The Orion Arm, also known as the Orion–Cygnus Arm, is a minor spiral arm within the Milky Way Galaxy spanning in width and extending roughly in length. This galactic structure encompasses the

Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

, 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 The Galactic Center is the barycenter of the Milky Way and a corresponding point on the rotational axis of the galaxy. Its central massive object is a supermassive black hole of about 4 million solar masses, which is called Sagittarius A*, a ...

, and the

Perseus Arm The Perseus Arm is one of two major spiral arms of the Milky Way galaxy. The second major arm is called the Scutum–Centaurus Arm. The Perseus Arm begins from the distal end of the long Milky Way central bar. Previously thought to be 13,000 ligh ...

'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. 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 The Local Bubble, or Local Cavity, is a relative superbubble, cavity in the interstellar medium (ISM) of the Orion Arm in the Milky Way. It contains the List of nearest stars and brown dwarfs, nearest stars and brown dwarfs and, among others, the ...

. It is approximately from the

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%. 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.

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. 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.

Messier objects

The Orion Arm contains a number of

Messier objects The Messier objects are a set of 110 astronomical objects catalogued by the French astronomer Charles Messier in his ' (''Catalogue of Nebulae and Star Clusters''). Because Messier was interested only in finding comets, he created a list of thos ...

: *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 The Pleiades (), also known as Seven Sisters and Messier 45 (M45), is an Asterism (astronomy), asterism of an open cluster, open star cluster containing young Stellar classification#Class B, B-type stars in the northwest of the constellation Tau ...

(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)

Maps

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 The Crab Nebula (catalogue designations M1, NGC 1952, Taurus A) is a supernova remnant and pulsar wind nebula in the constellation of Taurus (constellation), Taurus. The common name comes from a drawing that somewhat resembled a crab with arm ...

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 Polaris is a star in the northern circumpolar constellation of Ursa Minor. It is designated α Ursae Minoris (Latinisation of names, Latinized to ''Alpha Ursae Minoris'') and is commonly called the North Star or Pole Star. With an ...

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

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

rect 517 158 547 175

Messier 37 Messier 37 (also known as M37, New General Catalogue, NGC 2099, or the Salt and Pepper Cluster) is the brightest and richest open cluster in the constellation Auriga (constellation), Auriga. It was discovered by the Italian astronomer Giovanni Ba ...

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

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 A nebula (; or nebulas) is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral, or molecular hydrogen and also cosmic dust. Nebulae are often star-forming regions, such as in the Pillars of Creation in ...

desc bottom-left

References

External links

Messier Objects in the Milky Way (SEDS)A 3D map of the Milky Way Galaxy
{{Authority control Milky Way arms Galactic astronomy Spiral galaxies