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:

• "+II", Outer Arm
• "+I", Perseus Arm
• "0", Local Arm, Orion Arm
• "-I", Sagittarius Arm, Sagittarius-Carina Arm
• "-II", Scutum-Crux Arm
• Norma Arm

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

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 

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     

• "*": Listed as in the Orion Arm in

Last Modification: December 7, 2000