OB Associations
OB Associations
| Group Name | ob |
| Reference | Various |
| Prepared by | Glenn LeDrew
(Main-Sequence Software: Ottawa, Canada) |
| Labels | Yes |
| Files | ob.speck, ob.label |
| Dependencies | ob.cmap |
| Census | 58 OB Associations |
| Notes | Olive green = Sagittarius Arm
Blue and Purple = Orion Arm (or Spur) Orange = Perseus Arm |
OB associations are groups of O and B stars that have dispersed to the extent that their mutual gravity no longer holds them together. Although they may be far apart, stars in an association share a common motion in space because they were formed from the same gas cloud. This allows astronomers to easily determine OB association membership stars.
Associations typically have anywhere from 10 to 100 stars. If they are young (less than 50 million years old), they will be populated with their original O and B stars (although the O stars last only about 1 million years). For each O or B star that forms, tens of cooler M stars will form.
O and B stars are quite luminous, making OB associations visible to great distances. And since O and B stars are young stars, we know they form in the regions of the Galaxy where star formation occurs: the spiral arms. Therefore, OB associations are good tracers for spiral structure.
In the Atlas, seven-sided polygons of different colors represent the data. The colors are representative of the spiral arm structure. All the associations in this data set lie within 10,000 light-years, close enough to see our neighboring arms: the Sagittarius Arm toward Galactic center (orange), the Perseus Arm opposite Galactic center (olive green), and the Orion Arm (blue and purple). The Sun and Earth are located just on the inner edge of the Orion Arm, whose bright stars are scattered throughout Orion, Cygnus, and Centaurus.
Color and Size
There are two settings for the color and luminosity of the OB associations. The default has colors set to the color data variable in the ob.speck file. This produces olive green (Sagittarius Arm), blue and purple (Orion Arm), and orange (Perseus Arm) markers. The luminosity of each point is set to the association's diameter divided by 100 parsecs (326 light-years), rendering markers whose size no longer decreases with distance but is set to a scaling of the association's physical size.
| Data Variables for OB Associations | |||
| Number | Name | Description | Units |
| 0 | lumin | Diameter divided by 100 pc | pc |
| 1 | color | Color based on spiral arm membership | -- |
If you want to use a constant sizing, type lum const 50. Now, as with other data groups, the larger the marker is, the closer it is to you. If you would like a constant color as well, type color const 0.4 0.5 1.0, and the polygons will be a blue color. To return to the settings based on the data variables color and lumin, type color color and lum lumin at Partiview's Command Line.
The Continuing Star Formation Story
In the section “HII Regions,” we mentioned the Orion Nebula and the stars that are forming there. The stars that form from this cloud will eventually be part of a new OB association called Ori OB1d. It is the latest in a string of star formation events that have occurred in this region.
In the Atlas, you will see the OB associations Ori OB1a, Ori OB1b, and Ori OB1c in the constellation Orion. Because O stars live only about a million years before exhausting their nuclear fuel, we can place constraints on the relative time line of their formation. If the Orion Nebula (Ori OB1d) currently has O stars, then Ori OB1c must be about 1 million years old. We know this because it is the supernova explosions from Ori OB1c that likely caused the stars in the Orion Nebula to form. Ori OB1b likely followed this pattern too; its hot stars exploded and caused stars to form in Ori OB1c. This is not the end of the story either. At some point an Ori OB1e association and nebula will form and the cycle of birth and death in the Galaxy will continue.
© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott