Orion Arm

{{Short description|Minor spiral arm of the Milky Way galaxy; contains the Solar System}}

{{About|the galactic arm of the Milky Way Galaxy|the stars which form the arm of the Orion asterism|Orion (constellation)|other uses|Arm of Orion (disambiguation)}}

Image:Milky Way Arms ssc2008-10.svg's spiral arms[https://web.archive.org/web/20120326160806/http://planetquest.jpl.nasa.gov/system/interactable/7/index.html See the "Spiral Arms" part of this NASA animation for details].]]

The Orion Arm, also known as the Orion–Cygnus Arm, is a minor spiral arm within the Milky Way Galaxy spanning {{Convert|3,500|ly|pc|abbr=off|lk=on}} in width and extending roughly {{Convert|20,000|ly|pc|abbr=off}} in length.{{cite journal |last1=Xu |first1=Ye |last2=Reid |first2=Mark |last3=Dame |first3=Thomas |last4=Menten |first4=Karl |last5=Sakai |first5=Nobuyuki |last6=Li |first6=Jingjing |last7=Brunthaler |first7=Andreas |last8=Moscadelli |first8=Luca |last9=Zhang |first9=Bo |last10=Zheng |first10=Xingwu |title=The local spiral structure of the Milky Way |journal=Science Advances |date=28 September 2016 |volume=2 |issue=9 |pages=e1600878 |doi=10.1126/sciadv.1600878 |pmid=27704048 |pmc=5040477 |arxiv=1610.00242 |bibcode=2016SciA....2E0878X }} 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.

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.Dave Finley, [http://www.nrao.edu/pr/2013/localarm/ Earth's Milky Way Neighborhood Gets More Respect], National Radio Astronomy Observatory, 3 June 2013.

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 {{Convert|8,000|pc|ly|abbr=off}} 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|date=November 2023}} 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.{{cite journal | bibcode=2016SciA....2E0878X | title=The local spiral structure of the Milky Way | last1=Xu | first1=Y. | last2=Reid | first2=M. | last3=Dame | first3=T. | last4=Menten | first4=K. | last5=Sakai | first5=N. | last6=Li | first6=J. | last7=Brunthaler | first7=A. | last8=Moscadelli | first8=L. | last9=Zhang | first9=B. | last10=Zheng | first10=X. | journal=Science Advances | year=2016 | volume=2 | issue=9 | pages=e1600878 | doi=10.1126/sciadv.1600878 | pmid=27704048 | pmc=5040477 | arxiv=1610.00242 }}{{cite journal |last1=Xu |first1=Y. |last2=Li |first2=J. J. |last3=Reid |first3=M. J. |last4=Menten |first4=K. M. |last5=Zheng |first5=X. W. |last6=Brunthaler |first6=A. |last7=Moscadelli |first7=L. |last8=Dame |first8=T. M. |last9=Zhang |first9=B. |title=On the Nature of the Local Spiral Arm of the Milky Way |journal=The Astrophysical Journal |date=30 April 2013 |volume=769 |issue=1 |pages=15 |doi=10.1088/0004-637X/769/1/15 |arxiv=1304.0526 |bibcode=2013ApJ...769...15X }}{{cite journal | bibcode=2019ApJ...885..131R | title=Trigonometric Parallaxes of High-mass Star-forming Regions: Our View of the Milky Way | last1=Reid | first1=M. J. | last2=Menten | first2=K. M. | last3=Brunthaler | first3=A. | last4=Zheng | first4=X. W. | last5=Dame | first5=T. M. | last6=Xu | first6=Y. | last7=Li | first7=J. | last8=Sakai | first8=N. | last9=Wu | first9=Y. | last10=Immer | first10=K. | last11=Zhang | first11=B. | last12=Sanna | first12=A. | last13=Moscadelli | first13=L. | last14=Rygl | first14=K. L. J. | last15=Bartkiewicz | first15=A. | last16=Hu | first16=B. | last17=Quiroga-Nuñez | first17=L. H. | last18=Van Langevelde | first18=H. J. | journal=The Astrophysical Journal | year=2019 | volume=885 | issue=2 | page=131 | doi=10.3847/1538-4357/ab4a11 | arxiv=1910.03357 | s2cid=203904869 | doi-access=free }}{{cite journal |display-authors=3 |last1=Hirota |first1=Tomoya |last2=Nagayama |first2=Takumi |last3=Honma |first3=Mareki |last4=Adachi |first4=Yuuki |last5=Burns |first5=Ross A |last6=Chibueze |first6=James O |last7=Choi |first7=Yoon Kyung |last8=Hachisuka |first8=Kazuya |last9=Hada |first9=Kazuhiro |last10=Hagiwara |first10=Yoshiaki |last11=Hamada |first11=Shota |last12=Handa |first12=Toshihiro |last13=Hashimoto |first13=Mao |last14=Hirano |first14=Ken |last15=Hirata |first15=Yushi |last16=Ichikawa |first16=Takanori |last17=Imai |first17=Hiroshi |last18=Inenaga |first18=Daichi |last19=Ishikawa |first19=Toshio |last20=Jike |first20=Takaaki |last21=Kameya |first21=Osamu |last22=Kaseda |first22=Daichi |last23=Kim |first23=Jeong Sook |last24=Kim |first24=Jungha |last25=Kim |first25=Mi Kyoung |last26=Kobayashi |first26=Hideyuki |last27=Kono |first27=Yusuke |last28=Kurayama |first28=Tomoharu |last29=Matsuno |first29=Masako |last30=Morita |first30=Atsushi |last31=Motogi |first31=Kazuhito |last32=Murase |first32=Takeru |last33=Nakagawa |first33=Akiharu |last34=Nakanishi |first34=Hiroyuki |last35=Niinuma |first35=Kotaro |last36=Nishi |first36=Junya |last37=Oh |first37=Chung Sik |last38=Omodaka |first38=Toshihiro |last39=Oyadomari |first39=Miyako |last40=Oyama |first40=Tomoaki |last41=Sakai |first41=Daisuke |last42=Sakai |first42=Nobuyuki |last43=Sawada-Satoh |first43=Satoko |last44=Shibata |first44=Katsunori M |last45=Shizugami |first45=Makoto |last46=Sudo |first46=Jumpei |last47=Sugiyama |first47=Koichiro |last48=Sunada |first48=Kazuyoshi |last49=Suzuki |first49=Syunsaku |last50=Takahashi |first50=Ken |last51=Tamura |first51=Yoshiaki |last52=Tazaki |first52=Fumie |last53=Ueno |first53=Yuji |last54=Uno |first54=Yuri |last55=Urago |first55=Riku |last56=Wada |first56=Koji |last57=Wu |first57=Yuan Wei |last58=Yamashita |first58=Kazuyoshi |last59=Yamashita |first59=Yuto |last60=Yamauchi |first60=Aya |last61=Yuda |first61=Akito |title=The First VERA Astrometry Catalog |journal=Publications of the Astronomical Society of Japan |date=August 2020 |volume=72 |issue=4 |doi=10.1093/pasj/psaa018 |arxiv=2002.03089 }}

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.{{cite journal | bibcode=2019ApJ...882...48M | title=Stellar Overdensity in the Local Arm in Gaia DR2 | last1=Miyachi | first1=Yusuke | last2=Sakai | first2=Nobuyuki | last3=Kawata | first3=Daisuke | last4=Baba | first4=Junichi | last5=Honma | first5=Mareki | last6=Matsunaga | first6=Noriyuki | last7=Fujisawa | first7=Kenta | journal=The Astrophysical Journal | year=2019 | volume=882 | issue=1 | page=48 | doi=10.3847/1538-4357/ab2f86 | arxiv=1907.03763 | s2cid=195847953 | doi-access=free }} 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°.

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.{{cite journal |last1=Shen |first1=Juntai |last2=Zheng |first2=Xing-Wu |title=The bar and spiral arms in the Milky Way: structure and kinematics |journal=Research in Astronomy and Astrophysics |date=October 2020 |volume=20 |issue=10 |pages=159 |doi=10.1088/1674-4527/20/10/159 |arxiv=2012.10130 |bibcode=2020RAA....20..159S }}

Messier objects

The Orion Arm contains a number of Messier objects:

Maps

File:OrionSpur.png

File:Galaxymap.com, map 1000 parsecs (2022).pngs around the Sun inside the Orion-Cygnus Arm]]

=Interactive maps=

Image:Orion Arm.JPG|frame|center|Orion and neighboring arms (clickable map)

rect 126 149 188 182 Rosette Nebula

rect 285 116 327 145 Crab Nebula

rect 243 245 284 274 Orion Nebula

rect 299 288 345 312 Trifid Nebula

rect 343 304 384 333 Lagoon Nebula

rect 393 322 434 353 Omega Nebula

rect 445 322 494 353 Eagle Nebula

rect 424 244 483 280 North America Nebula

rect 293 248 319 266 Rigel

rect 225 179 299 246 Orion's Belt

rect 331 211 368 234 Polaris

rect 318 236 353 259 Sun

poly 302 176 303 241 315 242 361 177 Betelgeuse

rect 419 222 458 245 Deneb

poly 0 123 508 118 637 160 637 217 470 163 0 178 Perseus Arm

poly 2 202 460 201 633 261 637 326 408 260 1 258 Orion Arm

poly 1 284 397 293 633 360 637 477 541 475 357 413 0 400 Sagittarius Arm

desc bottom-left

Image:Nearest Nebulae and Star clusters.gif|thumb|800px|center|The nearest nebulae and star clusters (clickable map)

rect 396 142 447 173 Rosette Nebula

rect 376 230 426 258 Seagull Nebula

rect 463 264 501 292 Cone Nebula

rect 528 284 576 322 California Nebula

rect 695 117 741 149 Heart Nebula

rect 461 301 494 339 Orion Nebula

rect 691 154 739 182 Soul Nebula

rect 568 371 625 405 North America Nebula

rect 643 366 687 402 Cocoon Nebula

rect 688 392 761 429 Gamma Cygni Nebula

rect 594 404 625 444 Veil Nebula

rect 513 541 550 578 Trifid Nebula

poly 676 435 690 435 692 427 723 429 726 461 677 461 Crescent Nebula

rect 489 597 543 630 Lagoon Nebula

rect 555 592 595 626 Omega Nebula

rect 574 646 614 689 Eagle Nebula

rect 444 633 500 678 Cat's Paw Nebula

rect 90 502 161 529 Eta Carinae Nebula

rect 442 37 491 69 Crab Nebula

rect 517 158 547 175 Messier 37

rect 527 172 559 190 Messier 36

rect 533 191 563 208 Messier 38

rect 408 257 434 280 Messier 50

rect 327 232 357 257 Messier 46

rect 422 285 454 302 Messier 67

rect 553 321 582 338 Messier 34

rect 433 305 461 321 Messier 48

rect 409 314 435 330 Messier 41

rect 425 328 456 345 Messier 47

rect 474 343 500 365 Messier 44

rect 502 345 528 368 Messier 45

rect 542 378 568 399 Messier 39

rect 714 285 748 308 Messier 52

rect 352 285 379 308 Messier 93

rect 489 421 513 444 Messier 7

rect 495 452 518 473 Messier 6

rect 522 456 549 476 Messier 25

rect 512 478 539 500 Messier 23

rect 531 575 555 593 Messier 21

rect 556 564 589 580 Messier 18

rect 605 598 640 617 Messier 26

rect 630 618 654 639 Messier 11

rect 484 234 510 256 Messier 35

rect 287 248 316 276 NGC 2362

rect 370 359 411 381 IC 2395

rect 359 413 390 449 NGC 3114

rect 407 396 444 432 NGC 3532

rect 594 356 644 372 IC 1396

rect 458 392 502 406 IC 2602

rect 407 494 443 521 NGC 6087

rect 437 464 472 497 NGC 6025

rect 262 478 300 506 NGC 3766

rect 513 427 554 451 IC 4665

rect 181 445 226 472 IC 2581

rect 212 506 257 526 IC 2944

rect 213 565 246 598 NGC 4755

rect 128 463 154 494 NGC 3293

rect 362 591 392 628 NGC 6067

rect 404 548 437 587 NGC 6193

rect 425 595 453 630 NGC 6231

rect 461 556 498 585 NGC 6383

rect 58 506 91 521 Tr 14

rect 77 520 108 536 Tr 16

rect 797 130 831 153 Messier 103

rect 665 408 691 434 Messier 29

rect 746 139 782 157 hPer

rect 763 117 804 132 chi Per

rect 152 485 194 500 Col 228

rect 456 377 492 393 o Vel

poly 0 0 496 0 841 130 976 221 972 421 633 224 277 129 5 111 Perseus Arm

poly 2 230 444 239 688 340 970 495 971 639 559 452 317 387 1 380 Orion Arm

poly 2 481 423 492 694 606 922 757 470 761 234 669 1 668 Sagittarius Arm

rect 879 666 965 684 Star cluster

rect 878 684 944 699 Nebula

desc bottom-left

References

{{Reflist|refs=

{{cite journal |last1=Reid |first1=Mark |last2=Zheng |first2=Xing-Wu |title=New View of the Milky Way |journal=Scientific American |date=April 2020 |volume=322 |issue=4 |pages=28 |doi=10.1038/scientificamerican0420-28 |pmid=39014607 }}

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