OB Associations
OB Associations
Goals: Discover how the OB associations reveal the structure of the Galaxy.
Before starting, turn on: stars, ob, galac
You will be using: 10kly, galaxy
You've seen the night-sky view of the OB associations in the “Two-Dimensional Distributions” tutorial. Let's turn everything off in the Milky Way Atlas (gall off) and start with the stars, OB associations, and Galactic coordinates groups on.
OB Associations Are in the Galactic Disk
Again, it's easy to see that the OB associations are found in the Galactic plane. Rather than look at their distribution from Earth's perspective, let's look upon them from a point away from Earth. Fly backward from the Sun until you see all the OB associations, then orbit them from this vantage point.
You should still see the Galactic coordinates sphere encircling the Sun. It has a radius of 1,000 light-years. If you fly back toward the sphere, you will notice a few associations within that distance. For scale, turn on the 10,000-light-year grid. This grid is in the plane of the Galaxy and spans the OB associations. This data set represents not all the OB associations in the Galaxy but those that have known distances, those in the neighborhood.
OB Associations Are Loose Groupings of New Stars
Each of these points represents 10 to 100 stars that have dispersed to the point where they are no longer held together by their mutual gravity. O and B stars are among the hottest, most luminous stars and burn their nuclear fuel quickly. Because O stars live for only 1 million years and B stars for tens of millions of years, the lifetime of OB associations is not long compared with that of other objects in the Galaxy or the Galaxy itself.
As we have discussed in previous sections, the Galactic disk is the lifeblood of the Galaxy, where the process of stellar birth and death is ongoing. As you orbit the Sun from a distance such that you see all the associations, you can see they lie in a relatively flat plane. Turn on the Milky Way image to see how these fit into the larger picture.
OB Associations Trace Spiral Structure
Because OB associations contain young stars, they form in the spiral arms of the Galaxy and are good tracers of Galactic structure. Their color coding reflects this. The Sun is located on the inner side of the Orion Arm. Sometimes this is called the Orion-Cygnus Spur, as it may be a minor connector between major spiral arms. The OB associations in this arm are colored blue and purple. Toward the Galactic center, we look toward the constellation Sagittarius and the Sagittarius Arm of the Galaxy. The associations in this arm are olive green. Opposite this, we look away from the Galactic center toward the Perseus Arm. Here the associations are orange.
The OB associations allow you to examine the local spiral structure of the Galaxy. Now let's look at another type of star grouping called open clusters and see what they tell us about the structure of our Galaxy.
© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott