Orion Arm

The Orion Arm is a minor spiral arm that is across and approximately in length, containing the Solar System, including Earth. It is also referred to by its full name, the Orion–Cygnus Arm, as well as Local Arm, Orion Bridge, and formerly, the Local Spur and Orion Spur.

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.

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 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.

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. 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°. 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.Shen, j., and Zheng, X.
Shen, j., and Zheng, X., 2020, RAA, Vol.20, No.10,159

Messier objects

The Orion Arm contains a number of Messier objects:
*The Butterfly Cluster (M6)
*The Ptolemy Cluster (M7)
* Open Cluster M23
* Open Cluster M25
*The Dumbbell Nebula (M27)
* Open Cluster M29
* Open Cluster M34
* Open Cluster M35
* Open Cluster M39
*Winnecke 4 (M40)
* Open Cluster M41
*The Orion Nebula (M42)
*The De Mairan's Nebula (M43)
*The Beehive Cluster (M44)
*The Pleiades (M45)
* Open Cluster M46
*Open Cluster M47
*Open Cluster M48
* Open Cluster M50
*The Ring Nebula (M57)
*Open Cluster M67
* M73
*The Little Dumbbell Nebula (M76)
* Diffuse Nebula M78
* Open Cluster M93
*The Owl Nebula (M97)

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, 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

Other maps

See also

* Galactic disc
* Gould Belt
* Jon Lomberg's Milky Way painting used as background for Kepler Mission diagram, showing our location on the Orion Spur
* Local Bubble
* Loop I Bubble
* List of Messier objects
* List of nearest stars and brown dwarfs

References

External links

Messier Objects in the Milky Way (SEDS)A 3D map of the Milky Way Galaxy

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Milky Way arms
Galactic astronomy
Spiral galaxies