We will continue our efforts to become more familiar with our galactic neighborhood, this time we take a look at the open clusters that lie within our spiral arm (Orion) and the adjoining Perseus and Sagittarius arms. The images of the clusters come from the SEDS listing of open star clusters. The CMDs (explained below) are from WEBDA: a site devoted to stellar open clusters.
The following table shows some of the open clusters along with their color-magnitude diagrams (CMDs). The color of a star, given by B-V, compares the magnitudes of the stars as recorded through a Blue filter and then through a Visual filter. Larger values of B-V mean cooler, redder stars. This is just another way of presenting stars in a diagram. Note, these diagrams are from actual data.
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The first thing you may wish to do is open the map of our galactic neighborhood out to about 5000 light years. Then, as you review the map, the images of the clusters and the corresponding color-magnitude diagrams (CMDs), you can get an idea of just how far away the clusters are by looking at the apparent magnitudes, V. From how the main sequence looks in the CMD, you should get a rough idea of how old the cluster is; that is, young, middle-age, or old! Set up your desktop so that you can view the map and the images and CMDs of the clusters at the same time.
There are some other things to notice. Compare the number of stars that appear in each image to the number of data points in the CMD for that cluster. Compare how bright the stars look in the image to the general range of apparent magnitudes. Compare how far away the cluster is from us on the map to the general range of apparent magnitudes. Take a close look at M16 and its CMD as this is a very, very young cluster. If M16 is the youngest cluster out of this group, which cluster is the oldest? How many red giants are in the cluster? Were any white dwarfs observed? Unfortunately, the images were taken by different cameras, and so it is difficult to do any direct comparisons, just approximations.
Here is an image of the Hyades cluster and surrounding sky.
Now, take a look at the CMD.
You should immediately notice the four red giants, and numerous white dwarfs. Once we know the distance to this cluster, we can calculate the absolute magnitudes of the stars by substituting the apparent magnitudes shown here. Once we have the CMDs of the other clusters, we can compare the main sequences and adjust each one over the Hyades main sequence, making sure we don't shift the stars in color. Then, the difference between the apparent magnitude of the more distant cluster at a given (B-V) and the absolute magnitude of the Hyades at the same (B-V) gives us the distance to the farther cluster using our old friend the magnitude-distance equation. This method is called "main-sequence fitting" and it is another step in the distance ladder.
| Designation | Image | Color-Magnitude Diagram |
|---|---|---|
| M6 |  |
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| M7 |  |
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| M11 |  |
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| M16 |  |
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| M21 |  |
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| M23 |  |
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| M25 |  |
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| M34 |  |
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| M35 |  |
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| M36 |  |
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| M37 |  |
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| M39 |  |
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| M41 |  |
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| M44 |  |
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| M45 |  (Image by George Greaney) |
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| M47 |  |
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| M48 |  |
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| M50 |  |
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| M67 |  |
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| M93 |  |
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| M103 |  |
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