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Milky Way Spiral Structure

The disk of the Milky Way exhibits a spiral structure, which shows up in the distribution of the objects populating the disk component, i.e. population I stars and objects. These objects include, in particular, young stars, diffuse star-forming nebulae (H II regions) and open star clusters. Population I objects are very or rather young, in contrast to the very old population II objects of the Milky Way's Halo (globular clusters and old stars, including older planetary nebulae).

The origin of the spiral structure is thought to be in density waves which are triggered by gravitational disturbations, in particular during encounters with neighbor galaxies. The density waves first concern interstellar matter, which is compressed, forms diffuse nebulae which become starforming regions, and later form (open) clusters and associations of young stars, the most lumionous and conspicuous of which are massive, hot, blue and short-lived. Therefore, we see the most current density wave in the reddish H II regions, the elder next in the blueish clouds and associations of hot young stars, the next in age are characterized by type II (and Ib, Ic) supernova remnants and a bit older, less hot and blue open clusters. They can also be traced by the stars of the hotter spectral types, O, B, and A. The stars which are even older have dissipated into a more diffuse background; this includes intermediate population I stars, older star clusters and the younger representatives of the planetary nebulae.

The arms of the Milky Way, at least near the solar neighborhood in our Galaxy, are typically named for the constellations where (in the direction of which) more substantial parts of them are situated.

Our solar system is situated (by chance) in a smaller spiral arm, called the Local or Orion Arm, sometimes labelled "0". This small or intermediate arm connects the more substantial next inner arm and the next outer arm. The next inner arm, sometimes labelled "-I", is called the Sagittarius or Sagittarius-Carina Arm, the next outer arm, labelled "+I", is the Perseus Arm. At least one even outer arm, and two more inner arms have been identified.

A list of the Milky Way Spiral Arms follows, sorted from outside to inside near our solar neighborhood in the Galaxy:

These arms can be traced by deepsky objects, namely diffuse nebulae, open star clusters and associatons, (type II and Ib/Ic) supernova remnants, and main sequence stars of so-called "early" (hot) spectral types.

Perseus Arm

Object             Con Type  RA  (2000) Dec  dist

M 52 NGC 7654 Cas OC 23:24.2 +61:35 5.0 M103 NGC 581 Cas OC 01:33.2 +60:42 8.0 NGC 869 Per OC 02:19.0 +57:09 7.1 NGC 884 Per OC 02:22.4 +57:07 7.4 M 38 * NGC 1912 Aur OC 05:28.4 +35:50 4.2 M 1 NGC 1952 Tau SNR 05:34.5 +22:01 6.3 M 36 * NGC 1960 Aur OC 05:36.1 +34:08 4.1 M 37 * NGC 2099 Aur OC 05:52.4 +32:33 4.4
Orion Arm
Object             Con Type  RA  (2000) Dec  dist

M 6 NGC 6405 Sco OC 17:40.1 -32:13 2 M 7 NGC 6475 Sco OC 17:53.9 -34:49 0.8 M 23 NGC 6494 Sgr OC 17:56.8 -19:01 2.15 M 25 IC 4725 Sgr OC 18:31.6 -19:15 2 M 57 NGC 6720 Lyr PN 18:53.6 +33:02 2.3 M 27 NGC 6853 Vul PN 19:59.6 +22:43 1.25 M 29 NGC 6913 Cyg OC 20:23.9 +38:32 4.0 M 73 NGC 6994 Aqr OC? 20:58.9 -12:38 2.0 M 39 NGC 7092 Cyg OC 21:32.2 +48:26 0.825 M 76 NGC 650 Per PN 01:42.4 +51:34 3.4 M 34 NGC 1039 Per OC 02:42.0 +42:47 1.4 M 45 Tau C+N 03:47.0 +24:07 0.38 M 42 NGC 1976 Ori DN 05:35.4 -05:27 1.6 M 43 NGC 1982 Ori DN 05:35.6 -05:16 1.6 M 78 NGC 2068 Ori DN 05:46.7 +00:03 1.6 M 35 NGC 2168 Gem OC 06:08.9 +24:20 2.8 M 41 NGC 2287 CMa OC 06:46.0 -20:44 2.3 M 50 NGC 2323 Mon OC 07:03.2 -08:20 3 M 47 NGC 2422 Pup OC 07:36.6 -14:30 1.6 M 46 NGC 2437 Pup OC 07:41.8 -14:49 5.4 M 93 NGC 2447 Pup OC 07:44.6 -23:52 3.6 M 48 NGC 2548 Hya OC 08:13.8 -05:48 1.5 M 44 NGC 2632 Cnc OC 08:40.1 +19:59 0.577 M 67 NGC 2682 Cnc OC 08:50.4 +11:49 2.7 M 97 NGC 3587 UMa PN 11:14.8 +55:01 2.6 M 40 UMa DS 12:22.4 +58:05 0.51
Sagittarius Arm
Object             Con Type  RA  (2000) Dec  dist

M 20 NGC 6514 Sgr DN 18:02.6 -23:02 5.2 M 8 NGC 6523 Sgr DN 18:03.8 -24:23 5.2 M 21 NGC 6531 Sgr OC 18:04.6 -22:30 4.25 M 24 IC 4715? Sgr MWP 18:16.9 -18:29 10-15 NGC 6603 Sgr OC 18:18.4 -18:25 10 M 16 NGC 6611 Ser C/N 18:18.8 -13:47 7 M 18 NGC 6613 Sgr OC 18:19.9 -17:08 4.9 M 17 NGC 6618 Sgr DN 18:20.8 -16:11 5 M 26 NGC 6694 Sct OC 18:45.2 -09:24 5 M 11 * NGC 6705 Sct OC 18:51.1 -06:16 6
Notes: References:


Hartmut Frommert
Christine Kronberg
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Last Modification: December 7, 2000