Messier 6Messier 25Messier 23Messier 21Messier 18Messier 26Messier 11Messier 35NGC 2362IC 2395NGC 3114NGC 3532IC 1396IC 2602NGC 6087NGC 6025NGC 3766IC 4665IC 2581IC 2944NGC 4755NGC 3293NGC 6067NGC 6193NGC 6231NGC 6383Tr 14Tr 16Messier 103Messier 29HPerChi PerCol 228O VelPerseus ArmOrion ArmSagittarius ArmStar clusterNebula
The nearest nebulae and star clusters (clickable map)

Other maps

Close up on the Orion Arm, with major stellar associations (yellow), nebulae (red) and dark nebulae (grey) around the Local Bubble.

See also


  1. ^ See the "Spiral Arms" part of this NASA animation for details
  2. ^ Harold Spencer Jones, T. H. Huxley, Proceedings of the Royal Institution of Great Britain, Royal Institution of Great Britain, v. 38–39
  3. ^ Earth's Milky Way Neighborhood Gets More Respect, National Radio Astronomy Observatory, Dave Finley, 3 June 2013
  4. ^ Reid, Mark; Zheng, Xing-Wu (2020). A New Map of the Milky Way. Vol. April. Scientific American.
  5. ^ Xu, Y.; Reid, M.; Dame, T.; Menten, K.; Sakai, N.; Li, J.; Brunthaler, A.; Moscadelli, L.; Zhang, B.; Zheng, X. (2016). "The local spiral structure of the Milky Way". Science Advances. 2 (9): e1600878. arXiv:1610.00242. Bibcode:2016SciA....2E0878X. doi:10.1126/sciadv.1600878. PMC 5040477. PMID 27704048.
  6. ^ Xu, Y.; Li, J. J.; Reid, M. J.; Menten, K. M.; Zheng, X. W.; Brunthaler, A.; Moscadelli, L.; Dame, T. M.; Zhang, B. (2013). "On the Nature of the Local Spiral Arm of the Milky Way". The Astrophysical Journal. 769 (1): 15. arXiv:1304.0526. Bibcode:2013ApJ...769...15X. doi:10.1088/0004-637X/769/1/15. S2CID 119278638.
  7. ^ Reid, M. J.; Menten, K. M.; Brunthaler, A.; Zheng, X. W.; Dame, T. M.; Xu, Y.; Li, J.; Sakai, N.; Wu, Y.; Immer, K.; Zhang, B.; Sanna, A.; Moscadelli, L.; Rygl, K. L. J.; Bartkiewicz, A.; Hu, B.; Quiroga-Nuñez, L. H.; Van Langevelde, H. J. (2019). "Trigonometric Parallaxes of High-mass Star-forming Regions: Our View of the Milky Way". The Astrophysical Journal. 885 (2): 131. arXiv:1910.03357. Bibcode:2019ApJ...885..131R. doi:10.3847/1538-4357/ab4a11. S2CID 203904869.
  8. ^ Hirota, Tomoya; et al. (2020). "The First VERA Astrometry Catalog". Publications of the Astronomical Society of Japan. 72 (4). arXiv:2002.03089. doi:10.1093/pasj/psaa018.
  9. ^ Miyachi, Yusuke; Sakai, Nobuyuki; Kawata, Daisuke; Baba, Junichi; Honma, Mareki; Matsunaga, Noriyuki; Fujisawa, Kenta (2019). "Stellar Overdensity in the Local Arm in Gaia DR2". The Astrophysical Journal. 882 (1): 48. arXiv:1907.03763. Bibcode:2019ApJ...882...48M. doi:10.3847/1538-4357/ab2f86. S2CID 195847953.
  10. ^ Hirota, Tomoya; et al. (2020). "The First VERA Astrometry Catalog". Publications of the Astronomical Society of Japan. 72 (4). arXiv:2002.03089. doi:10.1093/pasj/psaa018.
  11. ^ Shen, Juntai; Zheng, Xing-Wu (2020). "The bar and spiral arms in the Milky Way: Structure and kinematics". Research in Astronomy and Astrophysics. 20 (10): 159. arXiv:2012.10130. Bibcode:2020RAA....20..159S. doi:10.1088/1674-4527/20/10/159. S2CID 229005996.
  12. ^ Vázquez, Ruben A.; May, Jorge; Carraro, Giovanni; Bronfman, Leonardo; Moitinho, André; Baume, Gustavo (January 2008). "Spiral Structure in the Outer Galactic Disk. I. The Third Galactic Quadrant". The Astrophysical Journal. 672 (2): 930–939. arXiv:0709.3973. Bibcode:2008ApJ...672..930V. doi:10.1086/524003. S2CID 55908842.