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This article is about the galactic arm of the Milky Way Galaxy. For the stars which form the arm of the Orion asterism, see Orion (constellation). For other uses, see Arm of Orion (disambiguation).
Figure 1.The observed structure of the Milky Way's spiral arms [1]

The Orion Arm, also known as the Orion–Cygnus Arm, is a minor spiral arm within the Milky Way Galaxy spanning 3,500 light-years (1,100 parsecs) in width and extending roughly 10,000 light-years (3,100 parsecs) in length. [2] This galactic structure encompasses the Solar System, including Earth. It is sometimes referred to by alternate names such as the Local Arm or Orion Bridge, and it was previously identified as the Local Spur or the Orion Spur. It should not be confused with the outer terminus of the Norma Arm, known as the Cygnus Arm.

Naming and brightness

The arm is named after the Orion Constellation, one of the most prominent constellations of the Northern Hemisphere in winter (or the Southern Hemisphere in summer). Some of the brightest stars in the sky as well as other well-known celestial objects of the constellation (e.g. Betelgeuse, Rigel, the three stars of Orion's Belt, and the Orion Nebula) are found within it, as shown on Orion Arm's interactive map.

Location

The Orion arm is located between the Carina–Sagittarius Arm, the local portion of which projects toward the Galactic Center, and the Perseus Arm's local portion, which forms the main outer-most arm. It is one of the two major arms of the galaxy. [ citation needed]

Scientists once believed the Orion arm to be a minor structure, namely a "spur" between Carina-Sagittarius and Perseus, but evidence presented in 2013 suggests the Orion Arm to be a branch of the Perseus Arm or possibly an independent arm segment. [3]

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

Composition

Recently, the BeSSeL Survey ( Bar and Spiral Structure Legacy Survey) analyzed 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. Their measurement has accuracy above ±10% and even 3%.[ citation needed] The accurate locations of interstellar masers in HMSFRs (high-mass star-forming regions) suggests 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 a length of ~20,000 ly in length and ~3,000 ly in width, with a pitch angle of 10.1° ± 2.7° to 11.6° ± 1.8°. These results suggest 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. [4] [5] [6] [7] [8]

Form

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]

The researchers have concluded that the Local Arm segment is associated only with gas and star-forming clouds, showing a significant overdensity of stars. They have also found that the pitch angle of the stellar arm is slightly larger than the gas-defined arm, and there is an offset between the gas-defined and stellar arm. These differences in pitch angles and offsets between the stellar and HMSFR-defined spiral arms are consistent with the expectation that star formation lags behind gas compression in a spiral density wave that lasts longer than the typical star formation timescale of 107 − 108 years. [11]

Messier objects

The Orion Arm contains a number of Messier objects:

Maps

A rough artist's depiction of the Orion Arm within the Milky Way, with features marked.
Molecular clouds around the Sun inside the Orion-Cygnus Arm

Interactive maps

Rosette Nebula Crab Nebula Orion Nebula Trifid Nebula Lagoon Nebula Omega Nebula Eagle Nebula North America Nebula Rigel Orion's Belt Polaris Sun Betelgeuse Deneb Perseus Arm Orion Arm Sagittarius Arm
Orion and neighboring arms (clickable map)
Rosette Nebula Seagull Nebula Cone Nebula California Nebula Heart Nebula Orion Nebula Soul Nebula North America Nebula Cocoon Nebula Gamma Cygni Nebula Veil Nebula Trifid Nebula Crescent Nebula Lagoon Nebula Omega Nebula Eagle Nebula Cat's Paw Nebula Eta Carinae Nebula Crab Nebula Messier 37 Messier 36 Messier 38 Messier 50 Messier 46 Messier 67 Messier 34 Messier 48 Messier 41 Messier 47 Messier 44 Messier 45 Messier 39 Messier 52 Messier 93 Messier 7 Messier 6 Messier 25 Messier 23 Messier 21 Messier 18 Messier 26 Messier 11 Messier 35 NGC 2362 IC 2395 NGC 3114 NGC 3532 IC 1396 IC 2602 NGC 6087 NGC 6025 NGC 3766 IC 4665 IC 2581 IC 2944 NGC 4755 NGC 3293 NGC 6067 NGC 6193 NGC 6231 NGC 6383 Tr 14 Tr 16 Messier 103 Messier 29 HPer Chi Per Col 228 O Vel Perseus Arm Orion Arm Sagittarius Arm Star cluster Nebula
The nearest nebulae and star clusters (clickable map)

See also

References

  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.

External links

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