HII Regions
HII Regions
Goals: Discover how star-birthing regions are distributed throughout the Galaxy.
Before starting, turn on: stars
You will be using: h2, galaxy, galGrid
Let's turn to one of the youngest objects in the Atlas. We've discussed OB associations as being generally older versions of open clusters, which themselves are typically between 5 million and a few billion years old. Now let's examine where the open clusters are born, in stellar nurseries called HII regions (pronounced “H-two”).
HII Regions Are Tightly Correlated to the Galactic Disk
From a perspective outside the Galaxy, turn on the h2 group. Each of these blue points represents an area of stellar birth in the Galaxy, a giant cloud of excited hydrogen gas, glowing from the stars born within them. Like the open clusters, they are concentrated in the disk of the Galaxy, more so than the clusters.
HII regions are also seen to greater distances in the Galaxy. As opposed to open clusters and OB associations, which are huddled around the Sun, we see HII regions across the Galaxy. Astronomers use radio telescopes to penetrate the gas and dust, allowing them to see signature elements, such as carbon monoxide, that are used to find these clouds across the Galaxy. The only part of the Galaxy invisible to us at these wavelengths is that part opposite the Galactic center, where a high concentration of gas and dust obscures our view to those objects beyond.
When you inspect the HII regions face-on (looking down on the Galactic disk), the data form a pattern that resembles a spider. There is a concentration near the Sun, as you might expect from observed data, but beyond the Sun the HII regions form in lines, like the long legs of a spider. Turn off the galGrid and galaxy to see this more clearly. Then bring back galaxy for reference.
Astronomers Use HII Regions to Map the Spiral Arms
These “legs” are in fact the spiral arms of our Galaxy. You may wonder why HII regions are seen to such great distances while we have distances to open clusters only near the Sun. The answer to this question involves the method for observing these objects. In the case of open clusters, we need to see the star and observe it with a telescope so that we can dissect its properties from their light. Because of the gas and dust in the Galaxy, those stars across the Milky Way are invisible to us.
HII regions mark the active part of the Galaxy, where the Galaxy rejuvenates itself through new star formation. But if stars are born in the Galaxy, that must mean they die too. Let's look at where stellar death occurs in the Galaxy.
© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott