The Orion Arm, also known as the Orion–Cygnus Arm, is a minor spiral arm within the Milky Way Galaxy spanning 3500abbr=offNaNabbr=off in width and extending roughly 20000ly in length.[1] 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.
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.
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.
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.[2]
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 8000pc from the Galactic Center.
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%. 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.[3] [4] [5] [6]
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.[7] 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°.[6]
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.[8]
The Orion Arm contains a number of Messier objects:
rect 126 149 188 182 Rosette Nebularect 285 116 327 145 Crab Nebularect 243 245 284 274 Orion Nebularect 299 288 345 312 Trifid Nebularect 343 304 384 333 Lagoon Nebularect 393 322 434 353 Omega Nebularect 445 322 494 353 Eagle Nebularect 424 244 483 280 North America Nebularect 293 248 319 266 Rigelrect 225 179 299 246 Orion's Beltrect 331 211 368 234 Polarisrect 318 236 353 259 Sunpoly 302 176 303 241 315 242 361 177 Betelgeuserect 419 222 458 245 Denebpoly 0 123 508 118 637 160 637 217 470 163 0 178 Perseus Armpoly 2 202 460 201 633 261 637 326 408 260 1 258 Orion Armpoly 1 284 397 293 633 360 637 477 541 475 357 413 0 400 Sagittarius Arm
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rect 396 142 447 173 Rosette Nebularect 376 230 426 258 Seagull Nebularect 463 264 501 292 Cone Nebularect 528 284 576 322 California Nebularect 695 117 741 149 Heart Nebularect 461 301 494 339 Orion Nebularect 691 154 739 182 Soul Nebularect 568 371 625 405 North America Nebularect 643 366 687 402 Cocoon Nebularect 688 392 761 429 Gamma Cygni Nebularect 594 404 625 444 Veil Nebularect 513 541 550 578 Trifid Nebulapoly 676 435 690 435 692 427 723 429 726 461 677 461 Crescent Nebularect 489 597 543 630 Lagoon Nebularect 555 592 595 626 Omega Nebularect 574 646 614 689 Eagle Nebularect 444 633 500 678 Cat's Paw Nebularect 90 502 161 529 Eta Carinae Nebularect 442 37 491 69 Crab Nebularect 517 158 547 175 Messier 37rect 527 172 559 190 Messier 36rect 533 191 563 208 Messier 38rect 408 257 434 280 Messier 50rect 327 232 357 257 Messier 46rect 422 285 454 302 Messier 67rect 553 321 582 338 Messier 34rect 433 305 461 321 Messier 48rect 409 314 435 330 Messier 41rect 425 328 456 345 Messier 47rect 474 343 500 365 Messier 44rect 502 345 528 368 Messier 45rect 542 378 568 399 Messier 39rect 714 285 748 308 Messier 52rect 352 285 379 308 Messier 93rect 489 421 513 444 Messier 7rect 495 452 518 473 Messier 6rect 522 456 549 476 Messier 25rect 512 478 539 500 Messier 23rect 531 575 555 593 Messier 21rect 556 564 589 580 Messier 18rect 605 598 640 617 Messier 26rect 630 618 654 639 Messier 11rect 484 234 510 256 Messier 35rect 287 248 316 276 NGC 2362rect 370 359 411 381 IC 2395rect 359 413 390 449 NGC 3114rect 407 396 444 432 NGC 3532rect 594 356 644 372 IC 1396rect 458 392 502 406 IC 2602rect 407 494 443 521 NGC 6087rect 437 464 472 497 NGC 6025rect 262 478 300 506 NGC 3766rect 513 427 554 451 IC 4665rect 181 445 226 472 IC 2581rect 212 506 257 526 IC 2944rect 213 565 246 598 NGC 4755rect 128 463 154 494 NGC 3293rect 362 591 392 628 NGC 6067rect 404 548 437 587 NGC 6193rect 425 595 453 630 NGC 6231rect 461 556 498 585 NGC 6383rect 58 506 91 521 Tr 14rect 77 520 108 536 Tr 16rect 797 130 831 153 Messier 103rect 665 408 691 434 Messier 29rect 746 139 782 157 hPerrect 763 117 804 132 chi Perrect 152 485 194 500 Col 228rect 456 377 492 393 o Velpoly 0 0 496 0 841 130 976 221 972 421 633 224 277 129 5 111 Perseus Armpoly 2 230 444 239 688 340 970 495 971 639 559 452 317 387 1 380 Orion Armpoly 2 481 423 492 694 606 922 757 470 761 234 669 1 668 Sagittarius Armrect 879 666 965 684 Star clusterrect 878 684 944 699 Nebula
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